GCSE · Physics · Edexcel · Spec 1PH0
Constant temperature: emission equals absorption
A can under a lamp has stopped getting warmer. So has it stopped taking in the lamp's radiation? Not even close, and the graph below shows why.
A dark can under a lamp: read the power balance
Slide the dot along the curve and ask: more, less or the same?
Higher tier (Edexcel): you need to explain that a body at constant temperature radiates the same average power as it absorbs.
Why it settles
Reason it through
Why does a cold object in steady radiation end up at a constant temperature?
First link · your turn
At the start the object is cold. Compare the average power it absorbs with the average power it radiates.
Higher tier (Edexcel): an explanation question like this is credited for linked statements, so write each step as its own short sentence.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
A flat line on a temperature graph is a balance, not a switch-off.
What you need to know
- A body is always absorbing radiation and always radiating it, both at once, not taking turns.
- Its temperature depends on two average powers: the power it absorbs and the power it radiates.
- Absorbs more average power than it radiates? Net energy flows in, so its temperature rises.
- Radiates more average power than it absorbs? Net energy flows out, so its temperature falls.
Have a goA mug of tea is cooling on a desk. Which is greater: the average power it absorbs, or the average power it radiates?
The average power it radiates is greater.
Its temperature is falling, so net energy is flowing out: more average power leaves than arrives.
- In steady radiation the absorbed power stays put, but a hotter object radiates more power, so warming slows.
Have a goThe lamp stays exactly as bright. As the can warms up, which power changes, absorbed or radiated, and which way?
The radiated power rises; the absorbed power stays the same.
The lamp hasn't changed, so the radiation arriving hasn't either, but a hotter can radiates more power.
- At constant temperature the average power radiated equals the average power absorbed, so there is no net energy transfer.
Have a goMaya looks at the flat part of the graph and announces, "The can's stopped absorbing anything." What do you tell her?
It is still absorbing. It is also still radiating, and the average powers are equal.
Flat means the two average powers match, not that either one has stopped.
- Constant temperature is a balance, not one particular temperature: change the absorbed power and the object settles at a different level.
The big picture
A body at constant temperature hasn't stopped absorbing or radiating. It radiates exactly as much average power as it absorbs. If it absorbs more than it radiates, it warms; if it radiates more than it absorbs, it cools.
Key points
Worked example
Problem
A black roof tile sits in steady sunshine. Its temperature is read every 10 minutes (illustrative numbers): 10 °C, 18 °C, 22 °C, 24 °C, 24 °C, 24 °C. Describe the balance between absorbed and radiated power in each interval.
⚠ Watch out
Reading a flat stretch as 'nothing is happening'. The object is still absorbing and still radiating. The average powers are just equal.
Memory hook
Flat doesn't mean off. Flat means power in equals power out.
Check yourself
The lamp is moved further away from the can. Straight away, which is bigger: the power absorbed or the power radiated? What happens to the temperature, and why does it eventually stop falling?
Flashcards
(6)At constant temperature, how do the average power radiated and the average power absorbed compare?
True or false: a body at constant temperature has stopped absorbing and radiating.
A body absorbs more average power than it radiates. What happens, and why?
In steady radiation, why does warming slow down as an object heats up?
Two objects each stay at a constant temperature, one hotter than the other. How can both be balanced?
What is the net transfer of energy to a body at constant temperature?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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