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Class 12 Physics Chapter 1 Thermal Physics: Important MCQs and Tips

Thermal Physics opens the Class 12 Physics book and sets the tone for the whole year. It links the everyday ideas of heat and temperature with the behaviour of molecules, and it brings together the kinetic theory of gases, the gas laws, internal energy, specific heat and the laws of thermodynamics. Board papers regularly include several MCQs from this chapter, and entry tests such as MDCAT and ECAT also borrow its ideas, so it is worth mastering early.

In this post you will find the key topics to revise, quick exam tips, and eight practice MCQs taken from our 150-MCQ chapter test. Try them honestly before looking at the answers.

Key topics to revise

  • Kinetic molecular theory and the assumptions of an ideal gas: when does a real gas behave ideally (high temperature, low pressure)?
  • Root-mean-square speed and how it depends on temperature and molar mass (v_rms is proportional to the square root of T/M).
  • Pressure of a gas from molecular collisions and the link between average kinetic energy and absolute temperature.
  • Internal energy, work done by a gas and heat, and the first law of thermodynamics.
  • Specific heat at constant volume and constant pressure and their difference.
  • Second law, heat engines, efficiency and the Carnot cycle.

Exam tips

  1. Always convert Celsius to kelvin before using any gas formula; most wrong answers in rms-speed questions come from forgetting this.
  2. For ratio questions, write the formula, cancel the constants, and only then put numbers in.
  3. Learn the sign convention for work and heat used in your textbook and stick to it.
  4. Remember that efficiency of a heat engine can never be 100%; options claiming this are traps.

Practice MCQs

Q1. Which condition is necessary for most gases to behave nearly ideally?

  • (a) Constant temperature and low pressure
  • (b) Low temperature and low pressure
  • (c) High temperature and constant pressure
  • (d) High temperature and low pressure

Q2. Ratio of the rms velocities of O₂ and H₂ at equal temperature will be:

  • (a) 1:4
  • (b) 1:1
  • (c) 2:1
  • (d) 4:1

Q3. What happens to a star when its gravitational force becomes greater than its internal pressure?

  • (a) The star remains stable with no changes
  • (b) The star expands and becomes a red giant
  • (c) Nuclear fusion stops immediately
  • (d) The star contracts and its core temperature increases

Q4. The life of a star depends mainly on its:

  • (a) Colour
  • (b) Distance from Earth
  • (c) Name
  • (d) Mass

Q5. If the density of a gas is doubled with the rms speed unchanged, its pressure becomes:

  • (a) Four times
  • (b) Half
  • (c) Double
  • (d) Unchanged

Q6. According to the book, red dwarfs will eventually evolve into:

  • (a) Black holes
  • (b) Low mass white dwarfs
  • (c) Neutron stars
  • (d) Red super giants

Q7. According to the book, k_B is effectively the gas constant per:

  • (a) Mole
  • (b) Kilogram
  • (c) Litre
  • (d) Molecule

Q8. In the hydrostatic equation, the negative sign indicates that:

  • (a) Gravity pushes outward
  • (b) Pressure decreases outward
  • (c) Pressure increases outward
  • (d) Density is negative

Answers

  1. (d) High temperature and low pressure
  2. (a) 1:4
  3. (d) The star contracts and its core temperature increases
  4. (d) Mass
  5. (c) Double
  6. (b) Low mass white dwarfs
  7. (d) Molecule
  8. (b) Pressure decreases outward

How to use these MCQs

Do not just read the answers. Cover them, attempt all eight questions within eight minutes, then check. For every wrong answer, open your textbook and read that paragraph again. Students who practise this way usually find that the same mistakes do not repeat in the board exam. Once you are comfortable with these, move on to a full timed test so that you can judge your preparation for the whole chapter of Thermal Physics, not just a few questions.

Practise the full chapter

These questions are only a small part of our complete Thermal Physics test. The full test has 150 MCQs, starting with all exercise MCQs from the textbook followed by carefully checked questions covering every topic of the chapter. You get an instant score, a full answer review, a printable result and up to three attempts in seven days.

Good luck with your preparation, from the Student Hub Pakistan team.