Heat is a form of energy that flows from a hotter body to a colder one. Temperature is the degree of hotness or coldness of a body. They are related but not the same.
Heat vs Temperature
| Heat | Temperature |
|---|---|
| A form of energy (joules) | A measure of hotness (degrees) |
| Depends on mass of substance | Does not depend on mass |
| Flows from hot to cold bodies | Measured with a thermometer |
Heat Capacity and Specific Heat Capacity
- Heat capacity is the heat required to raise the temperature of a body by 1 K (or 1 °C).
- Specific heat capacity (c) is the heat required to raise the temperature of 1 kg of a substance by 1 K.
Formula: Q = mcΔθ, where Q is heat energy, m is mass, c is specific heat capacity, and Δθ is the temperature change.
Latent Heat
Latent heat is the heat absorbed or released when a substance changes state without a change in temperature. It may be latent heat of fusion (solid ↔ liquid) or vaporisation (liquid ↔ gas).
Heat Transfer
- Conduction: transfer of heat through a solid without the movement of the material (e.g. metal rod in a flame).
- Convection: transfer of heat in liquids and gases by the movement of the heated fluid (e.g. boiling water).
- Radiation: transfer of heat as electromagnetic waves, needing no medium (e.g. heat from the sun).
Worked Example
How much heat is needed to raise 2 kg of water from 20 °C to 100 °C? (c of water = 4200 J/kg·K)
Q = mcΔθ = 2 × 4200 × (100 − 20) = 2 × 4200 × 80 = 672 000 J (672 kJ).
Summary
Heat is energy; temperature is hotness. Q = mcΔθ calculates heat transfer. Latent heat is absorbed during a change of state at constant temperature. Heat moves by conduction, convection and radiation.
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