Which one of the following is not a unit used for measuring energy in physics?

Introduction / Context:
In physics and everyday science, energy is measured in specific units that quantify the capacity to do work or produce heat. Recognising which quantities are units of energy and which are units of other related concepts such as power, force, or charge is essential for interpreting formulas and practical data. This question presents several commonly used physical units and asks you to identify the one that is not a unit of energy. It tests both your familiarity with standard units and your ability to distinguish energy from power and other quantities.

Given Data / Assumptions:
• The options include calorie, joule, electron volt, and watt. • We assume the SI system of units and some commonly used non SI units. • Energy is the ability to do work or produce heat. • Power is defined as the rate at which work is done or energy is transferred.

Concept / Approach:
The joule is the SI unit of energy and work, defined as one newton metre. The calorie is a non SI unit of energy, commonly used in chemistry and nutrition to measure heat and food energy. The electron volt is another energy unit, especially in atomic and nuclear physics, defined as the energy gained by an electron when it is accelerated through a potential difference of one volt. Watt, however, is the SI unit of power, not energy. One watt equals one joule per second and measures how quickly energy is used or produced. Therefore watt is not a unit of energy, but rather a unit describing the rate of energy transfer.

Step-by-Step Solution:
Step 1: Identify joule as the standard SI unit of energy; it clearly measures energy directly. Step 2: Recall that a calorie is defined as the amount of heat needed to raise the temperature of one gram of water by one degree Celsius, also a measure of energy. Step 3: Recognise that an electron volt is defined as the energy acquired by an electron when accelerated through one volt of potential difference and is widely used in particle and nuclear physics. Step 4: Note that watt is defined as one joule per second, which measures power rather than energy itself. Step 5: Conclude that watt is not a unit of energy, making it the odd one out in the list.

Verification / Alternative check:
You can verify by considering dimensional analysis. The dimension of energy is mass times length squared divided by time squared. Joules, calories, and electron volts all have these dimensions expressed in different unit choices. Power, on the other hand, has the dimensions of energy per unit time. Watt, which equals joule per second, therefore has dimensions that include an extra time in the denominator compared to energy. Checking physical contexts also helps: electric bills measure energy in kilowatt hours, where kilowatt is power and hours is time, combining to form an energy unit, not kilowatt alone. This confirms that watt by itself is not an energy unit.

Why Other Options Are Wrong:
Option A, calorie, is widely used as a unit of energy in thermodynamics and food nutrition, even though it is not an SI unit. Option B, joule, is the fundamental SI unit for work and energy. Option C, electron volt, is also a well established energy unit used for atomic scale processes. Because all three directly measure energy, they cannot be the correct answer to a question asking for a unit that is not energy. Only watt does not satisfy this requirement.

Common Pitfalls:
Learners sometimes confuse power and energy in everyday language, for example saying a device uses many watts of energy, when watts actually measure power. This confusion can cause mistakes when identifying units. It is useful to remember that energy tells you how much work is done, while power tells you how fast it is done. Another pitfall is thinking that unfamiliar units like electron volt are not energy, simply because they are less common than joules or calories. Always check definitions rather than relying only on familiarity.

Final Answer:
The correct choice is Watt, because watt is the SI unit of power (energy per unit time), whereas calorie, joule, and electron volt are all units of energy.