Electrical Power & Mains ElectricityEdexcel IGCSE Physics: Revision notes
Section 1
How is electrical power calculated?
Electrical power is the rate at which electrical energy is transferred. The relationship between power, current and voltage is:
P = I × V
where P is power (W), I is current (A), and V is voltage (V).
This relationship is used when selecting an appropriate fuse for an appliance: the fuse rating must be slightly higher than the normal operating current (calculated from P = IV), so it does not blow in normal use but still breaks the circuit if the current becomes dangerously high.
A 2300 W kettle operates at 230 V. I = P/V = 2300/230 = 10 A, so a 13 A fuse would be selected (the next standard rating above 10 A).
Section 2
How is energy transferred by an electrical appliance calculated?
The energy transferred by a current in a given time is found using:
E = I × V × t
where E is energy transferred (J), I is current (A), V is voltage (V), and t is time (s).
A current flowing through a resistor results in the electrical transfer of energy and a rise in temperature — this heating effect is used deliberately in devices such as kettles, toasters, electric heaters and irons.
A 10 A current flows through a 230 V heater for 60 s. E = I × V × t = 10 × 230 × 60 = 138,000 J.
Section 3
How is a.c. different from d.c.?
- Mains electricity is alternating current (a.c.) — the current regularly reverses direction
- Electricity from a cell or battery is direct current (d.c.) — the current flows in one direction only
UK mains supply is approximately 230 V a.c.
Section 4
How are people and devices protected from mains electricity?
Several safety features protect users and appliances from electrical hazards:
- Insulation — non-conducting material (e.g. plastic) covers wires and casings so users cannot touch live conductors
- Double insulation — appliances with two layers of insulation and no metal casing, so no earth wire is needed
- Earthing — connects the metal casing of an appliance to the ground via the earth wire; if a fault causes the casing to become live, a large current flows to earth, which blows the fuse and disconnects the appliance
- Fuses — a thin wire that melts and breaks the circuit if the current exceeds a safe level, protecting the appliance and wiring from overheating
- Circuit breakers — a switch that automatically trips (opens) if the current is too high, disconnecting the circuit; unlike a fuse, it can be reset and reused
Do not say a fuse 'stops electric shocks' directly — a fuse protects wiring/appliances from overheating and fire by breaking the circuit; earthing is what specifically directs a fault current away from a person touching the casing.
Must Know
- P = I × V links power, current and voltage; used to choose an appropriate fuse rating
- E = I × V × t gives the energy transferred by a current over time
- Current through a resistor transfers electrical energy and raises temperature (used in heaters, kettles, etc.)
- Mains electricity is alternating current (a.c.); cells/batteries supply direct current (d.c.)
- Safety features: insulation, double insulation, earthing, fuses and circuit breakers each protect users/devices in different ways
- A fuse melts and breaks the circuit on overcurrent; a circuit breaker trips and can be reset
That's the notes covered.
Carry on to the next subtopic.