An object left in the Sun does not heat up forever. As it warms, it radiates faster, until the energy it gives out each second matches the energy it takes in. At that point the temperature holds steady, even though energy is still flowing both ways.
An object stays at a constant temperature when the rate at which it absorbs energy equals the rate at which it emits energy. This balance, not a halt in energy flow, is what fixes the temperature. The same idea sets the average temperature of the Earth.
An object is in thermal equilibrium when the power it absorbs equals the power it emits, so its temperature stays constant.
A good emitter of radiation is also a good absorber.
Adjust how much energy flows in and watch the object settle at the temperature where emission matches absorption.
Four quick checks. Each correct answer earns XP and lights this skill on your star map.
An object stays at a constant temperature when...
A good emitter of radiation is also a good...
If an object absorbs energy faster than it emits it, its temperature...
At thermal equilibrium the object is...
Equilibrium is a balance of two flows, not the absence of them.
A balanced object has not stopped exchanging energy. At a constant temperature it is still absorbing and emitting all the time; the two rates are simply equal. Equilibrium means matched flows, not zero flow.
Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.
The Earth keeps a roughly steady average temperature because...
A dull black object placed in bright sunlight warms up until...
If a planet atmosphere traps more of the outgoing radiation, the surface temperature will...
That completes the heat-transfer strand. One topic left in the unit.