Constant temperature requires radiation balance
Definition
Thermal equilibrium
The condition in which an object has no net transfer of energy with its surroundings, so its temperature stays constant.
- An object continually absorbs electromagnetic radiation from its surroundings and emits electromagnetic radiation to its surroundings.
- Its temperature depends on the balance between the average power absorbed and the average power emitted.
- If average power absorbed is greater than average power emitted, the object's internal energy increases and its temperature rises.
- If average power emitted is greater than average power absorbed, the object's internal energy decreases and its temperature falls.
- At constant temperature, average power emitted equals average power absorbed, so the net energy transfer is zero.
Temperature changes restore the balance
- A warmer object emits radiation at a greater intensity than the same object when cooler.
- If absorption temporarily exceeds emission, the object warms and its emitted power rises until balance is restored.
- If emission temporarily exceeds absorption, the object cools and its emitted power falls until balance is restored.
- Equal powers do not mean that emission and absorption stop; both transfers continue at equal average rates.
Example
Radiation balance over one minute
- A surface absorbs 2400 J2400\,\text{J}2400J and emits 1800 J1800\,\text{J}1800J in 60 s60\,\text{s}60s.
- The net energy gain is 2400−1800=600 J2400-1800=600\,\text{J}2400−1800=600J.
- The average net power is P=Et=60060=10 WP=\dfrac{E}{t}=\dfrac{600}{60}=10\,\text{W}P=tE=60600=10W, so the surface warms.
- As it warms, its emitted power increases and the imbalance becomes smaller.
Common Mistake
Do not state that an object at constant temperature emits no radiation; it emits and absorbs equal average powers.
Exam technique
Use power, not a single photon
- Compare the average rates of energy transfer when explaining whether temperature rises, falls or stays constant.
- Write absorbed power equals emitted power for a constant temperature.
- Use P=EtP=\dfrac{E}{t}P=tE when energy and time are supplied.
Self review
- What two radiation transfers occur continuously?
- What happens when absorbed power exceeds emitted power?
- What condition keeps an object's temperature constant?
- Why does emission continue at thermal equilibrium?
