Unit 8, lesson 36. About 12 minutes.

Hot and cold

Which holds more energy: a boiling kettle, or a warm swimming pool?

Play

Getting the experiment ready…

Explain

Hot things feel warm.

Big things take longer to warm up.

Temperature tells you how hot something is. We measure it in degrees Celsius.

Heat is energy moving from hot things to cold things.

A cup of tea is hotter than a bath, but the bath holds much more energy. It takes more heat to warm more water.

Heat energy, Q, depends on mass, the material and the temperature rise: Q = m c ΔT.

Water's specific heat capacity, c, is about 4200 J/(kg °C). That is very large, which helps to keep the climate stable.

Worked example: warming 2 kg of water by 30 °C takes 2 × 4200 × 30 = 252 000 J.

Temperature: how hot
Heat: energy moving
More mass: more energy

Want the full story?

Thermal energy is the proper name for the energy stored in a hot object, and heat means energy being transferred. People use the words loosely. Temperature measures average particle energy, and thermal energy adds them all up.

The Formula

heat energy = mass × specific heat × temperature rise

Q = m \times c \times \Delta T

$Q$: the heat energy, measured in joules (J)
$m$: the mass, measured in kilograms (kg)
$c$: the specific heat capacity, 4200 for water, measured in joules per kilogram per degree (J/(kg °C))
$\Delta T$: the temperature rise, measured in degrees Celsius (°C)

Try the numbers

Water: 1 kgTemperature rise: 20 °C

Heat energy needed: 84,000 J

Try it at home

Ask a grown-up to help

You need two cups, warm (not hot) tap water, a thermometer if you have one.

You need: two cups, warm (not hot) tap water, a thermometer if you have one.

Feel warm water and cool water.

Fill one cup, and half-fill another, with the same warm tap water.
Feel or measure both.
Leave them for 15 minutes and compare again.

What's happening: the smaller amount cools faster, as it has less energy to lose.

Physics is life

Your body

Your body stays warm.

Your body is mostly water, so it takes a lot of energy to change your temperature.

Daily life

A big bath stays warm for ages.

Big hot water tanks stay warm because they hold so much energy.

Nature

The sea stays cool in summer.

The sea warms slowly and cools slowly, so coasts have milder weather.

Space

Space is very, very cold.

Far from any star, space is about −270 °C.

Quick check

Question 1 of 3

Which way does heat move?

Words to know

Temperature: How hot or cold something is.

Temperature: How hot or cold something is.
Heat: Energy moving from something hotter to something cooler.
Energy: What makes things able to happen. It is measured in joules.

Next up

How heat moves