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P5: ELECTRICITY IN THE HOME (Appliances and Efficiency (A domestic…
P5: ELECTRICITY IN THE HOME
Electricity Supply
There are two types of electricity supplies: AC (alternating current) and DC (direct current)
In AC supplies, the current is constantly changing direction
To measure the frequency of an AC supply, measure the time period of the waves and divide it by 1 (1/time taken for 1 cycle)
In DC supplies, the current flows in one direction only
Cells and batteries provide direct current
Direct current is created by direct voltage
The peak potential difference of an AC supply is the maximum voltage measured from 0 volts
A mains supply is an AC supply at around 230V
A mains circuit has a live wire, which is alternately positive and negative every cycle, and a neutral wire at 0 volts
Cables and Wires
Most electrical appliances are connected to the mains supply by three-core cables. They have three wires inside them, each with a core of copper and a coloured plastic coating- the colour of the coating shows the wire's purpose
LIVE WIRE (brown) The live wire provides the alternating potential difference (at about 230 V) from the mains supply
The body is at 0V (like the earth)- this means that if you touch a live wire, a large potential difference is produced across your body and current flows through you, causing an electric shock
Even if a plug socket or a light switch is turned off, there is still a danger of an electric shock. A current isn’t flowing but there’s still a pd in the live wire, so if you made contact with the live wire, your body would provide a link between the supply and the earth, and a current would flow through you
NEUTRAL WIRE (green and yellow) The neutral wire completes the circuit and carries away current — electricity normally flows in through the live wire and out through the neutral wire. It is around 0 V.
EARTH WIRE (green and yellow). It is for protecting the wiring, and for safety — it stops the appliance casing from becoming live. It doesn’t usually carry a current — only when there’s a fault. It’s also at 0 V
Electrical Power and Potential Difference
The power supplied to a device is the energy transferred to it each second
energy transferred = power (J) x time (s)
The electrical power supplied to an appliance = current (A) x potential difference (V)
Power rating = electrical power (W) / potential difference (V)
Electrical Current and Energy Transfer
Charge flow = current (A) x time (s)
When charge flows through a resistor, energy transferred through the resistor make it hot
Energy transferred = voltage x charge flow
When charge flows around a circuits for a given time, the energy supplied by the battery is equal to the energy transferred to all the components in the circuit
Appliances and Efficiency
A domestic electricity meter measure how much energy is supplied
The energy supplied to an appliance (J) = power (W) x time (s)
Useful energy used = efficiency x energy supplied
Power (J) = current (A) x Voltage (V)
Efficiency =(output power / input power) x100