2 3 electrical power and mains electricity

Electrical power is defined as the rate at which energy is transferred or converted per unit of time, typically measured in watts (W).

The two quantities that electrical power depends on are the voltage and current of a device.

The electrical power equation is: Where:

False. The unit of power is watts (W), where 1 watt is equivalent to 1 joule per second.

True. The unit watts (W) is the equivalent to joules per second (J/s).

An electrical fuse is a safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or surge).

If the current inside the wire becomes too large it heats up and melts.

The electrical symbol for a fuse looks like this:

The three fuse sizes are 3 A, 5 A and 13 A.

The correct fuse size for an appliance that requires a current of 4.5 A is 5 A.

True. Power is defined as the rate of doing work, or, the rate of energy transfer. It is measured in watts (W).

Work done is equal to the energy transferred in an electrical circuit.

True. Work is done when energy is transferred by moving charges in a circuit.

False. The energy supplied by the battery is equal to the energy transferred to all the components in an electrical circuit.

The equation for calculating energy transfers in terms of current, potential difference and time is: Where:

True. Mains electricity is potentially lethal.

An electrical safety hazard is a condition that poses a risk of electric shock or fire, such as damaged insulation or overheating cables.

Hazards that damp conditions pose to electrical circuits or appliances are that the moisture can conduct electricity, causing a short circuit or posing an electrocution risk.

Double insulation is when an appliance has insulation around the wires and a non-metallic case, eliminating the need for an earth wire.

Earthing is an electrical safety feature found in appliances with metal cases that provides a low resistance path to the earth to prevent electric shocks.

True. A fuse is an example of an electrical safety feature built into domestic appliances.

The purpose of the earth wire in an electrical appliance is to provide a low-resistance path to the Earth for the current.

The role of circuit breakers in a house is to quickly shut off electricity if the current exceeds a certain value, protecting the electrical system.

The materials commonly used to insulate wires are rubber and plastic.

Electrical cables overheat when too much current passes through the wire. This can potentially cause a fire or melt the insulation.

The electrons flowing in the wires around a circuit collide with the metal ions.

The temperature of an electrical resistor increases because of the collisions between the electrons flowing in the conductor and the lattice of metal ions within the conductor.

True. When current flows through an electrical wire, some energy is dissipated to the surroundings by heating.

A domestic example of when the heating effect of an electrical wire is utilised can be any one from:

Mains electricity supplies an alternating current (a.c.).

A battery or cell supplies a direct current to an electrical circuit.

Direct current is a steady current, constantly flowing in one direction through a circuit, from positive to negative.

False. The potential difference across a cell in a d.c. circuit travels in one direction only.

An alternating current is a current that continuously changes its direction, going back and forth around a circuit.

False. An alternating current power supply has two identical terminals that change from positive to negative and back again.

The frequency of mains electricity in the UK is 50 Hz.

The graph shows direct current (d.c.).

A current-time graph for alternating current (a.c.) looks like this:

Alternating current is used by a lamp plugged into a domestic plug socket.