Physics: Unit Five
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- Created by: hannahh.hm
- Created on: 13-08-17 07:35
Direct current - current in the circuit only flows in one direction (e.g. battery in a torch)
5.1 - Alternating current
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Alternating current - current repeatedly reverses its direction (e.g. switching on a light on at home, as mains electricity is an a.c. supply). The current flows one way then the opposite in successive cycles.
Frequency is the number of cycles a wave passes a certain point each second.
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In the UK, the mains frequency is 50 cycles per second (50Hz).
The current through a mains appliance alternates. That is because the mains supply provides an alternating potential difference between the live and neutral wire.
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The neutral wire is earthed at the local electricity substation. The potential difference between the live and earth wire is called the potntial or voltage of the live wire.
The live wire is dangerous because its potential repeatedly changes for + to - and back every cycle. In UK homes, it reaches about 325 V in each direction.
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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 25,000 V to about 132,000 V.
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 a 230 V direct-current supply. Factories use more so need 100kV
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P5.2 - Cables and plugs
When you plug a heater with a metal case into a wall socket, the metal case is automatiaclly connected through a wire called the earth wire. This stops the metal case becoming live if the wire breaks and touches the case.
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Plastic materials are good insulators. An appliance that has a plastic case is double-insulated, so it has no earth wire connection.
The earth wire of a circuit is connected to the ground at your home. It is at zero volts (0 V) and carries a current only if there is a fault.
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The longest pin of a three-pin plug is designed to make contact with the earth wire of a wall socket circuit. So, when you plug an appliance with a metal case into a wall socket, the case is automatically earthed.
The pins are made of brass because brass is a good conductor and doesn't rust of oxidise. Copper isn't as hard as brass even though it conducts better.
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The case material is an electrical insulator. The inside of the case is shaped so that the wires and pins can't touch each other when the plug is sealed.
The plug contains a fuse between the live pin and the live wire. If too much current passes through the wire in the fuse, it melts an cuts the live wire off.
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The brown wire is connected to the live pin.
The blue wire is connected to the neutral pin.
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The green and yellow striped wire is connected to the earth pin. A two-core cable doesn't have an earth wire.
If a live wire inside the appliance touches a neutral wire, a very big current passes between the two wires at the point of contact. This is called a short circuit. Provided the fuse blows, it cuts the current off.
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P5.3 - Electrical power and potential difference
power, P (watts, W) = energy transferred, E (joules, J) / time, t (seconds, s)
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For any electrical appliance: the current through it is that flows through it each second.
For any electrical appliance: the potential difference across it is the energy transferred to the appliance be each coulomb of charge that passes through it.
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For any electrical appliance: the power supplied to it is the energy transferred to it each second. This is the energy transferred be an electric current every second.
power supplied, P (watts, W) = current, I (amps, A) x potential difference, V (volts, V)
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Domestic appliances are often fitted with a 3A, 5A, or a 13A.
When an electric current passes through a resistor, the power supplied to the resistor heats it. The resistor heats the surroundings, so the energy supplied to it is dissipated to the surroundings.
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power, P (W) = current^2, I^2 (A) x resistance, R ( )
P5.4 - Electrical currents and energy transfer
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charge flow, Q (coulombs, C) = current, I (amps, A) x time, t (seconds, s)
When charge flows through a resistor, energy is transferred to the resistor, so the resistor becomes hotter.
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energy, E = charge flow, Q x potential difference, V
energy, E = power, P x time, t = potential difference, V x current, I x time, t
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When charge flows around a circuit for a given time, the energy supplied by the battery is equal to the enrgy transferred to all the components in the circuit.
P5.5 - Appliances and efficiency
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energy transferred from the mains, E (joules, J) = power, P (watts, W) x time, t (seconds, s)
power, P (watts, W) = current, I (amps, A) x potential difference, V (volts, V)
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efficiency = output power / input power x100%
Electrical appliances waste energy because the current in both the wires and the components of the appliance has a heating effect due to the resistance of the wires and the components. So they transfer energy by heating to the surroundings.
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Other cards in this set
Card 2
Front
Alternating current - current repeatedly reverses its direction (e.g. switching on a light on at home, as mains electricity is an a.c. supply). The current flows one way then the opposite in successive cycles.
Back
Frequency is the number of cycles a wave passes a certain point each second.
Card 3
Front
In the UK, the mains frequency is 50 cycles per second (50Hz).
Back
Card 4
Front
The neutral wire is earthed at the local electricity substation. The potential difference between the live and earth wire is called the potntial or voltage of the live wire.
Back
Card 5
Front
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 25,000 V to about 132,000 V.
Back
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