Chapter 5: Electricity in the home

Question 1

Alternating current

Answer 1

• Direct current (D.C.) flows in one direction only. Whereas alternating current (A.C.) repeatedly reverses its direction of flow
• A mains circuit has a live wire, which is alternately positive and negative every cycle, and a neutral wire at zero volts
• The peak voltage of an A.C. supply is the maximum voltage measured from zero volts
• To measure the frequency of an A.C. supply, measure the time period of the waves, then use the equation:

frequency = 1 / time taken for 1 cycle

Question 2

The national grid

Answer 2

• The electricity used in homes travels through the national grid (a nationwide network of cables and transformers); from power stations to home

• A typical power station generates 25 000V of potential difference
• Step-up transformers are used at power stations to transfer electricity to the national grid. These transformers are used to make the size of the alternating potential difference much bigger, typically from 25 000V to about 132 000V
• Step- down transformers are used to supply electricity from the national grid to consumers. Homes and offices in the UK are supplied with mains electricity that provides the same power as 230V direct-current supply. Factories use much more power than homes, so they are supplied with a p.d. of 100kV or 33kV
• Making the national grid so large means much less current is needed to transfer the same amount of power. So, the power loss to the resistance heating in the cables is much reduced

Question 3

Cables and plugs

Answer 3

• Sockets and plug cases are made of stiff plastic materials that enclose the electrical connections. Plastic is used because it is a good electrical insulator
• A mains cable is made up of two or three insulated copper wires surrounded by an outer layer of flexible plastic material
• In a three-pin plug or a three-core cable, the live wire is brown, the neutral wire is blue, and the earth wire is stripped green and yellow
• The earth wire is connected to the longest pin in a plug and is used to earth the metal case of a mains appliance

Question 4

Electrical power and potential difference

Answer 4

• The power supplied to a device is the energy transferred to it each second
• The energy transferred to a device is E= P/t
• The electrical power supplied to an appliance is equal to P = I x V

• The correct rating (A) for a fuse:
= electrical power / potential difference

Question 5

Electrical currents and energy transfer

Answer 5

• The charge flow is Q = I x t
• When charge flows through a resistor, energy transferred to the resistor makes it hot
• The energy transferred to a component is E = V x Q
• When charge flows around a circuit for a given time, the energy supplied by the battery is equal to the energy transferred to all components in a circuit

Question 6

Appliances and efficiency

Answer 6

• A domestic electricity meter
measures how much energy is supplied

• The energy supplied to an appliance is E = P x t
• Useful energy = efficiency x energy supplied