Energy, Year 7: food, resources and temperature

Kilojoules (kJ)

提示A thousand of the standard unit of energy.

为什么The 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

提示Compare like with like: both figures are for an equal mass.

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

800 kJ

提示The bowl is half of the amount on the label.

为什么50 g is half of 100 g, so it holds half the energy: 1,600 ÷ 2 = 800 kJ.

Active muscles transfer more energy

提示Moving the body uphill all day has to be paid for.

为什么The 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

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

为什么The 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

提示Wind and sunlight are examples; coal is not.

为什么Wind, sunlight, waves, tides and flowing water are all replaced by nature as fast as they are used.

fossil

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

为什么They 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

提示Think about a calm day.

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

The swimming pool

提示Think about how much water there is to warm up.

为什么The 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

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

为什么Temperature 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.