The kinetic particle model and the states
The kinetic particle model says all matter is made of particles that are always moving, and the hotter the substance, the faster they move. How closely those particles are packed and how freely they move gives us the three states.
| State | Arrangement | Movement |
|---|---|---|
| Solid | Tightly packed in a fixed pattern | Vibrate on the spot; fixed shape and volume |
| Liquid | Close together but disordered | Slide past each other; fixed volume, takes the shape of its container |
| Gas | Far apart and random | Move quickly in all directions; fill any container |
Heating a substance gives its particles more energy, so they move faster and can break free of their neighbours. That is why a solid melts to a liquid and a liquid boils to a gas.
Thermal (heat) energy and temperature
These two words are easy to muddle, but they are not the same thing.
- Temperature
- A measure of the average kinetic energy of the particles, measured in degrees Celsius, \( ^\circ\text{C} \). It tells you how hot something is, not how much energy it holds.
- Thermal (heat) energy
- The total energy of all the particles in an object, measured in joules, \( \text{J} \). It depends on temperature and on how much substance there is.
A cup versus a bath
A cup of boiling water is at a higher temperature than a warm bath, but the bath holds far more thermal energy because it contains many more particles. Temperature is an average; thermal energy is a total.
Heat always flows from a hotter region to a cooler one until both reach the same temperature, a state called thermal equilibrium.
Thermal expansion
When a substance is heated its particles move faster and push a little further apart, so the whole object gets slightly bigger. This is thermal expansion. It is largest in gases, smaller in liquids, and smallest in solids. Bridges are built with small expansion gaps so the metal has room to grow on hot days without buckling.
A steel bridge deck is \(200\ \text{m}\) long. On a hot day it expands by \(0.05\%\) of its length. How much longer does it become?
Heat transfer: conduction, convection and radiation
Thermal energy moves in three ways.
- Conduction
- Particles pass energy to their neighbours by vibrating and colliding, without moving position. Best in solids, especially metals. A metal spoon in hot soup heats up along its handle.
- Convection
- Warmer fluid (liquid or gas) becomes less dense, rises, and cooler fluid sinks to take its place, setting up a current. This is how a radiator warms a whole room and how hot air rises.
- Radiation
- Energy carried by infrared waves that need no particles at all, so it travels through a vacuum. This is how heat from the Sun reaches the Earth.
Where this is assessed
Explaining a familiar observation (a metal handle heating up, a hot-air balloon rising) using the particle model is Criterion A (knowing) and Criterion B (inquiring), especially when you predict and then test what will happen.
Dark, matt surfaces are the best emitters and absorbers of radiation, while shiny, light surfaces reflect it. That is why solar panels are dark and survival blankets are silver.
Check yourself
1. In which state are particles far apart and moving quickly in all directions? +
That describes a gas. The particles have the most energy, are furthest apart, and spread to fill any container.
2. Which holds more thermal energy: a \(100\ ^\circ\text{C}\) cup of water or a \(40\ ^\circ\text{C}\) bath, and why? +
The bath holds more thermal energy. Even though the cup is at a higher temperature (higher average particle energy), the bath has far more particles, so its total energy is greater. Temperature is an average; thermal energy is a total.
3. How does heat from the Sun reach the Earth across empty space? +
By radiation. Infrared waves need no particles to travel, so they cross the vacuum of space. Conduction and convection both need a material medium and so cannot work here.
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