Question 1

Which of the following is an example of a heterogeneous mixture?

Accepted Answer

Introduction / Context: Matter around us often exists as mixtures of different substances rather than pure compounds or elements. Mixtures can be classified as homogeneous, where composition is uniform throughout, or heterogeneous, where composition is not uniform and different phases may be visible. This question asks you to identify which given substance is best regarded as a heterogeneous mixture in basic school-level chemistry. Given Data / Assumptions: - Options are air, table salt, soda and brass. - We use standard textbook classifications for introductory chemistry. - Soda refers to carbonated soft drink or soda water, a mixture of water, dissolved carbon dioxide and sometimes flavouring. Concept / Approach: A homogeneous mixture has the same composition throughout and appears as a single phase to the naked eye. Air is usually considered a homogeneous mixture of gases. Brass is a homogeneous solid solution (alloy) of copper and zinc. Table salt, NaCl, is not a mixture at all; it is a pure compound. Soda, especially when carbon dioxide is actively coming out as bubbles, is more appropriately treated as a heterogeneous mixture of liquid and gas phases, with separate bubbles visible in the liquid. Therefore, among the given options, soda is the best example of a heterogeneous mixture at an everyday level. Step-by-Step Solution: Step 1: Analyse table salt. It is chemically sodium chloride, a single compound, not a mixture. Step 2: Consider air. It is a mixture of gases (mainly nitrogen and oxygen) that is uniform at ordinary scales and appears as a single gas phase, so it is treated as homogeneous. Step 3: Consider brass. It is a solid alloy of copper and zinc where the metals form a uniform solid solution, so it is homogeneous as well. Step 4: Look at soda. It contains liquid water, dissolved carbon dioxide and visible gas bubbles that are a separate phase, giving non-uniform composition when observed closely. Step 5: Conclude that soda is the heterogeneous mixture among the given options. Verification / Alternative check: When you pour soda into a glass, you can clearly see gas bubbles rising through the liquid. These bubbles are regions rich in carbon dioxide gas, distinct from the surrounding liquid water and solutes. This two-phase system (gas and liquid) is a defining characteristic of a heterogeneous mixture. In contrast, air at standard conditions appears as a single-phase gas and brass appears as a uniform solid, supporting their classification as homogeneous mixtures or solutions rather than heterogeneous mixtures. Why Other Options Are Wrong: Air: Generally treated as a homogeneous mixture of gases because its composition is uniform enough at the scales considered in basic chemistry and physics. Table salt: It is a pure compound, sodium chloride, not a mixture of different substances in variable proportions. Brass: This is a homogeneous solid alloy, a uniform mixture of copper and zinc atoms in one solid phase, so it is not heterogeneous. Common Pitfalls: Learners sometimes think that any mixture of gases or metals must be heterogeneous because its components are different. However, uniform gaseous mixtures and solid alloys are considered homogeneous. Another issue is ignoring the presence of visible bubbles or separate phases in soda and classifying it as homogeneous. Remember that heterogeneity relates to visible or detectable non-uniformity in composition or phase. Seeing more than one phase, such as gas bubbles in a liquid, is a clear sign of a heterogeneous mixture. Final Answer: Among the options given, soda is best regarded as a heterogeneous mixture.

Question 2

An object that possesses kinetic energy must necessarily be in motion rather than at rest, because kinetic energy is the energy associated with the motion of a body.

Accepted Answer

Introduction / Context: Kinetic energy is one of the most basic ideas in mechanics. It is the energy that a body has due to its motion. This question checks whether you clearly connect the presence of kinetic energy with the fact that an object must be moving, and not merely located at a certain height or kept at rest. Understanding this link helps in many topics, from work energy theorems to collision problems. Given Data / Assumptions: Kinetic energy is defined in classical mechanics as KE = 0.5 * m * v^2. Here m is the mass of the object and v is the speed or magnitude of velocity. We are told that an object has kinetic energy, so its KE is greater than zero. We compare different situations like being at rest, moving, falling and being at an elevated position. Concept / Approach: From the formula KE = 0.5 * m * v^2 it is clear that kinetic energy depends directly on the square of the speed. If the speed is zero, then v = 0 and KE = 0, so a body at complete rest has no kinetic energy in classical mechanics. As soon as a body starts moving, however slowly, its speed becomes non zero and therefore its kinetic energy becomes positive. Being at an elevated position gives the body potential energy, not kinetic energy, unless it is actually moving. So the correct way to think is that kinetic energy and motion always go together. Step-by-Step Solution: Step 1: Recall the kinetic energy formula KE = 0.5 * m * v^2.Step 2: Note that kinetic energy is zero if and only if the speed v is zero.Step 3: When an object is at rest, its speed v is zero, so KE = 0.Step 4: When an object is moving, v is non zero, so KE is positive.Step 5: Being at an elevated position only produces gravitational potential energy, not kinetic energy by itself.Step 6: A falling object is just one example of a moving body with kinetic energy, but motion in general is the key requirement. Verification / Alternative check: Think about everyday examples. A parked car has no kinetic energy until it starts moving. A rolling ball, a flying bird or flowing water all have kinetic energy because they are in motion. Similarly, when you lift an object and hold it still at a height, you give it potential energy, but it only gains kinetic energy when you let it move. These simple observations support the mathematical definition and confirm that motion is essential for kinetic energy. Why Other Options Are Wrong: An elevated position corresponds to gravitational potential energy, not necessarily kinetic energy, so option At an elevated position is not always correct. The option Falling describes a moving object, but it is only one special case of motion, not the general condition. An object At rest has zero speed, which directly implies zero kinetic energy, so that option is definitely incorrect. Only Moving correctly captures the general requirement for kinetic energy. Common Pitfalls: Many learners mix up potential and kinetic energy and may think that any high object automatically has kinetic energy. Others may focus only on special motions like falling instead of recognising that any motion, horizontal or vertical, gives kinetic energy. To avoid mistakes, always go back to KE = 0.5 * m * v^2 and check whether the speed is zero or not. Final Answer: An object that has kinetic energy must be moving.

Question 3

In optical instruments such as spectacles, cameras and microscopes, the lens is commonly made from which special type of optical glass?

Accepted Answer

Introduction / Context: Lenses are core components of many optical devices, including spectacles, cameras, microscopes and telescopes. They must be made from a type of glass that has well controlled optical properties such as refractive index and dispersion. This question checks whether you know the typical type of glass used to manufacture high quality lenses and introduces you to the idea that not all glass is the same from an optical point of view. Given Data / Assumptions: The object in question is a lens used in optical instruments. Several kinds of glass are listed: ordinary window glass, Pyrex glass, flint glass and cobalt glass. We assume we want good transparency, suitable refractive index and controllable dispersion for focusing light. The task is to identify the glass type most commonly associated with lens making in basic general knowledge questions. Concept / Approach: Optical glasses are broadly classified into crown glass and flint glass families. Flint glass is a type of optical glass with relatively high refractive index and significant dispersion due to the presence of lead oxide or similar components. These characteristics make flint glass useful in lens systems where designers combine different types of glass to correct chromatic aberration. Ordinary window glass and coloured glasses are not optimised for such optical performance. Pyrex is formulated for thermal resistance and is commonly used in laboratory glassware and cooking utensils, not as the main material for precision lenses. Step-by-Step Solution: Step 1: Recognise that lens glass must be optically clear and have a controlled refractive index.Step 2: Recall that optical engineers often use flint glass as one of the standard lens materials.Step 3: Note that ordinary window glass is not designed with precise dispersion properties in mind.Step 4: Remember that Pyrex glass is famous for being heat resistant, not for special optical qualities required in lenses.Step 5: Cobalt glass is simply glass coloured by cobalt compounds and is mostly decorative.Step 6: Conclude that flint glass is the most appropriate answer among the given options for lens manufacturing. Verification / Alternative check: In basic optics references, you will often see the terms crown glass prism and flint glass prism. Achromatic doublets, which are lens combinations designed to reduce colour fringing, usually pair crown and flint glasses because of their differing refractive indices and dispersions. This repeated use of flint glass in standard lens design supports the choice made here. Why Other Options Are Wrong: Ordinary window glass is inexpensive and transparent but does not offer the carefully controlled properties needed for precision lenses. Pyrex glass is tailored for high thermal shock resistance and is widely used in beakers and ovenware rather than in optical lenses. Cobalt glass is mainly used for coloured decorative items or filters and is not the standard base for lens manufacturing. Therefore these options do not match the requirement as closely as flint glass does. Common Pitfalls: Students may be tempted by Pyrex because it sounds technical or by ordinary glass because it seems commonly available. Others may think coloured glasses like cobalt glass are used in fancy instruments. To avoid confusion, remember that flint glass is a textbook example of optical glass used in lenses and prisms due to its optical properties, not just its availability or appearance. Final Answer: Lenses in many optical instruments are commonly made from flint glass.

Question 4

In basic thermodynamics, what is the key difference between thermal energy and heat energy when we describe the energy of a system and the energy transferred between systems?

Accepted Answer

Introduction / Context: Students often use the terms thermal energy and heat energy as if they were identical, but in thermodynamics they have related yet distinct meanings. Examiners like to test whether you understand that one term refers to energy stored inside a system, while the other refers to energy being transferred from one system to another. This question focuses on clarifying that difference in a precise, conceptual way. Given Data / Assumptions: We are talking about energy associated with the temperature of matter. Thermal energy is sometimes informally called internal energy due to microscopic motion. Heat energy is associated with energy transfer caused by a temperature difference. The question asks specifically for the difference between these two terms. Concept / Approach: Thermal energy is the total internal energy associated with the random motion and interactions of the particles inside a substance. It includes microscopic kinetic energies (vibration, rotation and translation of atoms and molecules) and parts of microscopic potential energy related to intermolecular forces. Heat, in contrast, is not a property that a system contains at an instant; it is energy in transit from one body to another because of a temperature difference. When two bodies at different temperatures are in contact, thermal energy flows from the hotter body to the colder body, and that transfer is what we call heat transfer. After the process ends, each body possesses internal or thermal energy, not heat as a stored quantity. Step-by-Step Solution: Step 1: Define thermal energy as the internal energy associated with the microscopic motion of particles inside a body.Step 2: Recognise that this energy is a state function; it depends on the state (temperature, volume, phase) of the system at a given moment.Step 3: Define heat energy as the energy that flows between systems because of a temperature difference.Step 4: Understand that heat is a process quantity; it refers to energy in transit, not to energy stored in a system.Step 5: Notice that after the transfer is complete, we no longer speak of heat inside a body, but of its internal or thermal energy.Step 6: Conclude that the correct distinction is that thermal energy is internal energy, whereas heat is energy transfer due to temperature difference. Verification / Alternative check: Thermodynamics textbooks emphasise that internal energy is a property of the system and is represented by symbols like U, while heat is denoted by Q and is defined only during a process. The first law of thermodynamics, written as ΔU = Q minus W, clearly treats Q as energy transferred and U as energy possessed. This formalism confirms that thermal energy belongs to the system, whereas heat is the mode by which thermal energy moves between systems. Why Other Options Are Wrong: The statement that thermal energy and heat energy are exactly the same ignores the important process versus state distinction and is therefore misleading. The idea that thermal energy applies only to solids and heat only to fluids is not supported by any physical theory. The claim that thermal energy is purely potential while heat is purely kinetic misunderstands internal energy, which usually has both kinetic and potential contributions. These incorrect options do not match the standard definitions used in physics. Common Pitfalls: A common mistake is to say that a body contains a certain amount of heat, when what is meant is internal energy or thermal energy. Another frequent confusion is to forget that heat must always be associated with a transfer between systems at different temperatures. To avoid these pitfalls, remember the simple rule: thermal energy is what the system has, heat is what is transferred because of a temperature difference. Final Answer: Thermal energy is the internal energy stored in the random motion of particles inside a system, whereas heat energy is the energy transferred between systems because of a temperature difference.

Question 5

In terms of its electronic configuration and molecular orbital structure, is the B2+ molecular ion paramagnetic or diamagnetic?

Accepted Answer

Introduction / Context: Paramagnetism and diamagnetism are important magnetic properties that depend on the presence or absence of unpaired electrons in atoms and molecules. This question asks specifically about the B2+ molecular ion, which is best analysed using molecular orbital theory. Understanding how to count electrons and place them into molecular orbitals is essential for predicting whether a molecule is paramagnetic or diamagnetic. Given Data / Assumptions: Boron has atomic number 5 and therefore 5 electrons in a neutral atom. The B2 molecule would have 10 electrons in total. The B2+ ion has one electron fewer than B2, so it has 9 electrons in total. Magnetic behaviour depends on whether there are unpaired electrons in the molecular orbitals. Concept / Approach: In simple molecular orbital theory for homonuclear diatomic molecules of light atoms, the order of orbitals up to the 2p level is sigma 1s, sigma 1s star, sigma 2s, sigma 2s star, then pi 2p and sigma 2p. For the B2 molecule with 10 electrons, the electrons fill up to the pi 2p orbitals, and there are unpaired electrons in these degenerate pi orbitals, giving paramagnetic behaviour. For B2+, one electron is removed from the highest occupied molecular orbital. Because the number of electrons becomes odd, it is not possible for all of them to be paired. Therefore at least one electron remains unpaired, which results in paramagnetism. Diamagnetism would require that all electrons are paired, which is not the case here. Step-by-Step Solution: Step 1: Count the electrons in neutral B2: each B contributes 5, so B2 has 10 electrons.Step 2: Remove one electron to form B2+, leaving 9 electrons available for the molecular orbitals.Step 3: Recall the approximate filling order for light diatomic molecules: sigma 1s, sigma 1s star, sigma 2s, sigma 2s star, then pi 2p orbitals.Step 4: Fill these orbitals with 9 electrons following the Pauli principle and Hund rule; the last occupied orbital will contain an unpaired electron.Step 5: Note that an odd total number of electrons makes complete pairing impossible, guaranteeing at least one unpaired electron.Step 6: Conclude that B2+ has unpaired electrons and is therefore paramagnetic rather than diamagnetic. Verification / Alternative check: A simple rule of thumb is that species with an odd number of electrons cannot have all electrons paired, so they must be paramagnetic. Since B2+ has 9 electrons, this rule alone suggests paramagnetic behaviour. More detailed molecular orbital diagrams found in standard inorganic or physical chemistry texts confirm that the appropriate highest occupied orbital for B2 and B2+ contains unpaired electrons, leading to observable paramagnetism. Why Other Options Are Wrong: The option Diamagnetic would require all electrons to be paired, which is impossible with 9 electrons distributed in molecular orbitals. The idea that the magnetism depends only on temperature ignores the key role of electron pairing and molecular structure. The suggestion that it cannot be determined from molecular orbital theory is incorrect because molecular orbital analysis is precisely the standard method used to determine the magnetic character of diatomic molecules like B2+. Common Pitfalls: Students sometimes confuse the neutral B2 molecule with its ion and may incorrectly apply the electron count of 10 instead of 9. Others rely on memory of specific examples without understanding the underlying orbital filling principles. A simple precaution is to always count the total electrons carefully and to remember that any species with an odd number of electrons must be paramagnetic in basic molecular orbital theory. Final Answer: The B2+ molecular ion is paramagnetic because it has an unpaired electron.

Question 6

In basic electrical terminology, electrical pressure in a circuit is commonly referred to by which name?

Accepted Answer

Introduction / Context: Everyday explanations of electricity often use analogies to water flow, in which electrical pressure is compared to water pressure. In formal electrical engineering and physics, this electrical pressure has a precise name and unit. This question checks whether you can correctly connect the informal phrase electrical pressure with the more standard term used in circuit theory. Given Data / Assumptions: The phrase electrical pressure refers to what pushes charges through a conductor. We are given four basic electrical quantities as options: current, voltage, resistance and power. We assume a direct current or simple circuit context, as in basic school level physics. Our task is to identify which technical term corresponds to electrical pressure. Concept / Approach: In a simple water flow analogy, pressure difference drives water through a pipe. In an electric circuit, potential difference or electromotive force drives electric charges through a conductor. This potential difference is measured in volts and is commonly called voltage. Many informal sources call it electrical pressure because it describes how strongly charges are pushed. Current, on the other hand, is the rate of flow of charge; resistance is the opposition to this flow; power is the rate at which electrical energy is converted. None of these directly represent the pushing pressure itself. Step-by-Step Solution: Step 1: Recall that voltage or potential difference is the work done per unit charge in moving a test charge between two points.Step 2: Recognise that this quantity is what provides the driving force that pushes charges through the circuit.Step 3: Understand that in analogies and informal teaching, this is often described as electrical pressure.Step 4: Note that current measures charge flow per unit time and is not the pressure itself.Step 5: Realise that resistance describes how much a material opposes current flow.Step 6: Conclude that electrical pressure is another way of describing voltage or potential difference. Verification / Alternative check: Textbooks and beginner guides to electronics frequently use the phrase water pressure for voltage in diagrams. Standard units further support this: voltage is measured in volts and is analogous to pressure in pascals in fluid mechanics. Current in amperes corresponds to flow rate, resistance in ohms represents restriction, and power in watts describes energy conversion rate. The analogy confirms that voltage is the correct technical term for electrical pressure. Why Other Options Are Wrong: Current is the amount of charge passing a point per unit time and depends on both voltage and resistance. Resistance is a measure of opposition to current and does not represent the driving cause of movement. Power describes how quickly energy is used or generated, but it is a consequence of both voltage and current, not a direct measure of the push itself. Therefore these options do not match the idea of electrical pressure. Common Pitfalls: Some students mix up voltage and current because both are discussed together in Ohm law. Others may have heard informal explanations that blur the distinction. To avoid confusion, remember that voltage is the cause or push, current is the effect or flow, resistance is the opposition, and power is the rate of doing work in the circuit. Final Answer: Electrical pressure is commonly called voltage or potential difference.

Question 7

In order to reduce friction between surfaces in contact, it is generally more effective to change the type of motion from sliding to which other form of motion?

Accepted Answer

Introduction / Context: Friction is a resisting force that acts between surfaces in contact. In many machines, vehicles and tools, it is desirable to reduce friction to save energy and prevent wear. One very common method of reducing friction is to replace sliding contact with rolling contact, as in the use of ball bearings and wheels. This question asks you to identify the correct change in type of motion that reduces friction effectively. Given Data / Assumptions: Two surfaces are in contact and can experience either sliding or rolling motion relative to each other. We assume normal conditions where lubrication or special materials are not the main focus. The aim is to reduce frictional force between these surfaces. Several choices mention static, dynamic, sliding and rolling motion. Concept / Approach: Sliding friction occurs when one surface moves tangentially past another in direct contact, like a block dragged over a table. Rolling friction occurs when a body such as a wheel or ball rolls over a surface. In general, rolling friction is much smaller than sliding friction for the same normal reaction, because deformation and micro locking between surfaces are reduced. That is why vehicles use wheels, and machines use ball bearings. Therefore, changing from sliding motion to rolling motion significantly reduces friction in many practical systems. Step-by-Step Solution: Step 1: Recall that sliding friction involves surfaces scraping past each other, producing a relatively large resistive force.Step 2: Recall that rolling friction involves a rolling object, like a wheel, where only a small area is in contact at any instant.Step 3: Compare typical coefficients of sliding friction and rolling friction; rolling friction coefficients are much smaller.Step 4: Identify that using rollers or ball bearings converts sliding motion into rolling motion.Step 5: Note that static and dynamic terms refer to whether the body is at rest or moving and do not by themselves guarantee lower friction.Step 6: Conclude that friction is reduced by changing from sliding to rolling motion, which matches the correct option. Verification / Alternative check: Everyday examples confirm this reasoning. Moving a heavy cupboard directly on the floor is difficult, but placing it on rollers makes it much easier to push. Bicycles and cars use wheels to roll rather than slide, which greatly reduces energy loss and wear. Ball bearings are explicitly designed to replace sliding contact between parts with rolling contact. All these applications support the principle that rolling motion leads to smaller friction than sliding motion. Why Other Options Are Wrong: Changing from static to dynamic or from dynamic to static does not clearly address whether contact is sliding or rolling. Rolling to sliding would actually increase friction, which is the opposite of what we want. Therefore only the option that specifically replaces sliding with rolling matches the correct strategy for friction reduction. Common Pitfalls: Some learners may confuse the terms static friction and dynamic friction with sliding and rolling friction and may believe that any change from static to dynamic reduces friction. However, friction is strongly influenced by the type of contact. To avoid confusion, focus on the physical mechanism: rolling motion reduces direct scraping and interlocking of surface asperities, making it more efficient than sliding motion for most engineering applications. Final Answer: Friction is most effectively reduced by changing from sliding motion to rolling motion.

Question 8

Among the common metals listed, which one is generally regarded as the best conductor of heat in practical applications?

Accepted Answer

Introduction / Context: Thermal conductivity is the property of a material that indicates how quickly it can transfer heat through itself. Metals are usually good conductors of heat, but some metals conduct heat better than others. This question asks you to identify which of the listed metals is generally regarded as the best conductor of heat, a simple yet important point in basic physics and engineering. Given Data / Assumptions: The metals mentioned are silver, gold and copper. We compare their thermal conductivities under normal conditions. All three are good conductors, but one stands out as the best among them. The answer is based on standard thermal conductivity tables used in physics. Concept / Approach: Most metals are good thermal conductors because of the free electrons that can carry energy through the lattice. Among common metals, silver has one of the highest thermal conductivities, slightly higher than copper and significantly higher than gold. For this reason, silver is often cited in textbooks as the best conductor of both heat and electricity among ordinary metals. Copper is used widely in industry mainly because it is cheaper and still has very high conductivity, but silver holds the top position theoretically. Step-by-Step Solution: Step 1: Recall or consult standard values: silver has very high thermal conductivity, closely followed by copper.Step 2: Recognise that gold, while a good conductor, is not as good as silver or copper in terms of thermal conductivity.Step 3: Understand that the purpose of the question is to identify the single best conductor among the listed metals.Step 4: Note that silver is traditionally given as the best conductor in physics reference data.Step 5: Accept that practical applications may favour copper due to cost, but the property being asked is pure thermal conductivity, not economic choice.Step 6: Conclude that silver is the correct answer as the best conductor of heat in this list. Verification / Alternative check: Looking at typical numerical values in data tables, silver shows a thermal conductivity around 400 W per metre per Kelvin, copper is slightly lower, and gold is lower again. This confirms that silver is the top performer in terms of heat conduction. Laboratory measurements and engineering handbooks consistently support this ordering of metals. Why Other Options Are Wrong: Gold is a good conductor but does not match silver in thermal conductivity and is also extremely expensive, so it is not usually chosen as the best conductor. Copper is widely used because it is cheaper and still very conductive, but the question is asking about the best conductor in terms of physical property, not popularity. The statement that all of the above conduct heat equally well is incorrect because there are measurable differences in their conductivities. Common Pitfalls: Students may think of copper as the best conductor purely because copper wires are common in domestic wiring and cookware. Others might be attracted by gold due to its high value and frequent use in electronics for corrosion resistance. To avoid such misconceptions, base your answer on physical data rather than everyday use. Silver is the best conductor of heat among these metals, even if it is used less often for cost reasons. Final Answer: Silver is generally regarded as the best conductor of heat among the listed metals.

Question 9

In astronomy and astrophysics, a light year is used as a convenient unit to measure which physical quantity?

Accepted Answer

Introduction / Context: Distances in the universe are enormous compared with everyday scales, so astronomers use special units that make these large numbers easier to handle. One of the most famous units is the light year. Because the term contains the word year, many people incorrectly think it measures time. This question checks whether you know that a light year is actually a measure of distance and not of time or speed. Given Data / Assumptions: We are dealing with the term light year, used in astronomy. Light travels at a nearly constant speed in vacuum, approximately 3.0 * 10^8 m per second. The term year usually refers to time, but here it is part of a compound unit. We must choose whether light year measures time, speed, distance or mass. Concept / Approach: A light year is defined as the distance that light travels in vacuum in one year. Because speed equals distance divided by time, multiplying the speed of light by the time of one year gives a huge distance. This distance is roughly 9.46 * 10^12 kilometres. Astronomers use it to express how far away stars and galaxies are. Even though the definition uses a time interval, the quantity itself is a distance, not a duration, not a speed and certainly not a mass. Step-by-Step Solution: Step 1: Recall the speed of light c ≈ 3.0 * 10^8 m per second.Step 2: Take one year as a time interval of about 365 days, which equals roughly 3.156 * 10^7 seconds.Step 3: Compute the distance travelled by light in one year using distance = speed * time.Step 4: This gives distance ≈ 3.0 * 10^8 m per second * 3.156 * 10^7 seconds, which is about 9.46 * 10^15 metres.Step 5: Recognise that this huge value is a distance and that astronomers name this distance a light year.Step 6: Conclude that a light year is a unit of distance, used especially for stellar and galactic scales. Verification / Alternative check: Astronomy books typically give statements such as the nearest star is about 4.3 light years away, or our galaxy is about 100,000 light years across. In each case, the context makes it clear that these numbers describe how far objects are from each other, not how long something takes. Additionally, when converting between astronomical units, one can translate light years into kilometres or metres, which are standard distance units, confirming that the underlying quantity is distance. Why Other Options Are Wrong: The option time is tempting because the word year usually indicates time, but the definition of light year multiplies speed by time to give distance. The speed of light is a constant measured in metres per second, not in light years. Mass has no relation to the term and is measured in kilograms, so that option is completely unrelated. Therefore, only distance correctly matches the definition of a light year. Common Pitfalls: Many students and members of the public mistakenly say that something takes so many light years, as if light year measured time. This confusion arises because the term includes year. To avoid this mistake, always remember the formula distance = speed * time and note that light year refers to the result of that multiplication for light travelling for one year, which is purely a distance. Final Answer: A light year is a unit of distance, not time.

Question 10

In atomic physics, why does a hydrogen atom not normally emit X rays in its line spectrum, even though many heavier atoms do?

Accepted Answer

Introduction / Context: Atomic spectra arise when electrons jump between energy levels within atoms, emitting or absorbing photons with energies equal to the difference between these levels. X rays are very high energy photons, corresponding to large energy level differences, usually associated with inner shell transitions in heavy atoms. This question asks why hydrogen, the simplest atom, does not normally emit X rays in its spectrum, highlighting the relationship between energy differences and photon energies. Given Data / Assumptions: Hydrogen has only one electron bound to a single proton. Hydrogen spectral lines lie mainly in the visible and ultraviolet regions. X rays have very high photon energies, typically in the kilo electron volt range. Photon energy equals the difference between initial and final energy levels of the electron. Concept / Approach: The energy of a photon emitted in a transition equals ΔE between two levels. For hydrogen, the bound electron energy levels are given by simple formulas that produce differences corresponding mainly to visible and ultraviolet wavelengths, not the much shorter X ray wavelengths. In heavier atoms, inner electrons are much more tightly bound, and transitions between these deep levels release high energy photons in the X ray region. Therefore hydrogen, with relatively small energy differences between its levels, does not normally generate X rays in its emission spectrum. Step-by-Step Solution: Step 1: Recall that the energy of emitted radiation is given by E = h * f, where f is frequency, and also equal to the energy level difference ΔE.Step 2: Recognise that X rays correspond to very high values of E and therefore very large ΔE between energy levels.Step 3: In hydrogen, the electron is bound in relatively low energy levels, and transitions such as from n = 2 to n = 1 give energies in the ultraviolet, not X ray range.Step 4: In multi electron heavy atoms, inner shell electrons are much more strongly bound and transitions from higher shells to these inner shells release large energies in the X ray range.Step 5: Conclude that the energy level separations in hydrogen are not large enough to produce X ray photons.Step 6: Thus, hydrogen does not emit X rays in its normal atomic line spectrum. Verification / Alternative check: Experimental observations of the hydrogen spectrum show familiar series such as Lyman (ultraviolet), Balmer (visible) and Paschen (infrared). None of these extend into the X ray region. X ray spectra and characteristic X ray lines are instead discussed in the context of heavier elements like copper or iron. This clear separation in observed spectra confirms that hydrogen transitions do not normally reach the high energies corresponding to X rays. Why Other Options Are Wrong: The small size of hydrogen does not by itself prevent X ray emission; what matters is energy level structure. Having only one electron is also not the fundamental reason; what is crucial is the binding energy and level separations. The statement that hydrogen is highly electropositive refers to chemical behaviour, not to its spectral line energies. The most direct and physically correct explanation is that the energy level separations are too small to emit X ray photons. Common Pitfalls: Students may focus on the number of electrons or atomic size rather than the key concept of energy differences. Others might confuse the presence of a single electron with an inability to emit high energy photons. To avoid such errors, always relate spectral frequencies to ΔE between levels and compare those differences with the typical energy ranges for infrared, visible, ultraviolet and X rays. Final Answer: A hydrogen atom does not normally emit X rays because its energy level separations are too small to produce the very high energy photons required for X rays.

Question 11

Which one of the following physical quantities is a scalar quantity, having magnitude only and no associated direction?

Accepted Answer

Introduction / Context: In physics, it is important to distinguish between scalar and vector quantities. Scalars have only magnitude, while vectors have both magnitude and direction. This distinction affects how we add quantities and how they behave in equations. The question asks you to identify the scalar quantity from a list that includes both vector and scalar physical quantities. Given Data / Assumptions: Displacement describes change in position and has direction. Electric field describes the force per unit charge at a point and has direction. Acceleration describes the rate of change of velocity and has direction. Work is defined as force multiplied by displacement in the direction of the force. Concept / Approach: Displacement, electric field and acceleration are all defined with an inherent directional aspect. Displacement is a vector pointing from initial to final position. Electric field is a vector that shows the direction in which a positive test charge would be pushed. Acceleration is a vector indicating how the velocity vector changes. Work, however, is defined as W = F * s * cos(theta), where F and s are vectors and theta is the angle between them. The dot product of two vectors is a scalar, so work has magnitude but no inherent direction, making it a scalar quantity. Step-by-Step Solution: Step 1: Recall that scalars have magnitude only; vectors have both magnitude and direction.Step 2: Consider displacement: it indicates how far and in which direction an object has moved, so it is a vector.Step 3: Consider electric field: it specifies the direction and magnitude of the force per unit positive charge, so it is a vector.Step 4: Consider acceleration: as the rate of change of velocity, it must point in a specific direction, making it a vector.Step 5: Consider work: it is calculated as the dot product of force and displacement, giving a scalar value.Step 6: Conclude that work is the only scalar quantity among the given options. Verification / Alternative check: In textbook notation, displacement, electric field and acceleration are written in bold or with arrows above to indicate vector nature. Work is written as a plain symbol without vector notation and is measured in joules, a scalar unit. When adding works from different processes, we simply add their numerical values, unlike vector addition where directions matter. This confirms that work is scalar. Why Other Options Are Wrong: Displacement cannot be scalar because knowing only the distance is not enough to specify change in position; direction is required. Electric field must specify both how strong and in which direction it acts, so it is vectorial. Acceleration describes how velocity changes in direction and magnitude, making it clearly a vector. Therefore, these three cannot be the correct scalar quantity in this question. Common Pitfalls: Students sometimes confuse distance with displacement and may incorrectly think displacement is scalar. Others may treat any quantity without obvious directional wording as scalar, ignoring the formal definitions. To avoid such confusion, always check whether direction is essential to the definition of the quantity. If it is, you have a vector; if not, you likely have a scalar. Final Answer: Work is a scalar quantity, while displacement, electric field and acceleration are vector quantities.

Question 12

In practice, large distances to stars in astronomy are most commonly expressed in which unit among the following?

Accepted Answer

Introduction / Context: Astronomers must measure extremely large distances to stars and galaxies, far beyond the range where everyday units like kilometres are convenient. To express these enormous scales, special units are used. This question asks which unit is most commonly used for the distance to stars, reinforcing the idea that astronomy uses its own natural distance scales. Given Data / Assumptions: We are concerned with distances to stars, not to planets within our solar system. The options include several invented sounding units, but only one is standard. A light year is a well known astronomical unit. We assume standard definitions from astronomy and physics. Concept / Approach: The most commonly used unit for distances to nearby stars is the light year, which is defined as the distance light travels in vacuum in one year. While astronomers also use parsecs and astronomical units, a light year is widely used in popular science communication and in many basic physics questions. Terms like galactic unit, stellar mile or cosmic kilometre are not standard scientific units for star distances and are not used in serious astronomical measurements. Step-by-Step Solution: Step 1: Recall that the distance to the nearest star, Proxima Centauri, is about 4.2 light years.Step 2: Recognise that distances to stars are usually given in light years or parsecs, not in kilometres or miles because numerical values would be too large.Step 3: Note that the term light year is specifically designed as a distance unit from the product of the speed of light and one year of time.Step 4: Understand that the other units listed such as galactic unit or cosmic kilometre are not standard names used in astronomy textbooks.Step 5: Match the correct and recognisable unit, light year, to the requirement of the question.Step 6: Conclude that distances of stars are measured in light years in this context. Verification / Alternative check: Open any introductory astronomy or astrophysics text and you will find phrases like the star is thirty light years away or the galaxy is many thousand light years across. Such texts rarely use kilometres for star distances because the numbers would be unwieldy. Also, standard scientific unit lists do not mention stellar mile or cosmic kilometre as accepted units, which confirms that light year is the correct choice here. Why Other Options Are Wrong: Galactic unit is not a widely defined or accepted term; while an astronomical unit (AU) exists for distances within the solar system, a galactic unit is not standard for stars. Stellar mile and cosmic kilometre sound like invented descriptive phrases rather than rigorous units and are not used in serious astronomical measurement. Therefore, these options are incorrect. Common Pitfalls: Some students may confuse astronomical unit (AU), used for distances between the Earth and the Sun, with units for star distances. Others may be attracted to impressive sounding names like galactic unit. To avoid mistakes, remember that light year and parsec are the standard for stars, while AU is for planetary scales within the solar system. Final Answer: Distances of stars are most commonly expressed in light years.

Question 13

In Newtonian mechanics, which of Newton laws of motion is also commonly known as the law of inertia?

Accepted Answer

Introduction / Context: Newton formulated three fundamental laws of motion that form the basis of classical mechanics. The concept of inertia, which is the tendency of a body to resist changes in its state of rest or uniform motion, is central to the first of these laws. This question checks whether you know which numbered law is usually referred to as the law of inertia in physics textbooks. Given Data / Assumptions: There are three Newton laws of motion: first, second and third. Inertia describes the tendency of a body to maintain its current state of motion unless a net external force acts. One of Newton laws explicitly formalises this idea. We must match the law of inertia with the correct numbered law. Concept / Approach: Newton first law states that a body remains at rest or moves with uniform velocity in a straight line unless acted upon by a net external force. This law directly expresses the concept of inertia and defines inertial frames of reference. Newton second law relates net force to acceleration and mass, and Newton third law deals with action reaction pairs. Therefore the first law is the one that is also called the law of inertia, because it formalises and quantifies the idea that bodies resist changes in motion. Step-by-Step Solution: Step 1: Recall the wording of Newton first law: a body continues in its state of rest or uniform straight line motion unless acted on by a net external force.Step 2: Recognise that this is a direct statement of the property of inertia.Step 3: Recall that Newton second law provides the equation F = m * a, relating force, mass and acceleration.Step 4: Recall that Newton third law states that every action has an equal and opposite reaction.Step 5: Identify that only the first law focuses explicitly on inertia and unforced motion.Step 6: Conclude that Newton first law is also called the law of inertia. Verification / Alternative check: Introductory physics books sometimes introduce inertia even before giving the formal statement of Newton first law, then explain that the law of inertia is an alternative name for the first law. Historical discussions of Galileo and Newton also emphasise that understanding inertia was a key step towards formulating this first law. You rarely see the phrase law of inertia applied to the second or third law, which confirms the correct association. Why Other Options Are Wrong: Newton second law describes how force causes acceleration and introduces the concept of momentum, but it does not itself define inertia. Newton third law deals with the mutual forces between interacting bodies, the action reaction pair, which again is distinct from inertia. Saying that all of the above are the law of inertia is misleading because each law addresses a different fundamental aspect of motion. Common Pitfalls: Some students mix up the numbering of Newton laws or think that inertia is part of all laws equally. Others may remember F = m * a from the second law and incorrectly associate inertia with it because mass appears in that equation. To avoid confusion, remember that the first law emphasises what happens when net force is zero, which is exactly the behaviour that defines inertia. Final Answer: The law of inertia is Newton first law of motion.

Question 14

A person is sitting in a train moving forward and facing the engine. He tosses a coin straight up and the coin falls behind him on the seat. From this observation, what can we conclude about the motion of the train?

Accepted Answer

Introduction / Context: This is a classic conceptual mechanics problem about motion in non inertial frames. When a person tosses a coin inside a moving train, the path of the coin relative to the person depends on whether the train is moving uniformly or accelerating or decelerating. This question tests your understanding of relative motion and how horizontal acceleration of the train affects the apparent fall of the coin for an observer inside the train. Given Data / Assumptions: The passenger is facing the engine, so forward direction is in front of him. He tosses the coin vertically upward relative to the train compartment. The coin falls behind him instead of in his hand or in front of him. We neglect air resistance and assume a short toss inside the compartment. Concept / Approach: If the train were moving with constant velocity (uniform speed in a straight line), both the passenger and the coin would share the same forward velocity. When he tosses the coin straight up, it retains this initial horizontal velocity, so it would come back to his hand. If the train accelerates forward while the coin is in the air, the train and the passenger gain extra forward speed, but the coin continues with its original forward speed. As a result, by the time the coin comes down, the passenger has moved ahead relative to the coin, so the coin appears to fall behind him. In contrast, if the train were decelerating, the coin would tend to fall in front of him. Step-by-Step Solution: Step 1: Initially, both the passenger and the coin share the same forward velocity of the train.Step 2: When the passenger tosses the coin straight up, he gives it no extra horizontal speed, so it retains the original horizontal velocity.Step 3: If the train maintains uniform speed, the coin and the passenger remain aligned horizontally and the coin falls back into his hand.Step 4: If the train accelerates forward during the flight of the coin, the passenger and train increase their horizontal speed.Step 5: The coin keeps its old forward speed, so relative to the passenger it lags behind.Step 6: Therefore, the observation that the coin falls behind indicates that the train is moving forward and gaining speed. Verification / Alternative check: Consider the opposite case of a train that is slowing down while the coin is in the air. The coin would keep its larger initial forward speed while the train slows, so it would land ahead of the passenger. This is the opposite of what is described. Another check is to imagine the same experiment in a car that suddenly accelerates; loose objects tend to slide backward relative to the car, which is consistent with the coin appearing behind the passenger in an accelerating train. Why Other Options Are Wrong: If the train were moving forward with uniform speed, the coin would fall back into the thrower hand, not behind. If the train were losing speed, the coin would fall ahead of him, because it would retain a higher horizontal speed than the decelerating train. The option that the train is moving backward with uniform speed does not match the given situation because both the initial conditions and the appearance of the coin do not support backward motion. Common Pitfalls: A common mistake is to assume that any motion of the coin away from the hand implies that the train speed is changing in an unspecified way. Another frequent error is to forget that inside a uniformly moving vehicle, simple experiments like dropping objects behave as if you were at rest. To answer correctly, focus on how extra forward acceleration of the train changes the relative position of the passenger and the coin during its flight. Final Answer: If the tossed coin falls behind the passenger facing forward, the train must be moving forward and gaining speed.

Question 15

In a simple improvised explosive device (IED), which component is responsible for providing the electrical energy (electrical charge) that powers the firing circuit?

Accepted Answer

Introduction / Context: An improvised explosive device, often abbreviated as IED, is a simple bomb constructed from available materials. Understanding its basic components is important for safety awareness and basic security education. This question focuses on which part of an IED actually supplies the electrical energy that runs the firing or initiation circuit, as distinct from the explosive material itself or the switching mechanism. Given Data / Assumptions: A simple IED typically consists of a power source, a switch or activator, a detonator or fuse and the main explosive charge. The power source is often a battery or similar device. The phrase creates an electrical charge in the question refers to providing electrical energy for the circuit. We must distinguish between providing energy, triggering the circuit and being the explosive itself. Concept / Approach: In an electrical circuit of any kind, including that of an IED, the power source is the component that supplies electrical energy. Common examples are batteries or small accumulators. The switch or activator simply opens or closes the circuit, allowing current to flow at a chosen moment, but it does not create the charge. The detonator converts electrical energy into a small explosive impulse that triggers the main charge. The main explosive charge is the bulk material that releases large amounts of chemical energy, not electrical energy. Therefore, the correct answer is the power source, which provides the electrical charge and energy needed to operate the firing system. Step-by-Step Solution: Step 1: Identify the standard components of a simple IED: power source, activator, detonator and main charge.Step 2: Recall that a power source such as a battery supplies electrical energy to a circuit.Step 3: Understand that the activator or switch only controls whether the circuit is open or closed.Step 4: Note that the fuse or detonator is a device that initiates the explosion when current flows through it.Step 5: Remember that the explosive main charge contains stored chemical energy, not electrical charge.Step 6: Conclude that the part responsible for creating or providing electrical charge is the power source. Verification / Alternative check: In any basic circuit diagram, the symbol for a battery or cell is shown as the origin of potential difference that drives current through wires, switches and loads. Without this source, no current would flow and no electrical action would happen. Detonators, switches and explosives are passive components that require the power source to function. This general understanding of circuits supports identifying the power source as the relevant component in an IED. Why Other Options Are Wrong: The activator or switch simply provides a way to turn the circuit on or off; it does not create electrical energy. The fuse or detonator is designed to respond to current by producing heat or a small explosion, but it still relies on energy supplied by the power source. The explosive main charge does not produce electrical charge, but rather releases chemical energy when triggered. Therefore, none of these options match the task of providing electrical energy to the circuit. Common Pitfalls: Because the words fuse and explosive are strongly associated with the idea of an explosion, some learners may assume they are responsible for everything, including electrical energy. Others may not fully understand the distinction between a power source and a trigger. To avoid confusion, always separate in your mind the roles of supplying energy, controlling the circuit and releasing chemical energy in explosive devices. Final Answer: In an IED, the power source such as a battery is the component that provides the electrical energy or electrical charge to the circuit.

Question 16

In a fluorescent tube light, what is the main purpose of the choke or ballast in the electrical circuit?

Accepted Answer

Introduction / Context: Fluorescent tube lights, commonly called tube lights, use a gas discharge to produce light. The electrical circuit that drives a tube light includes a component called a choke or ballast. Understanding the role of this choke is a standard topic in basic electricity and household appliance physics. This question focuses on the main function of the choke during the starting of the tube light. Given Data / Assumptions: The circuit of a typical fluorescent tube includes a choke (ballast), a starter and the tube itself. The tube contains low pressure gas and requires a high voltage to initiate discharge. Normal supply voltage is not always sufficient to start the discharge without assistance. The choke is an inductive component that can briefly generate higher voltages when current is interrupted. Concept / Approach: A fluorescent tube needs a higher voltage to strike an arc across its length than to keep that arc running once it has started. The choke or ballast is basically an inductor. When the starter opens the circuit after preheating the filaments, the rapid interruption of current through the inductor produces a momentary high induced voltage according to the law of electromagnetic induction. This increased voltage appears across the tube and helps to ionise the gas and start the discharge. After ignition, the choke also helps limit the current to a safe value, but the most commonly emphasised function in school level questions is to provide a high starting voltage. Step-by-Step Solution: Step 1: Recognise that gas discharge tubes require a higher voltage to start than to continue operating.Step 2: Identify the choke as an inductor placed in series with the tube.Step 3: When the starter opens, the current through the inductor is suddenly interrupted.Step 4: This sudden change in current produces a large voltage spike across the inductor due to electromagnetic induction.Step 5: This high induced voltage appears across the tube ends and helps ionise the gas and start the arc.Step 6: Once the tube is lit, the choke continues to limit current, but its primary textbook purpose is to provide high starting voltage. Verification / Alternative check: Practical descriptions of tube light circuits usually show the choke as a series inductor and explain that it generates an inductive kick when the starter opens, used to strike the tube. Modern electronic ballasts perform similar functions using solid state circuits, but the underlying need for a starting overvoltage remains. This supports the conclusion that inducing high voltage is the primary role of the choke in traditional tube lights. Why Other Options Are Wrong: Providing very low resistance is not the main purpose; in fact, without current limiting the tube would draw excessive current once started. Providing a very high constant resistance would waste power and is not how a choke functions, since inductors resist changes in current rather than acting as simple resistances. Inducing a low voltage would not help start the tube; it is the high voltage that is needed to ionise the gas and strike the discharge. Common Pitfalls: Some learners confuse the choke with a simple resistor and talk only about current limiting, forgetting the starting role. Others think of it as a transformer and assume it always steps down voltage. To avoid such misunderstandings, remember that a choke in a tube light acts as an inductor that produces a brief high voltage pulse when current is interrupted, making it easier to start the discharge in the tube. Final Answer: The main purpose of the choke in a tube light is to induce a high voltage across the tube at the time of starting.

Question 17

For accurately measuring the ground distance between two points on the Earth surface, especially over longer ranges, which of the following methods is generally the most reliable and precise?

Accepted Answer

Introduction / Context: Ground distance measurement is important in surveying, navigation and mapping. Over small distances, simple tools such as tapes or chains may be adequate, but over larger distances or rough terrain, more advanced methods offer far greater precision and convenience. This question asks you to identify the most reliable and precise way to measure ground distance from among a set of simple and advanced methods. Given Data / Assumptions: The goal is to measure the actual ground distance between two points on the Earth surface. The options include pacing, simple tape measurement, vehicle odometer readings and satellite based or electronic distance measurement. We assume that accuracy and reliability are the main criteria. Modern surveying and navigation tools are available and in common use. Concept / Approach: Pacing and step counting are rough estimation techniques and depend heavily on the individual stride length and terrain, so they are not very accurate. Using a simple measuring tape or chain can give good accuracy over short, flat distances, but it becomes impractical and less precise over long or uneven ground. Vehicle odometers measure wheel rotations and can suffer from calibration errors, tyre wear differences and path deviations. Modern satellite based Global Positioning System receivers and dedicated electronic distance measurement instruments can use radio signals or lasers to determine distances with high precision, even over long ranges. Therefore, for reliability and precision, the modern electronic or GPS based methods are superior. Step-by-Step Solution: Step 1: Evaluate pacing: it is quick and requires no instruments, but is approximate and depends on personal stride.Step 2: Evaluate simple measuring tapes or chains: they can be precise over short distances but are difficult to apply accurately over long, rough or obstructed paths.Step 3: Evaluate vehicle odometers: they depend on wheel size and calibration and may not follow the shortest ground path exactly.Step 4: Evaluate GPS and electronic distance measurement instruments: they use signals from satellites or laser beams and are designed for surveying accuracy.Step 5: Recognise that these instruments can correct for many sources of error and provide repeatable, precise measurements.Step 6: Conclude that modern satellite based GPS and electronic distance measurement devices provide the best way to measure ground distance. Verification / Alternative check: Surveying practice today routinely uses total stations, electronic distance meters and GPS based methods to measure distances and coordinates in civil engineering, highway planning and land records. These tools have specified accuracy levels and are trusted for legal and scientific work. Traditional methods such as chains and pacing are now mainly used for rough estimates or teaching, which confirms the superiority of the electronic and GPS methods. Why Other Options Are Wrong: Pacing is too approximate for precise work and varies from person to person. Simple tape or chain measurements are laborious and error prone over long distances, especially if the terrain is uneven or obstacles are present. Vehicle odometer readings are influenced by tyre wear, slipping and detours, making them unreliable as precision instruments. Hence, while these methods can give rough distances, they do not match the accuracy and reliability of modern GPS and electronic distance measurement devices. Common Pitfalls: Some learners may think that measuring tapes are always accurate without considering practical difficulties over long ranges. Others might assume that vehicle odometers are precise enough for survey work simply because they display distances. To avoid such misunderstandings, always ask whether the method is designed and calibrated for precise measurement in professional contexts. In modern surveying, GPS and electronic distance measurement tools are the standard for accurate ground distance measurement. Final Answer: The best and most reliable way to measure ground distance between two points is to use modern satellite based GPS or electronic distance measurement instruments.

Question 18

According to Archimedes principle, the buoyant force acting on a body immersed in a fluid depends primarily on which of the following?

Accepted Answer

Introduction / Context: When a body is placed in a fluid, it experiences an upward force called buoyant force or upthrust. Archimedes principle gives a simple rule for calculating this force and explains why objects float or sink. This question tests whether you know that the buoyant force depends on the fluid displaced, not directly on the mass of the body or the immersion depth alone. Given Data / Assumptions: A body is partially or fully immersed in a liquid such as water. The liquid exerts an upward buoyant force on the body. Archimedes principle is valid: fluid is incompressible and at rest. We compare factors such as the mass of liquid displaced, mass of the body, depth of immersion and shape of the body. Concept / Approach: Archimedes principle states that the buoyant force on a body immersed in a fluid is equal to the weight of the fluid displaced by the body. Since weight equals mass multiplied by gravitational acceleration, this means that buoyancy is directly proportional to the mass of the displaced fluid, or equivalently to the volume of the displaced fluid times its density. The buoyant force does not depend directly on the mass of the body itself; a heavy object can float if it displaces enough fluid. Similarly, depth of immersion and shape affect buoyancy only through the volume of fluid displaced, not independently. Step-by-Step Solution: Step 1: Recall Archimedes principle: upthrust equals the weight of the fluid displaced.Step 2: Express weight as W = m * g, where m is the mass of displaced fluid and g is gravitational acceleration.Step 3: Recognise that buoyant force is therefore proportional to the mass of displaced liquid.Step 4: Understand that the mass of the body matters only in comparison to the buoyant force when deciding float or sink, not in calculating the buoyancy itself.Step 5: Note that depth of immersion can change the volume displaced, but there is no direct formula linking buoyancy solely to depth.Step 6: Realise that shape influences how much fluid is displaced for a given part of the body, but again buoyancy is fundamentally tied to the mass of displaced fluid. Verification / Alternative check: Consider two blocks of different materials but identical volumes placed in the same liquid. Each displaces the same volume of fluid and therefore the same mass of fluid, so they experience the same buoyant force, even if their masses differ. This observation supports the idea that buoyancy depends on displaced fluid, not on the body mass itself. Experiments with floating and sinking objects in water also confirm that what matters is whether the weight of the fluid displaced is enough to balance the weight of the object. Why Other Options Are Wrong: The mass of the body alone does not determine the buoyant force; rather it determines whether the body sinks or floats when compared with buoyancy. Depth of immersion does not independently determine buoyancy unless it changes the volume of displaced fluid, and in very deep water the buoyant force is still about the same for the same displacement. The exact shape of the body matters only because it affects how much fluid is displaced for a given part of the body; it is not the primary variable in the expression for buoyancy. Common Pitfalls: Students often confuse the rule for floating or sinking with the formula for buoyancy. They may think heavy objects cannot float, forgetting that ships made of steel float because they are shaped to displace enough water. To avoid this confusion, remember that buoyant force is determined by the fluid displaced, not directly by the mass of the body, and use Archimedes principle as your guide. Final Answer: Buoyancy depends primarily on the mass (or weight) of the liquid displaced by the body.

Question 19

Blotting paper is able to soak up and absorb ink efficiently mainly due to which physical phenomenon acting in the tiny pores of the paper?

Accepted Answer

Introduction / Context: When you place blotting paper on a wet ink spot, the paper quickly soaks up the ink. This everyday observation can be explained using basic fluid physics. The key concept is capillary action, which allows liquids to rise or move through narrow spaces without external forces. This question tests whether you can correctly identify capillary action as the main reason blotting paper works so effectively. Given Data / Assumptions: Blotting paper is porous, with many tiny capillary like channels. Ink is a liquid that wets the paper, meaning it tends to spread on its surface. The absorption happens even against gravity to some extent. The options mention capillary action, diffusion, siphon action and viscosity. Concept / Approach: Capillary action occurs when adhesive forces between a liquid and the walls of a narrow tube, combined with surface tension, cause the liquid to rise or move in the tube. Blotting paper contains a network of fine pores that behave like many tiny capillary tubes. When the edge of the paper touches ink, the ink is drawn into these pores by capillary forces, allowing the paper to absorb the liquid efficiently. Diffusion involves random motion of molecules and is much slower and not the main mechanism here. Siphon action requires a continuous path and height difference, which is not the case. Viscosity is a measure of internal friction and would oppose flow if it were very high. Step-by-Step Solution: Step 1: Observe that blotting paper has a fibrous, porous structure with many tiny channels.Step 2: When the paper touches ink, the ink spreads into these pores and rises above the original level of ink.Step 3: Recognise that this upward or sideways movement of liquid into narrow pores is a characteristic of capillary action.Step 4: Understand that adhesive forces between ink and paper, together with surface tension, pull the ink into the pores.Step 5: Note that diffusion alone would be too slow and random to explain the rapid absorption observed.Step 6: Conclude that capillary action is the dominant phenomenon that allows blotting paper to absorb ink. Verification / Alternative check: Similar behaviour can be seen when you dip the corner of a paper towel into water; the water climbs up the towel against gravity. Classic experiments with thin glass tubes in water show the liquid rising to a certain height due to capillary action. These examples match the behaviour of blotting paper with ink and support the explanation that capillary forces in narrow pores are responsible. Why Other Options Are Wrong: Diffusion of ink molecules through the paper plays a minor role but is not the main driver of rapid bulk liquid movement into the pores. Siphon action requires a U shaped tube and flow driven by gravity difference in connected columns, which is not the setup here. High viscosity would actually slow the movement of ink rather than help it; low viscosity assists flow. Therefore, these options do not correctly describe the primary phenomenon. Common Pitfalls: Some students confuse diffusion with any spreading of a liquid and may choose that answer. Others might not be familiar with the term capillary action. To avoid this, remember that capillary action specifically refers to movement of liquids in narrow tubes or pores due to adhesive forces and surface tension, and blotting paper is a classic example of this effect. Final Answer: Blotting paper absorbs ink mainly because of capillary action acting in the fine pores of the paper.

Question 20

The humidity of the air at a place depends on several factors. Which of the following combined choice best represents important influences on atmospheric humidity?

Accepted Answer

Introduction / Context: Humidity describes the amount of water vapour present in the air and is an important element of weather. It affects comfort, rainfall patterns and even the performance of some instruments. The actual humidity at any place and time is influenced by several interconnected factors. This question asks you to recognise that weather, temperature and geographic location all play roles in determining humidity levels. Given Data / Assumptions: Relative humidity is the ratio of actual water vapour content to the maximum possible at that temperature. Weather includes wind, cloud cover and rainfall, which influence water vapour supply. Temperature affects how much water vapour air can hold before condensation occurs. Location includes proximity to oceans, altitude and regional climate. Concept / Approach: Humidity is not controlled by a single factor. Temperature strongly influences the saturation vapour pressure of water; warmer air can hold more water vapour before reaching saturation, so relative humidity changes with temperature. Weather patterns such as rain, storms and wind bring in moist or dry air masses and can increase or decrease humidity. Location matters because coastal regions with warm seas can have higher humidity compared with inland deserts or high mountain areas. Therefore, when asked which factors humidity depends upon, the correct answer is that all of the listed factors contribute in practice. Step-by-Step Solution: Step 1: Recognise that temperature affects the maximum water vapour capacity of the air.Step 2: Understand that weather conditions such as rain, cloud cover and wind can add or remove moisture from the air locally.Step 3: Note that geographic location influences sources of moisture, such as oceans, lakes and vegetation.Step 4: See that coastal tropical locations usually have higher humidity than inland deserts, demonstrating the location effect.Step 5: Realise that none of these factors alone is sufficient to fully determine humidity.Step 6: Conclude that humidity depends on weather, temperature and location considered together. Verification / Alternative check: Weather reports often mention that humidity is higher on warm, cloudy days with onshore winds in coastal cities, and lower on cool, dry days with offshore winds. This shows that both weather patterns and temperature are important. Climate classification also recognises that tropical coastal zones, arid deserts and high altitude regions differ greatly in typical humidity levels, confirming the role of location. These observations support choosing all of the above as the correct answer. Why Other Options Are Wrong: Weather alone cannot explain humidity without considering temperature; for example, cool rainy weather may have lower absolute humidity than very hot clear weather, even if relative humidity is high. Temperature alone also fails to account for the availability of water vapour sources, which depends on location and weather. Location alone does not capture day to day changes caused by passing weather systems. Thus, no single factor is sufficient by itself. Common Pitfalls: Some learners focus only on temperature because many definitions of relative humidity mention temperature explicitly. Others may think only of coastal versus inland location without considering daily weather changes. To avoid such narrow views, remember that humidity is a combined result of temperature, the presence of moisture sources and the current weather conditions moving air masses around. Final Answer: The humidity of the air depends on weather conditions, temperature and geographic location taken together, so all of the above are relevant factors.

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