Science 7 · Heat and temperature
Heat and temperature are not the same thing
A bathtub of warm water holds more heat than a cup of hot tea, even though the tea is hotter. This unit explains why, using one idea — that everything is made of moving particles — and then follows heat as it moves, changes the state of things, and warms our homes.
- 1. The words, first
- 2. Heat, temperature and the particle model
- 3. Expansion and changes of state
- 4. Conduction, convection and radiation
- 5. Heating our homes, and the energy it takes
- 6. What costs marks
The words, first
The idea: Temperature, thermal energy and heat are three different things. Most of this unit depends on keeping them apart.
| Word | What it means |
|---|---|
| Particle model of matter | The idea that all matter is made of tiny particles that are always moving, with spaces between them. Heating makes them move faster. |
| Temperature | How hot or cold something is: a measure of how fast its particles are moving, on average. Measured in degrees Celsius (°C). |
| Thermal energy | The total energy of all the moving particles in an object. More particles, or faster ones, means more thermal energy. |
| Heat | Thermal energy moving from a warmer object to a cooler one. On its own, it always flows from warm to cool. |
| Expansion / contraction | Taking up more space as it warms / less space as it cools. |
| Changes of state | Melting: solid to liquid. Freezing: liquid to solid. Evaporation and boiling: liquid to gas. Condensation: gas to liquid. Sublimation: solid straight to gas. Deposition: gas straight to solid. |
| Melting point / boiling point | The temperature at which a substance melts / boils. For water, 0 °C and 100 °C at sea level. |
| Fluid | Anything that can flow: a liquid or a gas. |
| Conduction / convection / radiation | The three ways heat moves: through touching particles / carried by a moving fluid / as waves, like sunlight, that need no particles. |
| Conductor / insulator | A conductor lets heat through easily, as most metals do. An insulator slows it down, as air, wool and foam do. |
Heat, temperature and the particle model
The idea: Temperature is about how fast the particles move. Thermal energy is about how fast and how many. Heat is thermal energy on the move.
| State | The particles | So it |
|---|---|---|
| Solid | Packed close, held in place, vibrating | Keeps its shape |
| Liquid | Close together, but able to slide past each other | Flows and takes the shape of its container |
| Gas | Far apart, moving freely | Spreads out to fill any container |
Worked: the bathtub and the tea. A cup of tea is at 90 °C; a bathtub of water is at 40 °C. The tea has the higher temperature: its particles are moving faster, on average. But the bathtub has far more thermal energy, because it holds hundreds of times as many particles. The tea could melt only a handful of ice cubes before it cooled right down; the bath could melt a whole bucket of ice.
Heat flows one way, from warmer to cooler, until both are at the same temperature. Hold an ice cube and cold does not flow into your hand: heat flows out of your hand into the ice.
Thermometers work by expansion: the liquid inside expands as it warms and climbs the thin tube.
Expansion and changes of state
The idea: Heating makes particles move faster and spread out, so things expand. During a change of state, the energy goes into pulling particles apart instead, and the temperature stops rising.
Expansion. Heat a material and its particles move faster and push farther apart, so it takes up more space. The particles themselves do not get bigger; the spaces between them do. Bridges have expansion joints, gaps that let the deck grow and shrink: a 100 m steel bridge is roughly 8 cm longer on a +30 °C summer afternoon than on a −40 °C winter night. Water is the odd one out: it expands by about a tenth when it freezes, which is why ice floats and a frozen pipe can burst.
Worked: heating water to boiling. A beaker of water is heated steadily on a hot plate, and its temperature is read every minute:
minute 0: 20 °C · 1: 34 °C · 2: 48 °C · 3: 61 °C · 4: 74 °C · 5: 86 °C · 6: 96 °C · 7: 100 °C · 8: 100 °C · 9: 100 °C
The temperature climbs to 100 °C and stops, though the hot plate is still on and the water is still taking in energy. That energy now goes into pulling particles apart into a gas — boiling — instead of making them move faster, so the temperature stays at 100 °C until the water has boiled away. Melting works the same way: a glass of ice water stays at about 0 °C until the ice is gone.
The less familiar changes. Evaporation happens only at a liquid's surface, at any temperature — a puddle drying — while boiling happens all through the liquid, at its boiling point. Snow can shrink on a cold, sunny day without ever melting: sublimation. Frost on a window on a winter night grows straight from water vapour in the air: deposition.
Conduction, convection and radiation
The idea: Heat moves three ways: through touching particles, carried by a moving fluid, or as waves that need no particles at all. Keeping heat in means blocking all three.
Conduction is strongest in solids, and in metals most of all: a metal spoon in hot chocolate warms right up to the handle; a wooden one does not. Convection needs a fluid: air above a baseboard heater warms, expands and rises, cooler air sinks to take its place, and a current circulates round the room. Radiation is why a campfire warms your face while your back stays cold. Dark, dull surfaces absorb it well and light, shiny ones reflect it, so a black car seat gets hotter in July than a white one.
Worked: the cold tile floor. On a winter morning, a tile bathroom floor feels much colder under bare feet than the carpet in the hall. A thermometer shows both are at 20 °C. The difference is conduction: tile is a much better conductor than carpet, so it carries heat away from your feet faster. “Feels colder” tells you how fast heat is leaving you, not the temperature of the floor.
Insulators work by trapping air. Air is a poor conductor, and air held still cannot carry heat by convection. A down jacket, a foam cup, attic fibreglass and double-pane windows all hold air still. A vacuum flask goes further: there is almost no air between its two walls, so neither conduction nor convection can cross, and its shiny walls reflect radiation back.
Heating our homes, and the energy it takes
The idea: Every heating technology moves thermal energy into a building. They differ in where the energy comes from and how much is wasted.
| Technology | How it heats | The trade-off |
|---|---|---|
| Wood stove | A closed metal box radiates heat and warms the air around it | Far less wasteful than an open fireplace, which sends most of its heat up the chimney; smoke |
| Natural-gas furnace | Burns gas to heat air, which a fan blows through ducts | The most common heating in Alberta homes; gives off carbon dioxide, and a faulty one can leak carbon monoxide |
| Heat pump | Uses electricity to move heat from the outdoor air or the ground into the house | Moves more heat than the electrical energy it uses; less efficient in deep cold |
| Solar | Sunlight through south-facing windows, or collectors that heat water or air | No fuel and no emissions; nothing at night, weakest in December |
How a thermostat decides. A thermostat switches the furnace on when the house cools below the set temperature, and off once it is reached. Older ones use a bimetallic strip: brass and steel joined along their length. Brass expands more than steel, so as the strip warms it curls, with the brass on the outside of the curve, and as it cools it curls back, opening or closing a switch.
Where the energy comes from. Natural gas, coal and oil are non-renewable: they formed from living things over millions of years, and cannot be replaced. Sunlight, wind and flowing water are renewable: nature replaces them as fast as we use them. Wood counts as renewable only if trees are replanted as fast as they are cut.
Using less. Attic insulation, weatherstripping round doors and a thermostat turned down at night all cut how much heat a house loses, and so how much fuel it burns.
What costs marks
The idea: Six, and the first two are the same mistake in different words.
- Treating heat and temperature as the same. Temperature is average particle speed; heat is energy moving.
- Saying the hotter object has more thermal energy. The bathtub beats the tea.
- Saying particles get bigger when heated. They move faster and spread apart.
- Expecting the temperature to keep rising while water boils. It stays level until the change is finished.
- Saying cold flows into things. Heat flows out of you, faster into a good conductor.
- Mixing up conduction and convection. Convection needs a liquid or gas that moves.