5.17P Explain what happens to a body if the average power it radiates is…

The idea

Every body radiates electromagnetic radiation and absorbs radiation from its surroundings. The net change in temperature depends on the balance of average power radiated and average power absorbed. If the average power radiated is less than the average power absorbed, the body gains energy overall, so its internal energy and temperature rise. If the average power radiated is more than the average power absorbed, the body loses energy overall, so its temperature falls. When the two are equal, the temperature stays constant.

For example: A cool object placed in a warm room absorbs more than it radiates at first, so it warms up until the two rates match.

What earns the marks

• States that a body whose average radiated power is less than its average absorbed power gains energy and its temperature rises.
• States that a body whose average radiated power is more than its average absorbed power loses energy and its temperature falls.
• Identifies that equal average radiated and absorbed power leaves the temperature constant.
• Links the net power difference to a change in internal energy of the body.

Common mistakes and how to put them right

• Confuses the direction of the imbalance.

  Put it right: Net absorption warms the body and net radiation cools it.

• Assumes a body always cools because it radiates.

  Put it right: Check whether absorbed power exceeds radiated power first.

• Treats the two powers as always equal.

  Put it right: Recognise that a difference drives a temperature change.

Exam tips

• Read the option carefully to check which power is larger before deciding the temperature change.
• Link the answer to net energy gain or loss rather than to radiation alone.
• Use the equal-power case as a reference point for constant temperature.