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Heat and Temperature

Heat always flows from hot to cold, and it can travel three different ways.

Why does a metal spoon in hot cocoa get hot while a wooden one doesn't? Why is beach sand burning hot when the ocean feels cool? On this page you'll learn the difference between heat and temperature, how to switch between °C, °F and kelvin, how heat moves by conduction, convection and radiation, and how to stay safe around hot things.

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Temperature, heat and thermal energy

Everything is made of tiny particles that are always moving, as you saw in the Matter unit. The faster they jiggle, the hotter something is.

Temperature measures the average energy of motion of the particles: how hot or cold something is.
Thermal energy is the total energy of all the moving particles in an object. A bathtub of warm water has more thermal energy than a cup of hot cocoa, because it has so many more particles, even though the cocoa has a higher temperature.
Heat is thermal energy moving from a warmer object to a cooler one. Heat is measured in joules (J).

Temperature scales

Celsius (°C): water freezes at 0 °C and boils at 100 °C (at sea level). Used in science and most countries.
Fahrenheit (°F): water freezes at 32 °F and boils at 212 °F. Used for weather in the United States.
Kelvin (K): starts at absolute zero, 0 K = −273.15 °C, the coldest anything could ever be, when particles have the least possible motion. Kelvin steps are the same size as Celsius degrees, and there is no degree sign.

Conversions: °F = °C × 9/5 + 32 · °C = (°F − 32) × 5/9 · K = °C + 273.15

Worked examples

Water boils at 100 °C: 100 × 9/5 + 32 = 180 + 32 = 212 °F, and 100 + 273.15 = 373.15 K.
A typical body temperature is about 37 °C: 37 × 9/5 + 32 = 98.6 °F.
Made-up number: a spring day is 68 °F. (68 − 32) × 5/9 = 36 × 5/9 = 20 °C.

Try it: the temperature converter and heat flow

Slide the temperature, or tap a preset, to see it in all three scales.

Temperature: 20 °C

20 °C = 68 °F = 293.15 K.

Two blocks touching

Two identical metal blocks: one at 80 °C, one at 20 °C. Push them together and watch heat flow until they reach the same temperature. This is a simplified model with no heat lost to the air.

The left block is 80 °C and the right block is 20 °C.

Heat flows from hot to cold

Heat always moves on its own from a warmer object to a cooler one, never the other way. A cold drink warms up on a summer day, and hot soup cools down. When two objects reach the same temperature, heat stops flowing between them. This is called thermal equilibrium. Your fridge can make things colder, but only by using energy to pump heat out.

Conduction, convection and radiation

Conduction: heat passes by direct touch, particle to particle. A metal pan handle gets hot because heat travels along the metal from the burner.
Convection: heat is carried by a moving liquid or gas. Warm air from a radiator rises and cool air sinks to take its place, making a loop. A sea breeze happens when land warms faster than water in the daytime: warm air over the land rises, and cooler air from the sea blows in.
Radiation: heat travels as infrared and visible light, even through empty space. Sunlight warms Earth this way, and you feel the warmth of a campfire on your face from a distance.

Conductors and insulators

Thermal conductors, like most metals, let heat move through them quickly. A metal spoon in hot soup soon gets a hot handle. Insulators, like wood, plastic, and trapped air, slow heat down. A wooden spoon stays cool to hold.

A thermos has a vacuum between two walls, so conduction and convection can barely happen, and shiny surfaces reflect radiation. It keeps hot drinks hot and cold drinks cold.
A down jacket traps lots of still air between feathers. The jacket doesn't make heat; it slows your body heat from escaping.

Thermal expansion

Most materials take up more space when they get warmer, because their particles move more and spread apart. This is thermal expansion.

Bridges have expansion joints, gaps that let the metal and concrete grow on hot days without cracking.
Liquid thermometers work because the liquid inside expands and rises up the thin tube as it warms.
A stuck metal lid on a glass jar often opens after an adult runs it under warm tap water, because the metal lid expands more than the glass.

Specific heat

Specific heat is how much energy it takes to warm 1 kilogram of a material by 1 degree. Water has a very high specific heat: it takes several times more energy to warm water than the same mass of sand or rock. That's why, on a sunny day, the beach sand can be too hot to walk on while the water still feels cool. At night the sand cools quickly too, while the sea stays warmer.

Safety around hot things

Only use the stove, oven, or kettle with an adult. Turn pan handles in so no one bumps them.
Use oven mitts or pot holders. Steam can burn as badly as boiling water, so keep your face away from pots and kettles.
Very hot tap water can burn skin, so test it carefully before you put your hands in.
Metal outside in the summer sun, like slides and car seat buckles, can get hot enough to burn.
If you get a small burn, cool it under cool (not icy) running water and tell an adult right away.

Experiments to try at home

Ask an adult before you start. Use only warm tap water, never boiling water, and no stove or flames.

1. Spoon race
You need: a metal spoon, a wooden spoon, and a plastic spoon; a mug of warm tap water (an adult should fill it); a small dab of butter.
Put a tiny dab of butter near the top of each handle. Stand all three spoons in the mug and watch for 5 minutes.
Why: The butter on the metal spoon softens first, because metal is a good conductor. Wood and plastic are insulators.
2. Convection colors
You need: two clear glasses, warm and cold tap water, food coloring.
Fill one glass with warm water and one with cold. Let them sit still for a minute, then gently add one drop of food coloring to each.
Why: The color spreads faster in the warm water because its particles move faster and convection currents swirl it around.
3. Dark cup, light cup
You need: two paper cups, black and white paper, tape, tap water, a sunny window, and a thermometer if you have one.
Wrap one cup in black paper and one in white. Fill both with the same amount of cool water and set them in the sun for 30 minutes. Compare the temperatures.
Why: Dark colors absorb more of the Sun's radiation, so the water in the black cup warms more.
4. Balloon on a bottle
You need: an empty plastic bottle, a balloon, a bowl of warm tap water, a bowl of ice water. An adult should handle the warm water.
Stretch the balloon over the bottle's mouth. Stand the bottle in the warm water for a few minutes, then move it to the ice water.
Why: The air inside expands when it warms and puffs up the balloon, then contracts when it cools. Keep balloon pieces away from little kids: they're a choking hazard.

Sources

OpenStax, College Physics 2e, 13.1: Temperature (scales, absolute zero, thermal equilibrium).
OpenStax, College Physics 2e, 13.2: Thermal Expansion of Solids and Liquids.
OpenStax, College Physics 2e, 14.1: Heat.
OpenStax, College Physics 2e, 14.2: Temperature Change and Heat Capacity (specific heat).
OpenStax, College Physics 2e, 14.5: Conduction.
OpenStax, College Physics 2e, 14.6: Convection.
OpenStax, College Physics 2e, 14.7: Radiation.

Credits: All illustrations on this page are original, hand-coded SVG by Galileo. The share image was drawn in code.