Energy, Year 7: food, resources and temperature

Kilojoules (kJ)

HintA thousand of the standard unit of energy.

The whyThe joule is the scientific unit of energy, and food stores so much that labels use thousands of them. Kilocalories are an older unit still printed beside it.

Food A

HintCompare like with like: both figures are for an equal mass.

The whyBoth labels quote energy for 100 g, so the numbers can be compared directly. Fatty and sugary foods have the highest values.

800 kJ

HintThe bowl is half of the amount on the label.

The why50 g is half of 100 g, so it holds half the energy: 1,600 ÷ 2 = 800 kJ.

Active muscles transfer more energy

HintMoving the body uphill all day has to be paid for.

The whyThe energy stored in food is transferred when muscles move the body and keep it warm. More activity means more is transferred, so more has to be eaten.

5

Hint'Kilo' means the same as in kilogram and kilometre.

The whyThe prefix kilo- means a thousand, so 5,000 J ÷ 1,000 = 5 kJ. Food labels use kilojoules because food stores so many joules.

A renewable resource

HintWind and sunlight are examples; coal is not.

The whyWind, sunlight, waves, tides and flowing water are all replaced by nature as fast as they are used.

fossil

HintThe same word names the remains of ancient life found in rocks.

The whyThey are non-renewable: they take millions of years to form and are being burned far faster than that.

It is unreliable — the wind does not always blow

HintThink about a calm day.

The whyTurbines generate nothing in still air, so another source or a store of energy is needed as a back-up.

The swimming pool

HintThink about how much water there is to warm up.

The whyThe teaspoon is hotter, but the pool contains millions of times more water. The total energy depends on the amount of material as well as how hot it is.

How hot it is

HintA thermometer reads the same in a cup or a bath of the same water.

The whyTemperature is measured in degrees Celsius and energy in joules. A cup and a bath of water can be equally hot while the bath stores far more energy.