Section 1 of 6Why a current causes heating
1Current flows
- Current flows A potential difference makes free electrons drift through a wire or resistor.
- Current is the rate of flow of charge.
2Collisions occur
- Collisions occur As the electrons move they collide with the vibrating ions of the metal lattice and with each other.
3Ions vibrate more
- Ions vibrate more Each collision transfers kinetic energy to the ions, so they vibrate with greater amplitude.
4Temperature rises
- Temperature rises Greater vibration means a higher internal energy, so the temperature of the conductor goes up.
5Energy is dissipated
- Energy is dissipated Because the conductor is hotter than its surroundings, thermal energy spreads out into the air around it.
Section 2 of 6Energy transfer pathway
Electrical transfer
- Electrical transfer Charge moving round a circuit is an electrical energy transfer pathway: energy from the cell or mains supply is carried to the components.
- In a resistor the energy is transferred to the thermal store of the resistor.
Heating the surroundings
- Heating the surroundings Some of this thermal energy is dissipated to the surroundings by conduction, convection and radiation.
- Dissipated energy spreads out and cannot easily be recovered.
Wanted and unwanted heating
- Wanted and unwanted heating In a heater the thermal energy is the useful output.
- In a computer or a phone charger the same effect wastes energy and may make the device get warm.
Section 3 of 6Everyday applications
| Appliance | How the heating effect is used |
|---|---|
| Electric heater | A high-resistance element glows and warms the room by radiation and convection |
| Electric oven and grill | Heating elements raise the temperature of the air or food |
| Electric hob | A hot coil or plate transfers thermal energy to the pan by conduction |
| Toaster | Thin wires heat up and radiate infrared to brown the bread |
| Kettle | An element immersed in water transfers energy to the water, which then heats by convection |
Section 4 of 6What affects the amount of heating
Heating effect of a current
Larger current
- Larger current More charge passes each second, so there are more collisions and more energy is transferred per second.
Higher resistance
- Higher resistance A wire with a higher resistance has more collisions per electron, so for the same current it heats more.
Longer time
- Longer time The longer the current flows, the more thermal energy is transferred in total.
Power
- Power The rate of energy transfer is the power, P = I × V, measured in watts (W),
- and for a resistor also P = I2 × R.What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
- and for a resistor also P = I2 × R.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Current | The rate of flow of electric charge, measured in amperes (A). |
| Resistor | A component that opposes the flow of current and transfers electrical energy to thermal energy. |
| Free electrons | Electrons in a metal that are not bound to one atom and can move through the structure. |
| Dissipate | To spread energy out into the surroundings, usually as thermal energy. |
| Heating effect | The rise in temperature of a conductor when a current passes through it. |
Section 6 of 6Exam technique
1
Saying that electrons 'get hot' or that the current is 'used up'. The current is the same all round a series circuit; it is the energy that is transferred.
2
Forgetting to mention collisions with the ions of the lattice when asked to explain why a wire gets hot.
3
Describing the energy as 'lost' or 'destroyed'. It is transferred to the thermal store of the surroundings, and is conserved.
4
Assuming heating is always wasteful. In heaters, toasters and kettles it is the useful output.