Electrical power and safetyIB MYP Sciences: Revision notes

Power is the rate at which energy is transferred. It is measured in watts (W); 1 W is 1 joule per second. For an electrical appliance:

P = I × V

where P is power (W), I is current (A) and V is potential difference (V). A kilowatt (kW) is 1000 W.

Worked example: a hairdryer on 230 V draws a current of 5.0 A. P = I × V = 5.0 × 230 = 1150 W.

The energy transferred by an appliance depends on its power and the time it is on:

E = P × t

With power in watts and time in seconds, the energy is in joules (J). With power in kilowatts and time in hours, the energy is in kilowatt-hours (kWh).

Worked example: a 2.0 kW kettle runs for 3.0 minutes (0.050 h). E = 2.0 × 0.050 = 0.10 kWh. In joules: 2000 × 180 = 360 000 J.

Households pay for the energy they use, measured in kWh.

cost = energy in kWh × price per kWh

Worked example: a 1.5 kW heater runs for 4.0 hours. Energy = 1.5 × 4.0 = 6.0 kWh. If one kWh costs $0.30, the cost is 6.0 × 0.30 =$1.80.

A lower-power appliance that gives the same result, such as an LED instead of a filament lamp, costs less to run.

Mains electricity is an alternating current (a.c.) supply, of about 230 V in the UAE and many other countries. A three-pin plug connects an appliance to three wires:

• Live (brown): carries the alternating p.d. from the supply; it is dangerous
• Neutral (blue): completes the circuit
• Earth (green and yellow): a safety wire, which does not normally carry current

The plug also contains a fuse in the live wire.

• A fuse contains a thin wire that melts if the current becomes too large, breaking the circuit. Choose a fuse rating just above the normal current.
• A circuit breaker is an automatic switch that trips when the current is too large. It can be reset, so is more convenient than a fuse.
• The earth wire connects a metal case to the ground. If the live wire touches the case, a large current flows through the earth wire, which blows the fuse and makes the case safe.
• A double-insulated appliance has a plastic case and extra insulation, so it has no earth wire.

Never touch electrical switches with wet hands and never overload sockets.

When two insulating materials are rubbed together, electrons move from one to the other. The material that gains electrons becomes negatively charged; the one that loses them becomes positively charged. Like charges repel and unlike charges attract.

A charged object can attract small pieces of paper, and a build-up of charge can cause a spark or shock. This is why aircraft and fuel tankers are earthed when refuelling.