5.16P Explain that for a body to be at a constant temperature it needs to…

The idea

A body's temperature stays constant only when the energy it gains balances the energy it loses. Every body absorbs radiation from its surroundings and radiates its own; if the average power radiated equals the average power absorbed, the net energy transfer is zero and the temperature does not change. If a body radiates more than it absorbs, it cools; if it absorbs more than it radiates, it warms. The word 'average' matters because absorption and emission fluctuate, yet the balance holds over time. A cup of tea in a room eventually settles at room temperature because at that point it radiates the same average power it absorbs from the room. This balance explains why objects reach a steady temperature rather than changing indefinitely.

What earns the marks

• States that a body at constant temperature absorbs and radiates energy at equal average rates.
• Explains that equal average power radiated and absorbed gives zero net energy transfer, so temperature stays constant.
• Explains that a body radiating more than it absorbs cools down.
• Explains that a body absorbing more than it radiates warms up.
• Applies the balance to an example, such as an object settling at the temperature of its surroundings.

Common mistakes and how to put them right

• Claims a body at constant temperature neither absorbs nor radiates.

  Put it right by saying it does both, at equal average rates.

• Ignores the word 'average' and treats the balance as instantaneous.

  Put it right by noting the balance holds on average over time.

• States that a body radiating more than it absorbs warms up.

  Put it right by saying it cools.

• Assumes constant temperature means no energy transfer occurs.

  Put it right by describing equal, opposing transfers.

Exam tips

• Look for the pairing of 'radiates' and 'absorbs' in options — the correct one balances them.
• Check the direction: more radiated than absorbed means cooling, more absorbed than radiated means warming.
• Remember 'average' power, since real transfers vary moment to moment.
• Use a steady-temperature example such as an object in a room to test whether an option is consistent.