Energy stored in food can be released by combustion(burning) or by respiration in our cells. The labels on packets of food show how much energy is available from the food.
The amount of energy available may be shown in a unit called the calorie, as in the photograph. However, the scientific unit for energy is the joule, which has the symbol J.
A lot of energy is available from most foods, so food labels usually show kJ (kilojoules) instead of J:
1 kJ = 1000 J
For example, 2000 J = 2000 ÷ 1000 = 2 kJ.
To give you an idea of what 2 kJ can do, it is the energy needed to lift a 100 kg mass by 2 m, or to keep a 20 W electric lamp alight for 100 seconds.
The table shows the typical amount of energy available from 100 g of several different foods. The larger the number, the more energy is available.
|Food||Energy in kJ per 100 g|
Brisk walking typically uses 14 kJ per minute. This means that:
A typical adult woman needs around 8400 kJ per day. You could get this from just 375 g of potato crisps (fifteen 25 g bags).