How does current flow in a circuit, what is resistance, and how is mains electricity made safe?
Charge and current, potential difference, resistance and Ohm's law, series and parallel circuits, alternating and direct current, the three wires of a mains plug with fuses and earthing, and electrical power.
A focused CCEA GCSE Single Award Science answer on electricity, covering charge and current, potential difference, resistance and Ohm's law, series and parallel circuits, alternating and direct current, the mains plug with fuses and earthing, and electrical power.
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What this dot point is asking
CCEA wants you to define charge and current, explain potential difference, use resistance and Ohm's law, compare series and parallel circuits, distinguish AC and DC, describe the three wires with fuses and earthing, and calculate electrical power.
Charge, current and voltage
Resistance and Ohm's law
Series and parallel circuits
Mains electricity and safety
A mains cable has three wires: live (brown), which carries the dangerous voltage; neutral (blue), which completes the circuit; and earth (green and yellow), a safety wire to the metal case. A fuse melts and breaks the circuit if the current gets too high; it should be rated just above the appliance's normal current. Double-insulated appliances have a plastic case and no earth wire, because the case can never become live.
Electrical power
Examples in context
Example 1. Why house lights are wired in parallel. If lights were in series, switching off or removing one bulb would break the whole circuit and all the lights would go out. Wiring them in parallel gives each light the full mains voltage and its own switch, so each can be controlled on its own and one failing bulb does not affect the rest. This is a direct, practical use of the difference between series and parallel.
Example 2. Choosing the right fuse. A fuse should be rated just above the appliance's normal working current. A kettle drawing 8 A needs a 13 A fuse, not a 3 A one, which would blow in normal use, nor a much larger one, which would not blow soon enough in a fault. Matching the fuse to the current is a common CCEA safety question that tests the idea of the fuse blowing only on a fault.
Try this
Q1. State Ohm's law as an equation. [1 mark]
- Cue. (voltage equals current times resistance).
Q2. Name the three wires in a mains cable. [1 mark]
- Cue. Live, neutral and earth.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20213 marksA 12 V supply is connected across a 48 ohm resistor. Calculate the current using Ohm's law.Show worked answer →
Three marks for the formula, the rearrangement and the answer.
Ohm's law is voltage equals current times resistance, V equals IR.
Rearrange to find the current: I equals V divided by R.
I equals 12 divided by 48, which equals 0.25 A.
So the current is 0.25 A. Markers reward the correct formula, the rearrangement and the answer with the unit amps.
CCEA SAS 20194 marksExplain the job of the earth wire and the fuse, and how they work together to make an appliance safe.Show worked answer →
Four marks for the two jobs and how they act together.
The earth wire is a safety wire connected to the metal case, normally carrying no current.
The fuse is a thin wire that melts and breaks the circuit if the current gets too high.
If the live wire touches the metal case, a large current flows through the earth wire to earth.
This large current blows (melts) the fuse, disconnecting the supply, so the case cannot stay live and give a shock. Markers reward the earth providing a low-resistance path, the large fault current, and the fuse blowing to cut off the supply.
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)