3.6.2.1
Specific latent heat: energy to change state without changing temperature
Thermal Energy Transfer — AQA A-Level Physics
Key Definition
Specific latent heat — The thermal energy required to change the state of 1 kg of a substance without any change in temperature. Symbol: L. Unit: J kg^{-1}.
$$Q = mL$$
- Where:
- $Q$ = thermal energy for change of state (J)
- $m$ = mass (kg)
- $L$ = specific latent heat (J kg^{-1})
Key Definition
Specific latent heat of fusion — The thermal energy required to convert 1 kg of solid to liquid with no change in temperature.
Key Definition
Specific latent heat of vaporisation — The thermal energy required to convert 1 kg of liquid to gas with no change in temperature.
$$Q = mL$$
- $Q$: thermal energyThe capacity to do work. Measured in joules (J). for change of state (J)
- $m$: mass (kg)
- $L$: specific latent heatThe energyThe capacity to do work. Measured in joules (J). required to change the state of 1 kg of a substance without changing its temperature. (J kg^{-1})
- During a change of state, temperature stays constant. All energyThe capacity to do work. Measured in joules (J). goes into changing potential energy (particle separation), not kinetic energyThe energy an object possesses due to its motion..
- Latent heat of vaporisation is much larger than latent heat of fusion because particles must be completely separated from each other, not just allowed to slide past.
- Water: $L_{\text{fusion}} = 330 kJ kg^{-1}. L_{\text{vaporisation}} = 2.26 MJ kg^{-1} (roughly 7 times larger)$.
- Vaporisation also requires work against atmospheric pressureForce per unit area. Measured in pascals (Pa), where 1 Pa = 1 N m⁻²..
Worked Example
0.6 MJ is needed to boil 530 g of a liquid. Calculate the specific latent heatThe energy required to change the state of 1 kg of a substance without changing its temperature..
Show Solution
1
Rearrange Q = mL
$$L = \frac{Q}{m}$$
2
Substitute
$$L = \frac{0.6 \times 10^6}{530 \times 10^{-3}} = 1.1 \times 10^6 \text{ J kg}^{-1} = 1.1 \text{ MJ kg}^{-1}$$
Answer
$L = 1.1$ MJ kg$^{-1}$ (latent heat of vaporisation, since the change is liquid to gas)