If two systems are each in thermal equilibrium with a third, they are in thermal equilibrium with each other. Defines temperature operationally.
-- NCERT Class 11 Physics, Ch. 11, p. 228Concept of Temperature
8 MCQs9-step worked example
Source: NCERT ThermodynamicsOfficial key: NTA-verifiedLast updated: 25 Sep 2026
Concept of Temperature, explained for NEET
Topic: Concept of Temperature
Temperature is one of the most familiar physical quantities, yet NEET aspirants often stumble on its precise thermodynamic definition. The core idea: temperature determines the direction of heat flow between two systems in thermal contact. Heat flows from higher temperature to lower temperature — and when it stops, the systems are in thermal equilibrium.
The Zeroth Law of Thermodynamics (NCERT Class 11 Physics, Chapter 11, page 227) formalises this: if system A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C, then A and B are in thermal equilibrium with each other. This transitive property is what makes thermometers meaningful — system C acts as the thermometer.
Temperature is a scalar state function. It depends only on the current state of the system, not how it got there. For an ideal gas, temperature is directly proportional to the average translational kinetic energy of molecules: ⟨KE⟩ = (3/2)kT, where k is Boltzmann's constant.
Temperature scales and conversions appear regularly in NEET:
- Celsius to Kelvin: T(K) = T(°C) + 273.15
- Fahrenheit to Celsius: T(°C) = (5/9)[T(°F) − 32]
- The Kelvin scale is the SI thermodynamic scale; 0 K is absolute zero.
A common confusion: students treat temperature as a measure of "total heat" in a body. Temperature measures average molecular kinetic energy per degree of freedom — two bodies at the same temperature can hold vastly different amounts of thermal energy depending on mass and specific heat.
Watch out for conversion errors in numerical problems — forgetting the 273.15 offset or misapplying the (5/9) vs (9/5) factor in Fahrenheit conversions costs easy marks.
Can you answer these Concept of Temperature MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The Zeroth Law of Thermodynamics is the basis for the concept of:
Show answer and why every option is right or wrong
Answer: B. The Zeroth Law establishes thermal equilibrium as a transitive relation, which is the foundation for defining temperature and for the use of thermometers (NCERT Class 11 Physics, Chapter 11, page 227).
Why A is wrong: A is wrong. Internal energy is related to the First Law of Thermodynamics (energy conservation), not the Zeroth Law.
Why C is wrong: C is wrong. Entropy is defined through the Second Law of Thermodynamics, not the Zeroth Law.
Why D is wrong: D is wrong. Pressure is a mechanical state variable defined independently through force per unit area; it does not require the Zeroth Law for its definition.
MCQ 2Easy RecallPractice
What is the SI unit of temperature?
Show answer and why every option is right or wrong
Answer: A. The Kelvin (K) is the SI base unit for thermodynamic temperature, defined via the Boltzmann constant. Celsius and Fahrenheit are non-SI scales (NCERT Class 11 Physics, Chapter 11, page 227).
Why B is wrong: B is wrong. Degree Fahrenheit is used in some countries for everyday measurement but is neither SI nor commonly used in physics.
Why C is wrong: C is wrong. Degree Celsius is widely used but is not the SI unit of temperature; it is a derived scale offset from Kelvin by 273.15.
Why D is wrong: D is wrong. Joule is the SI unit of energy, not temperature.
MCQ 3Easy RecallPractice
If system A is in thermal equilibrium with system C, and system B is in thermal equilibrium with system C, then according to the Zeroth Law:
Show answer and why every option is right or wrong
Answer: C. The Zeroth Law states precisely this transitive property: if A is in equilibrium with C and B is in equilibrium with C, then A and B are in equilibrium with each other (NCERT Class 11 Physics, Chapter 11, page 227).
Why A is wrong: A is wrong. This directly contradicts the Zeroth Law, which guarantees the transitive nature of thermal equilibrium.
Why B is wrong: B is wrong. If both A and B are in thermal equilibrium with C, they are at the same temperature, so no heat flows between them.
Why D is wrong: D is wrong. Thermal equilibrium implies equal temperatures, not zero internal energy. Systems in equilibrium can have any amount of internal energy.
MCQ 4Direct ApplicationPractice
Convert 37°C (normal human body temperature) to Kelvin.
Show answer and why every option is right or wrong
Answer: A. T(K) = T(°C) + 273.15 = 37 + 273.15 = 310.15 K. This is a direct application of the Celsius-to-Kelvin conversion formula.
Why B is wrong: B is wrong. This results from subtracting 273.15 instead of adding (236.15 = 273.15 − 37), a sign-reversal error in the conversion formula.
Why C is wrong: C is wrong. Adding 300 instead of 273.15 is an arithmetic approximation error; the correct offset is 273.15, not 300.
Why D is wrong: D is wrong. 98.6 is the body temperature in Fahrenheit, not Kelvin. This confuses the Fahrenheit scale value with the Kelvin result.
MCQ 5Direct ApplicationPractice
A temperature of −40°F is equivalent to what value on the Celsius scale?
Show answer and why every option is right or wrong
Answer: D. T(°C) = (5/9)[T(°F) − 32] = (5/9)(−40 − 32) = (5/9)(−72) = −40°C. The point −40 is the unique temperature where Celsius and Fahrenheit scales coincide.
Why A is wrong: A is wrong. −72 is the intermediate value (−40 − 32) before multiplying by 5/9. Stopping at the subtraction step without applying the fraction is a common procedural error.
Why B is wrong: B is wrong. −20 comes from treating 5/9 as ½ and leaving out the subtraction of 32: (1/2)(−40) = −20. The exact conversion (5/9)(−40 − 32) gives −40°C.
Why C is wrong: C is wrong. 0°C corresponds to 32°F, not −40°F. This conflates the freezing point of water with the coincidence point of the two scales.
MCQ 6Direct ApplicationPractice
Two bodies A and B are at temperatures 300 K and 400 K respectively. When placed in thermal contact, heat will flow:
Show answer and why every option is right or wrong
Answer: B. Heat flows spontaneously from higher temperature to lower temperature. Since B (400 K) > A (300 K), heat flows from B to A until they reach thermal equilibrium.
Why A is wrong: A is wrong. Heat flows from higher temperature to lower temperature. A is at a lower temperature (300 K), so it receives heat, not gives it.
Why C is wrong: C is wrong. Whether temperatures are above or below 273 K is irrelevant to heat flow direction. The only criterion is the temperature difference between the two bodies.
Why D is wrong: D is wrong. In classical thermodynamics, net heat flow is unidirectional — from the hotter body to the cooler body. While molecular collisions occur in both directions microscopically, the net macroscopic flow is one-way.
MCQ 7Concept TrapPractice
A large block of iron and a small cup of water are both at 80°C. Which statement is correct?
Show answer and why every option is right or wrong
Answer: C. Temperature measures average kinetic energy per molecule, not total energy. At the same temperature, the iron block has far greater mass and therefore much more total thermal energy than the small cup of water. Temperature equality does not imply energy equality.
Why A is wrong: A is wrong. Both are explicitly stated to be at 80°C. Temperature is identical; the question tests the distinction between temperature and thermal energy.
Why B is wrong: B is wrong. This is the common confusion the question targets. Same temperature means same average molecular kinetic energy, NOT same total thermal energy. Total energy depends on mass and specific heat capacity as well.
Why D is wrong: D is wrong. Temperature is a universal scalar quantity; it can be compared between any two systems regardless of material composition.
MCQ 8CalculationPractice
A thermometer reads 50°F. Convert this to the Kelvin scale.
Show answer and why every option is right or wrong
Answer: D. Step 1: Convert to Celsius: T(°C) = (5/9)(50 − 32) = (5/9)(18) = 10°C. Step 2: Convert to Kelvin: T(K) = 10 + 273.15 = 283.15 K. This is a two-step conversion requiring both the Fahrenheit-to-Celsius and the Celsius-to-Kelvin formulas.
Why A is wrong: A is wrong. This results from adding 273.15 directly to 50 (treating °F as °C), skipping the Fahrenheit-to-Celsius conversion entirely.
Why B is wrong: B is wrong. 255.37 K is 0 °F in kelvin, (5/9)(0 − 32) + 273.15: it drops the 50 and converts only the −32 offset. The correct conversion is (5/9)(50 − 32) + 273.15 = 10 + 273.15 = 283.15 K.
Why C is wrong: C is wrong. This results from using (5/9)(50 − 32) incorrectly as −10°C (sign error in the subtraction or fraction), then adding 273.15 to get 263.15 K.
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How do you solve a Concept of Temperature question? A worked example
- 1
Given
A thermometer reads 212°F.
- 2
Required
Express this temperature in (a) Celsius and (b) Kelvin.
- 3
Concept
Temperature scale conversion uses the Fahrenheit-to-Celsius formula and the Celsius-to-Kelvin offset. These are direct applications of the definitions of the three common temperature scales.
- 4
Formula
• T(°C) = (5/9)[T(°F) − 32]• T(K) = T(°C) + 273.15
- 5
Substitution
• T(°C) = (5/9)(212 − 32) = (5/9)(180)• T(K) = T(°C) + 273.15
- 6
Calculation
• T(°C) = (5/9) × 180 = 100°C• T(K) = 100 + 273.15 = 373.15 K
Note on exact values: 32 and 273.15 are defined conversion constants (exact by definition). The input 212°F is the exact boiling point of water at 1 atm. These do not introduce significant-figure limitations. - 7
Final answer
212°F = 100°C = 373.15 K
- 8
Common trap
The most common error is using (9/5) instead of (5/9) when converting from Fahrenheit to Celsius — that would give T(°C) = (9/5)(180) = 324°C, a wildly incorrect answer. Remember: Fahrenheit-to-Celsius uses the fraction 5/9; Celsius-to-Fahrenheit uses 9/5.
A second trap: forgetting to subtract 32 first. If you compute (5/9)(212) = 117.8°C, you get a wrong intermediate value. - 9
Similar NEET-style question
"The temperature of a furnace is 1832°F. What is the temperature in Kelvin?"
(Answer: T(°C) = (5/9)(1832 − 32) = (5/9)(1800) = 1000°C → T(K) = 1273.15 K)
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What to remember before solving Concept of Temperature questions
More in Thermodynamics: 2 exam traps and mistakes · 4 formulas · 2 question patterns from its other lessons.
Concept of Temperature questions from past NEET papers
No question in our NEET 2020–2025 set targets this topic directly.
Sources
NCERT refs: Class 11 Physics Chapter 11, p.227
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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