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Energy & Heat: fun facts and quiz
The essentials in 7 ideas, with fun facts, an illustrated story and a quiz to see how much you know.
The essentials
Forms of Energy
Energy comes in many forms: movement, heat, light, sound, chemical energy in food and fuel, and stored energy like a stretched spring. It changes form but is never destroyed.
Key fact
Energy can’t be created or destroyed, only changed.
Heat vs Temperature
Temperature measures how fast particles are jiggling. Heat is energy flowing from hot to cold. A warm bath holds more heat than a hot cup of tea.
Key fact
Heat always flows from hotter things to colder things.
How Heat Moves
Heat moves by conduction through touching materials, by convection as warm liquids and gases rise, and by radiation, like the Sun’s warmth crossing space.
Key fact
Metal feels colder than wood because it carries heat away faster.
Energy in Food
Food energy is measured in calories or kilojoules. An adult needs around 2,000 to 2,500 kilocalories a day.
Key fact
A food “Calorie” is really a kilocalorie: 1,000 calories.
Absolute Zero
Absolute zero, −273.15 °C, is the coldest possible temperature, where particles move as little as they possibly can. Scientists have come within billionths of a degree of it.
Key fact
The Kelvin scale starts at absolute zero.
Wasted Energy
No machine turns all its energy into useful work; some always escapes as heat. A petrol engine turns only about 20 to 30% of its fuel’s energy into motion.
Key fact
Electric motors can be over 90% efficient.
Melting & Boiling
Adding heat can change a substance’s state: ice melts at 0 °C and water boils at 100 °C at sea level. On high mountains, water boils at a lower temperature.
Key fact
On the summit of Everest, water boils at about 70 °C.
Fun facts
Almost all the energy you use began as sunlight: in food, in wood, even in coal and oil made from ancient plants.
A sparkler’s sparks are over 1,000 °C, yet they rarely burn you: each one is so tiny it holds very little heat.
Emperor penguins survive Antarctic winters by huddling in their thousands, taking turns on the cold outer edge.
Shivering can make your body burn energy several times faster than at rest, all just to keep you warm.
The coldest known natural place is the Boomerang Nebula, at about −272 °C.
Your body wastes energy too: only about a quarter of what your muscles burn becomes movement; the rest is heat.
Pasta takes longer to cook high in the mountains, because water boils there at a lower temperature.
Story: The Brewer Who Weighed Heat
The Brewer Who Weighed Heat
1840s, Manchester. Brewer James Joule suspects that motion and heat are really two forms of the same thing.
He lets falling weights turn a paddle wheel in water, and measures the water warming by a tiny fraction of a degree.
The same work always made the same heat. Today’s unit of energy, the joule, is named after him.
True or false?
Tap your answer to see if you got it, and why.
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Question 1 of 10
Energy can change from one form to another.
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True. A torch turns chemical energy into light.
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Question 2 of 10
Heat flows from hot things to cold things.
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True. Never the other way on its own.
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Question 3 of 10
The Sun warms the Earth by radiation.
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True. It crosses empty space.
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Question 4 of 10
The Calories on food labels are really kilocalories.
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True. A capital C means 1,000 calories.
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Question 5 of 10
Nothing can be colder than absolute zero.
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True. Particles would have the least possible motion.
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Question 6 of 10
Car engines waste much of their fuel energy as heat.
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True. That’s why they need radiators.
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Question 7 of 10
Water boils at a lower temperature on high mountains.
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True. Lower air pressure lets it boil sooner.
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Question 8 of 10
Energy can be destroyed completely.
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False. It only changes form.
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Question 9 of 10
Heat and temperature mean exactly the same thing.
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False. Temperature is how hot; heat is energy on the move.
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Question 10 of 10
Metal feels cold because it’s always colder than wood.
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False. At the same temperature, it just draws heat from your hand faster.
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