Module 2.3 Thermodynamics

Knowledge levels

Item	A	B1	B2 / B2L	B3
2.3(a) Temperature: thermometers, scales (Celsius, Fahrenheit, Kelvin); definition of heat	2	2	2	2
2.3(b) Heat capacity, specific heat, heat transfer, expansion, laws of thermodynamics, gas laws, cp and cv, work by expanding gas, isothermal/adiabatic processes, engine cycles, latent heat, thermal energy, heat of combustion	1	2	2	1

Temperature and heat

• Temperature measures how hot a body is - the average kinetic energy of its molecules.
• Heat is energy transferred because of a temperature difference (joule, J). Heat flows from hot to cold.
• Thermometers: liquid-in-glass (expansion), bimetallic strip, resistance thermometer (RTD - resistance rises with temperature), thermocouple (EGT measurement, Module 3.5), infrared (radiation).

Temperature scales

Scale	Water freezes	Water boils	Absolute zero
Celsius	0 °C	100 °C	−273.15 °C
Fahrenheit	32 °F	212 °F	−459.67 °F
Kelvin	273.15 K	373.15 K	0 K
Rankine	491.67 °R	671.67 °R	0 °R

Conversions: °F = (°C × 9/5) + 32; °C = (°F − 32) × 5/9; K = °C + 273.15. A temperature difference of 1 °C = 1 K = 1.8 °F.

Worked example

ISA sea-level temperature is 15 °C. Convert to °F and K.

• °F = 15 × 9/5 + 32 = 27 + 32 = 59 °F
• K = 15 + 273 = 288 K

Key fact: −40 °C = −40 °F, the only point where the two scales agree.

Heat capacity and specific heat

• Heat capacity: heat needed to raise a whole body by 1 K (J/K).
• Specific heat capacity c: heat needed to raise 1 kg by 1 K (J/kg·K). Water ≈ 4 186 J/kg·K (high - good coolant); aluminium ≈ 900; steel ≈ 450.
• Q = m c ΔT

Worked example

How much heat raises 2 kg of water from 20 °C to 80 °C?

• Q = 2 × 4 186 × 60 = 502 kJ