Physics
Thermodynamics & heat transfer — quick study summary
The four laws of thermodynamics govern energy and heat flow. Zeroth law: if A and B are each in thermal equilibrium with C, they're in equilibrium with each other (defines temperature). First law: ΔU = Q − W (energy is conserved). Second law: entropy of an isolated system never decreases. Third law: entropy of a perfect crystal at 0 K is zero. Heat moves by conduction (solids), convection (fluids), and radiation (electromagnetic, even through vacuum).
Key points
- First law: ΔU = Q − W (energy conserved; heat in, work out)
- Second law: entropy of isolated system never decreases — sets time's direction
- Three heat-transfer modes: conduction, convection, radiation
- Specific heat capacity c: Q = mcΔT (energy to raise mass m by ΔT)
- Latent heat L: Q = mL (energy for phase change with no temperature change)
Practice quiz
Click each question to reveal the answer.
1. How much heat is needed to raise 2 kg of water by 20 °C (c = 4200 J/kg·K)?
- 8,400 J
- 84,000 J
- 168,000 J
- 1,680,000 J
Answer: 168,000 J (168 kJ)
Q = mcΔT = 2 × 4200 × 20 = 168,000 J.
2. Why does sweating cool you down?
Answer: Evaporating water absorbs heat from your skin (latent heat of vaporisation)
Phase change from liquid to gas absorbs a lot of energy without raising temperature — that energy comes from your body.
3. Which heat transfer mode doesn't need a medium?
Answer: Radiation
Radiation transfers heat as electromagnetic waves — it works through a vacuum, which is why we feel the Sun.
Last reviewed: May 2026