Domestic electricity for OCR GCSE Physics
This page covers the following topics:
1. Direct potential difference
2. Alternating potential difference
3. Mains electricity
4. Three-core cable
5. The National Grid
Direct current is electric current that always flows in one direction. Direct current can be described as a constant flow of electrons from an area of high density of electrons to an area of low density. Direct current in a circuit causes a constant voltage circuit. Direct current is used in appliances and devices that use batteries as a power supply.
Alternating current is electric current that periodically changes direction. It is the type of current used by most power distribution systems and generated by most power plants. Alternating current holds the advantage over direct current that it can be generated for cheaper and it suffers less energy losses when transmitted over long distances.
The mains electricity from a power station in the UK is generated at a frequency of 50 Hz and is transmitted to households at a voltage of 230 V. The mains electricity is supplied through alternating current, which is created using magnets. Generators in power stations contain coils of wire, within which is a magnet. As a magnet spins, the coils of wire cut through its magnetic field and thus a current through them is induced. Since the magnet is spinning, its direction is periodically changing, therefore the current induced is alternating.
A mains electricity cable is made up of copper wires, since copper is a good electrical conductor, whereas the outside of the cable is made up of plastic, since it is a good insulator. The plug is made up of three wires: the brown live wire, the blue neutral wire and the green and yellow striped earth wire. The live wire has a potential difference of 230 V, provides the current and goes to the right. The earth wire is for safety purposes, so as to avoid the appliance becoming live, and goes at the top. The neutral wire is to complete the circuit and goes to the left. In the case that the live wire goes loose and comes into contact with the casing of the plug, the appliance could electrocute whoever touches it. Therefore, the earth wire is connected to the case and a metal plate or water pipe underground, providing a low resistance path for the current to go to the ground rather than the person touching the appliance. The cable grip is placed within cable so that the wires are tightly held in place. The fuse is used such that a wire melts if the current gets too high.
The National Grid provides households across the UK with electricity by connecting them to power stations. During the transmission of electricity, transformers are used to alter voltages and currents. A transformer is made up of coils of wires wrapped around a magnetic core. A step-up transformer is used to increase voltage and decrease current, since less current will mean that less energy will be lost during transmission through the heating of wires. A step-down transformer reduces the voltage to 230 V, which is the voltage required for using eletcricity in homes. The type of transformer is determined through the number of coils in it.
Describe the set-up of a transformer.
A transformer is made up of coils of wires wrapped around a magnetic core.
State the three wires that are found within a three-core cable.
The three wires are the live wire, the earth wire and the neutral wire.
Explain the direction of alternating current.
The direction of alternating current will periodically change.
What is the frequency of mains electricity from a power station in the UK?
The frequency of mains electricity from a power station in the UK is 50 Hz.
Explain how mains electricity is created in generators in power stations.
Generators in power stations contain coils of wire, within which is a magnet. As a magnet spins, the coils of wire cut through its magnetic field and thus a current through them is induced.
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