In this physical quantity analogy, “Luminous intensity is to Candela as Pressure is to ______”. Select the SI unit that stands in the same physical quantity to SI unit relationship for pressure that candela does for luminous intensity.

Introduction / Context:
This analogy tests knowledge of SI units and their corresponding physical quantities. The pair “Luminous intensity : Candela” connects a physical quantity with its SI base unit. To complete the analogy, we must choose the SI unit that corresponds to pressure in the same way. Such questions are common in basic physics and general science sections of competitive exams and reinforce familiarity with standard units.

Given Data / Assumptions:

• Physical quantity: Luminous intensity. • SI unit: Candela (symbol cd). • Another quantity: Pressure. • Options for possible units: Radian, Newton, Pascal, Joule. • We assume standard SI definitions: Newton for force, Pascal for pressure, Joule for energy, and radian as a unit of plane angle.

Concept / Approach:
The relationship is clearly “physical quantity : its SI unit”. Luminous intensity is measured in candela. For the second pair, we need the SI unit in which pressure is measured. In the International System of Units, pressure is defined as force per unit area and its derived SI unit is Pascal (Pa), which equals one Newton per square metre. Therefore the correct completion of the analogy is Pressure : Pascal.

Step-by-Step Solution:
Step 1: Restate the first pair. Luminous intensity, one of the base physical quantities, is measured in candela. Step 2: Identify the nature of pressure. Pressure is defined as force acting per unit area. Step 3: Recall the SI unit of pressure. Since pressure = force / area and the SI unit of force is Newton while the SI unit of area is square metre, the SI unit of pressure is Newton per square metre. This combined unit is given the special name Pascal (Pa). Step 4: Match with the options. Radian measures angles, Newton measures force, Pascal measures pressure, and Joule measures energy or work. Thus, Pascal is the correct SI unit for pressure.

Verification / Alternative check:
Verify by checking each candidate unit against its standard physical quantity. Radian is used for angular measure, such as the angle around a circle. Newton is used for force (mass times acceleration). Joule is used for energy, for example in mechanics and thermodynamics. Only Pascal is reserved for pressure, defined as one Newton per square metre. This confirms that Pascal is the unique choice that fits the analogy structure.

Why Other Options Are Wrong:
• Radian: Unit of plane angle; unrelated to pressure. • Newton: Unit of force, not pressure; pressure uses Newton in combination with area in the numerator. • Joule: Unit of energy or work, not force per area.

Common Pitfalls:
A common error is to choose Newton because pressure involves force. However, Newton alone measures force, not force per unit area. Remember that pressure is a derived quantity, and its correct SI name is Pascal. Keeping a small list of key SI units in memory, like metre, kilogram, second, ampere, kelvin, mole, candela, and some derived units such as Newton and Pascal, greatly helps with these questions.

Final Answer:
The SI unit that correctly completes the analogy is Pascal.