Definition
Heating a solid, liquid or gas generally makes it expand — the particles vibrate/move more and take up more space on average. Gases expand the most for a given temperature rise, then liquids, then solids, since gas particles are least tightly held together.
Linear expansion of a rod: ( = coefficient of linear expansion, = original length)
Example 1 — linear expansion
A steel rod is m at C, with . Find its length at C.
- C.
- m.
- New length .
Answer: m
Remember
Bridges need expansion joints because their large length means even a small produces a significant over a hot day; a glass jar's metal lid is short enough that its expansion is negligible in comparison — this is also why electric transmission wires sag more in summer and pull tight in winter.
( = specific heat capacity — heat needed to raise kg of a substance by C)
Example 2 — heat to raise temperature
Find the heat required to raise a kg iron bar from C to C ( J/kgC).
- C.
- .
Answer: J
Definition
Latent heat is the energy needed to change a substance's state (melt or vaporize it) at a constant temperature — this is why a melting ice cube stays at C the whole time it's melting, even though heat keeps being supplied: the energy goes into breaking bonds between particles, not raising temperature.
( = specific latent heat of fusion or vaporization)
Example 3 — latent heat of fusion
Find the heat required to melt g of ice at C into water at C ( J/kg).
- kg.
- .
Answer: J
Evaporation cools a liquid because the fastest, most energetic particles escape from the surface, leaving the remaining liquid with lower average kinetic energy (and so lower temperature) — this is why evaporation is faster in dry air (less competition from water vapour already in the air re-condensing) and in wide, shallow containers (more surface area for particles to escape from).
Quick Recap
- ●Thermal expansion: gases liquids solids for a given temperature rise;
- ●Specific heat:
- ●Latent heat: — temperature stays constant during a change of state
- ●Evaporation cools a liquid; faster with dry air, more surface area, higher temperature
