Heat Transfer
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 5 practice questions.
Heat is energy that flows because of a temperature difference, always from hot to cold, until the objects reach the same temperature (thermal equilibrium). It moves by conduction, convection or radiation.
1. Three ways heat moves
Conduction: through a material, molecule to molecule (a metal spoon in hot soup).
Convection: by a moving fluid carrying energy (warm air rising).
Radiation: by electromagnetic waves, even through a vacuum (sunlight).
2. The two equations
A large specific heat means it takes a lot of energy to change the temperature. Water’s c is about 10 times iron’s, so the same heat warms iron about 10 times as much.
Double the thickness of a wall and half as much heat leaks through each second; double its area and twice as much does.
3. Example, then try it
How much heat does it take to warm 0.20 kg of water from 25 °C to 75 °C?
Q = mcΔT = (0.20)(4200)(50) = 42,000 J.
A 0.50 kg block of aluminum (c = 900 J/(kg·°C)) at 200 °C is dropped into 1.0 kg of water (c = 4200 J/(kg·°C)) at 20 °C in an insulated container. What is the final temperature?
Set up: heat lost by the aluminum = heat gained by the water: (0.50)(900)(200 − T) = (1.0)(4200)(T − 20).
Expand: 90,000 − 450T = 4200T − 84,000.
Solve: 174,000 = 4650T → T ≈ 37 °C.
Check: the aluminum drops about 163 °C but the water rises only 17 °C, because water’s mc is about 10 times larger.
Answer: about 37 °C.
4. Common mistakes
A wall is replaced with one made of the same material that is twice as thick and has twice the area. The temperatures on each side stay the same. How does the rate of heat flow through it compare?
Q/Δt = kAΔT/L. Doubling A doubles the rate; doubling L halves it. The two cancel.
Ready to practice?
5 questions on Heat Transfer, each with a hint and an explanation for every wrong answer.