Heat, Units & the Calorimeter
What we're measuring, in which units, and the one rule that does the measuring.
Lesson at a glanceShow the key points
- Heat: energy in transit because of a temperature difference; once transferred it is internal energy. 'Heat in a body' is meaningless.
- SI unit joule; calorie = heat to warm 1 g of water by 1°C (14.5 → 15.5°C at 1 atm); 1 cal = 4.186 J.
- Calorimeter: insulated copper vessel, stirrer and thermometer.
- Principle of calorimetry: heat given by hot bodies = heat received by cold bodies (no losses).
Calorimetry is the bookkeeping of heat: measuring how much energy flows when hot and cold things meet. First, what heat is, and the units we count it in.
Heat is energy in transit
When bodies at different temperatures touch, energy flows from the hotter to the colder. Energy transferred because of a temperature difference is heat. Once it arrives, it is part of the receiving body’s internal energy.
Units: the joule and the calorie
Being energy, heat is measured in joules. An older unit, devised before heat was known to be energy, is still widely used:
A 1200 kg car moves at 54 km/h. Express its kinetic energy in kilocalories.
The principle of calorimetry
A simple calorimeter is a copper vessel with a copper stirrer, kept in a wooden box to insulate it, plus a thermometer. Objects at different temperatures are brought together inside it. If no heat is exchanged with the surroundings:
- 1Heat is energy transferred because of a temperature difference; afterwards it is internal energy.
- 21 cal = 4.186 J; 1 kcal = 1000 cal.
- 3Calorimetry: in an insulated calorimeter, heat lost by hot bodies = heat gained by cold ones.
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Keep exploring
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