(b) Mains electricity
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
2.2Electrical safety devicesUnderstand how the use of insulation, double insulation, earthing, fuses and circuit breakers protects the device or user in a range of domestic appliances2.3Heating effect of currentUnderstand why a current in a resistor results in the electrical transfer of energy and an increase in temperature, and how this can be used in a variety of domestic contexts2.4Power, current and voltageKnow and use power = current × voltage, P = IV, and apply it to selecting appropriate fuses.2.5Electrical energy transferUse energy transferred = current × voltage × time, E = IVt.2.6AC and DCKnow the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied by a cell or battery
How mains safety devices protect
Electrical safety features either stop a user reaching a live conductor, provide a safe path during a fault, or disconnect the supply when the current becomes dangerous.
| Safety method | How it protects | Typical boundary |
|---|---|---|
| insulation | a non-conducting covering stops contact with a live conductor | one protective insulating layer |
| double insulation | two independent insulating barriers separate live parts from the user | an insulating outer case means no earth wire is required |
| earthing | a low-resistance earth wire connects an exposed metal case to earth | acts when a live wire faults onto the case |
| fuse | excessive current heats and melts a thin fuse wire, breaking the circuit | must be replaced after operating |
| circuit breaker | detects excessive current and trips to break the circuit | can be reset and usually operates faster than a fuse |
For an earthed metal-cased appliance: if the live wire touches the case, the earth wire provides a low-resistance path. A large fault current then flows and the fuse melts or circuit breaker trips, disconnecting the live supply and reducing the risk of electric shock.
The earth wire does not normally carry the appliance current. A fuse or circuit breaker does not make an excessive current safe while it keeps flowing: it opens the circuit, so the protective device must be in the live side of the supply.
Why a resistor heats up
When current flows through a resistor, moving electrons collide with ions in its lattice. The electrons transfer energy to the lattice, increasing the resistor's thermal energy store and temperature.
A heating element is designed to use this effect. It becomes hot and transfers energy to its surroundings: a kettle heats water, a toaster heats bread, an electric heater warms a room, and an iron heats its soleplate.
The resistor transfers electrical energy; it does not create energy. Heating is useful in the element but usually unwanted in connecting wires, where it wastes energy and excessive heating can damage insulation.
Calculate power and choose a fuse
Power is the rate at which an appliance transfers energy. For an electrical device, power equals current multiplied by voltage.
P=IV
| Quantity | Symbol | SI unit |
|---|---|---|
| power | P | watt (W) |
| current | I | ampere (A) |
| voltage | V | volt (V) |
Example: a 2.8 kW toaster operates at 230 V. First convert 2.8kW=2800W. Then I=P/V=2800/230=12.2A. From 3 A, 5 A, 10 A and 13 A fuses, choose 13 A: it is the smallest available rating above the normal operating current.
A fuse rated below the normal current would melt during ordinary use. A much larger rating may fail to disconnect soon enough when current becomes unsafe, so select the smallest suitable rating above the operating current.
Calculate electrical energy transfer
The electrical energy transferred depends on current, voltage and how long the device operates.
E=IVt
| Quantity | Symbol | SI unit |
|---|---|---|
| energy transferred | E | joule (J) |
| current | I | ampere (A) |
| voltage | V | volt (V) |
| time | t | second (s) |
Example: a device draws 2.0 A from a 12 V supply for 30 s. E=IVt=2.0×12×30=720J. If time is given in minutes or hours, convert it to seconds before using SI units.
Energy and power are different: energy is measured in joules, while power is the rate of energy transfer in watts. The time factor in E=IVt is what turns the transfer rate IV into an amount of energy.
Distinguish alternating and direct current
Direct current (d.c.) flows in one direction only. Alternating current (a.c.) repeatedly reverses direction.
| Feature | Direct current (d.c.) | Alternating current (a.c.) |
|---|---|---|
| current direction | one direction only | repeatedly changes direction |
| supply polarity | does not reverse | repeatedly reverses |
| required source example | cell or battery | mains electricity |
Direct current does not have to keep the same magnitude; the defining feature is that it does not reverse direction. For alternating current, crossing zero is part of changing direction, not evidence that the supply has permanently switched off.