What is heat?
Everything is made of particles that are always moving. The faster they move, the more energy they have. Thermal energy is the total energy of all these moving particles.
Heat is thermal energy that moves from a warmer object to a cooler one. It always flows from hot to cold, never the other way on its own. When you hold an ice cube, heat flows from your hand into the ice, which is why your hand feels cold.
Heat and temperature
| Heat | Temperature |
|---|---|
| Energy that moves from hot to cold | A measure of how hot or cold something is |
| Measured in joules (J) | Measured in degrees Celsius (°C) or kelvins (K) |
| Depends on mass and temperature | Measures the average energy of particles |
A swimming pool at 25 °C has much more thermal energy than a cup of tea at 80 °C, because the pool has so many more particles.
Three ways heat moves
Conduction
Conduction is heat transfer through direct contact. Fast-moving particles bump into slower neighbours and pass energy along.
- A metal spoon in hot soup gets hot along its handle.
- Your feet feel cold on a tile floor.
- A frying pan heats food.
Metals are good conductors. Wood, plastic and air are poor conductors (insulators).
Convection
Convection is heat transfer by the movement of liquids or gases.
- Warm fluid expands, becomes less dense, and rises.
- Cooler, denser fluid sinks to take its place.
- This creates a circular flow called a convection current.
Examples:
- Water circulating in a pot on the stove
- Warm air rising from a heater and spreading across a room
- Sea breezes: during the day, land heats faster, warm air rises over land, and cool air from the sea moves in
- Ocean currents and weather systems
- Hot rock slowly circulating in Earth’s mantle
Radiation
Radiation is heat transfer by electromagnetic waves (mostly infrared). It doesn’t need any material; it can travel through empty space.
- The Sun warms Earth across 150 million km of space.
- You feel the warmth of a campfire on your face.
- A thermal camera "sees" infrared radiation.
Dark, dull surfaces absorb and give off radiation well. Light, shiny surfaces reflect it. That’s why a black car gets hotter in summer than a white one.
| Method | Needs contact? | Needs a material? | Example |
|---|---|---|---|
| Conduction | Yes | Yes | Touching a hot pan |
| Convection | No | Yes (liquid or gas) | Boiling water |
| Radiation | No | No | Sunlight |
Heating things up
Different substances need different amounts of energy to warm up. This is their specific heat capacity.
| Substance | Specific heat capacity (J/kg·°C) |
|---|---|
| Water | 4180 |
| Aluminum | 900 |
| Iron | 450 |
| Sand | About 800 |
Water has a very high specific heat capacity. It heats up and cools down slowly. That’s why:
- beach sand gets scorching hot while the water stays cool
- coastal cities like Vancouver and Halifax have milder climates than inland cities
- water is used to cool car engines
The heat needed can be calculated: Q = m × c × ΔT
Example: Heating 2 kg of water from 20 °C to 70 °C needs Q = 2 × 4180 × 50 = 418 000 J (418 kJ).
Effects of heat
| Effect | Example |
|---|---|
| Temperature rises | Water warms in a kettle |
| Change of state | Ice melts; water boils |
| Expansion | Bridges have expansion joints; power lines sag in summer |
| Chemical changes | Cooking an egg |
Keeping heat in, or out
Insulation slows heat transfer. Good insulators trap air.
| Design | How it controls heat |
|---|---|
| Winter coat | Traps warm air close to your body |
| Double-paned windows | Air or gas between panes slows conduction |
| Attic insulation | Fibreglass or foam traps air |
| Thermos | Vacuum stops conduction and convection; shiny walls reflect radiation |
| Igloo | Snow traps air and insulates |
Good insulation is especially important in Canada, where heating homes uses a lot of energy.
Common mistakes
- Thinking cold flows. Heat flows from warm to cold; "cold" is just less heat.
- Thinking heat and temperature are the same.
- Thinking only metals conduct. All materials conduct some heat; insulators just conduct poorly.
Try it yourself
- Put butter blobs along a metal spoon, a plastic spoon and a wooden spoon. Stand them in hot water. Which loses its butter first?
- Drop food colouring into hot and cold water. Watch convection currents.
- Place black and white paper in the sun with a thermometer under each. Compare after 20 minutes.
Quick recap
- Heat is thermal energy moving from hot to cold.
- Heat moves by conduction, convection and radiation.
- Water has a high specific heat capacity, so it warms and cools slowly.
- Heat can raise temperature, change states and cause expansion.
- Insulation slows heat transfer by trapping air.