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P5 Electricity in the home, Screenshot 2021-02-25 132528 , image -…
P5 Electricity in the home
5.1 Alternating current
Direct current
Flows in one direction only
Alternating current
The current from the mains supply repeatedly reverses its direction.
Frequency
Frequency is the rate at which an alternating current reverses its direction. The frequency of the UK mains supply is 50 hertz. Direction of flow is 50 times each second
Frequency= 1/time taken for 1 cycle
National grid
When you use mains appliances, the electricity is supplied from power stations to homes and buildings through the national grid
A nationwide network of cables and transformers.
The households main supply
The live wire carries an alternating potential difference. This means it is alternatively positive and negative every cycle.
The neutral wire is at zero volts. It is the return wire.
5.2 Cables and Plugs
Live wire (brown)
This carries the current to the appliance. Touching this can be deadly.
Earth wire (green yellow)
This 'earths' the appliance in case one of the wires touches the casing.
The socket
Sockets and plugs are often made from stiff plastic materials that enclose the electrical connections. Plastic is used since it is a good electrical insulator. It prevents electric shock.
Most appliances are supplied with a three-core cable, this means the cable is made up of three separate wires
Appliances with a plastic as an insulator often do no require an earth wire as there is no danger if the live wire touches the casing. These are two-core cables.
Neutral wire (blue)
This completes the circuit and is usually at 0V
Fuse (red)
This stops the flow of the current if it gets too high.
Components are earthed to make sure you don't get an electric shock if the live wire accidentally touches the casing. The electricity will flow harmlessly through the earth wire instead of through you when touching the casing.
5.3 Electrical power and potential difference
Power
Power can be calculated using the equation p=e/t
The equation can also be rearranged
P is power in watts, W
E is energy in joules, J
t is time in seconds, s
Calculating power with current and potential difference
P= I x V
P is the power appliance in watts, W
I is the current in amperes, A
V is the potential difference in volts, V
Can also be rearranged
5.4 Electrical currents and energy transfer
An electric current is a flow of charge
The charge that flows in a given time can be calculated using the equation Q= I x t
Q is the charge in coulombs, C
I is the current in amperes, A
t is the time in seconds, s
When charge flows through a resistor, energy is transferred to the resistor. The thermal energy store of the resistor increases, so the resistor becomes hotter. This thermal energy is transferred to the surroundings.
This energy transferred to a component by a flow of charge can be calculated using the equation E= V x Q
E is the energy in joules, J
V is the potential difference in volts, V
Q is the charge in coulombs, C
5.5 Appliances and efficiency
The energy transferred to an appliance by the mains supply can be calculated using the equation E= P x t
E is the energy transferred to the device from the mains supply in joules, J
P is the power of the electrical appliance in watts, W
t is time in seconds, s
Electrical appliances waste energy because of the heating effect of the current in the wires. Appliances with electric motors also waste energy due to the friction between the moving parts
efficiency= useful energy transferred by appliance/total energy supplied to appliance x100
or efficiency= output power/ input power x 100
The useful energy transferred by the appliance is sometimes called the output energy
Keywords
direct current- current that passes round the circuit in one direction
alternating current- the current from the mains supply repeatedly reverses its direction
live wire
neutral wire
national grid- is a system of cables and transformers linking power stations to consumers
earth wire
fuse
