Thermodynamics: heat, work and the gas laws
Thermodynamics punishes sloppy sign conventions. Fix the convention once and the first law becomes routine.
What this covers
- Temperature, heat and internal energy
- Specific and latent heat
- The ideal gas law
- First law of thermodynamics
- Heat engines and efficiency
Worked example
Problem
How much heat raises 0.5 kg of water by 20 °C? (c = 4180 J/kg·K)
- 1Choose the equationNo phase change, so Q = mcΔT.
- 2SubstituteQ = 0.5 × 4180 × 20.
Answer: 41 800 J (41.8 kJ)
Mistakes that cost marks
- Using Q = mcΔT during a phase change instead of Q = mL.
- Mixing Celsius and Kelvin in gas-law calculations.
- Getting the sign of work done on versus by the gas backwards.
Questions students ask
When do I use Kelvin?
Always in gas laws and efficiency; temperature differences can stay in Celsius because the interval is identical.
Practise thermal physics until it sticks
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