(HT only) A body at constant temperature is absorbing radiation at the…

The idea

This is Higher tier only. A body at a steady temperature has not stopped exchanging radiation; it is absorbing and emitting at equal rates, so there is no net transfer and the internal energy stays constant. Both rates are large, not zero. A mug of tea left on a table cools at first because it emits faster than it absorbs, and as it cools its rate of emission falls until the two rates match at room temperature. From then on the mug still emits infrared and still absorbs infrared from the room, in equal measure. This is what is meant by being in thermal equilibrium with the surroundings, and answers that say the object stops emitting score nothing.

What earns the marks

• Stating that the rate of absorption of radiation equals the rate of emission.
• Stating that there is therefore no net transfer of energy, so the temperature stays constant.
• Stating that the object continues to emit and to absorb rather than stopping.
• Applying the balance to a named situation, such as an object left at a steady room temperature.

Common mistakes and how to put them right

• Saying an object at constant temperature has stopped emitting radiation.

• Comparing total energies rather than rates of emission and absorption.

• Assuming equal rates require the object and its surroundings to be different temperatures.

• Confusing a constant temperature with there being no energy transfer of any kind.

Exam tips

• Use the word rate in both halves of the sentence; 'absorbs as much as it emits' is not enough.
• This statement is Higher tier only, so expect it in an explain question worth two or three marks.
• Finish by saying what that means for the internal energy of the body.