Mains Electricity (Edexcel GCSE Combined Science: Physics): Flashcards
Exam code: 1SC0
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- FrontAC & DC
Define direct current (d.c.).
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Define direct current (d.c.).
A direct current is a current that is steady, constantly flowing in the same direction in a circuit, from positive to negative.
Define alternating current (a.c.).
An alternating current is a current that continuously changes its direction, going back and forth around a circuit.
What type of terminals does a d.c. power supply have?
A d.c. power supply has a fixed positive terminal and a fixed negative terminal.
Give an example of a device that produces direct current.
Electric cells, or batteries, produce direct current.
What is meant by the frequency of an alternating current?
The frequency is the number of times the current changes direction back and forth each second.
In the UK, mains electricity is an alternating current with a frequency of ______ and a potential difference of around ______.
In the UK, mains electricity is an alternating current with a frequency of 50 Hz and a potential difference of around 230 V.
True or False?
On an oscilloscope trace, a direct current is shown as a wave that oscillates above and below the time axis.
False.
A direct current is shown as a flat line; it is the alternating current that oscillates above and below the time axis.
State the frequency and potential difference of the UK domestic mains electricity supply.
Frequency = 50 Hz
Potential difference ~ 230 V
What are the two equivalents of positive and negative in a mains a.c. supply?
Live and neutral, which form either end of the electrical circuit.
Define the role of the live wire.
The live wire carries the alternating potential difference from the supply to a circuit.
Define the role of the neutral wire.
The neutral wire forms the opposite end of the circuit to the live wire, to complete the circuit.
The three-core cable insulation is colour coded: live is ______, neutral is ______ and earth is ______.
The three-core cable insulation is colour coded: live is brown, neutral is blue and earth is green and yellow stripes.
What is the role of the earth wire?
The earth wire acts as a safety wire to stop the appliance from becoming live.
Explain how a fuse protects an appliance if the current becomes too large.
The thin metal wire in the fuse heats up and melts, breaking the circuit and stopping the current.
True or False?
A circuit breaker melts and must be replaced after it has cut off the current, just like a fuse.
False.
A circuit breaker is an automatic electromagnetic switch that does not melt, so it can be reset and used again, and it works faster than a fuse.
Why are switches and fuses always connected to the live wire?
The live wire is the most dangerous wire (around 230 V), so connecting switches and fuses to it means a device is no longer connected to the live supply when a switch is off or a fuse blows.
The live wire has a potential difference of around ______, the neutral wire is close to ______, and the earth wire is ______.
The live wire has a potential difference of around 230 V, the neutral wire is close to 0 V, and the earth wire is 0 V.
Why does the neutral wire still pose a small shock risk, despite being much safer than the live wire?
The neutral wire has a potential difference close to, but not exactly, 0 V, so it can still deliver a small shock.
Explain why a very large current is created if a person connects the live wire directly to earth.
There is a large potential difference between live (230 V) and earth (0 V), so connecting them creates a very large current, which can give a lethal shock.
Why do electricians switch off the mains supply before working on appliances?
Working on live wires would create a large potential difference between the wire (230 V) and the electrician (0 V), so a current would pass through their body to reach earth.
True or False?
Switching a device off makes it completely safe to touch its internal wiring, even if the mains supply is still on.
False.
Even if a device is switched off, the live wire can still cause an electric shock if the mains supply is still on.
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