AC and DC, Mains Electricity

This lesson covers the difference between alternating current (AC) and direct current (DC) and the key features of the UK mains electricity supply, as required by the Edexcel GCSE Combined Science specification (1SC0).

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Direct Current (DC)

Direct current is an electric current that flows in one direction only.

Feature	Detail
Direction	Always the same (one direction)
Provided by	Cells, batteries, solar cells, DC power supplies
Polarity	Fixed positive and negative terminals

DC on an Oscilloscope

On an oscilloscope (or voltage-time graph), a steady DC supply appears as a horizontal straight line above or below the zero line.

graph LR
    subgraph "DC Trace"
        A["Time -->"] --- B["Constant voltage line"]
    end

The height of the line above the zero line shows the voltage. The line is flat because the voltage does not change with time.

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Alternating Current (AC)

Alternating current is an electric current that repeatedly reverses direction.

Feature	Detail
Direction	Continuously changes — first one way, then the other
Provided by	Mains electricity, generators
Waveform	A smooth sine wave on an oscilloscope

AC on an Oscilloscope

An AC supply produces a sine wave on a voltage-time graph. The wave oscillates equally above and below the zero line.

Key features of the AC wave:

Feature	What It Shows
Peak voltage	The maximum height of the wave above (or below) the zero line
Time period (T)	The time for one complete cycle (one full wave)
Frequency (f)	The number of complete cycles per second, measured in hertz (Hz)

The relationship between frequency and time period:

$f = \frac{1}{T}$f=T1​

Worked Example 1

An AC supply has a time period of 0.02 s. What is the frequency?

$f = \frac{1}{T} = \frac{1}{0.02} = 50 \text{ Hz}$f=T1​=0.021​=50 Hz

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UK Mains Electricity

The UK domestic electricity supply is an AC supply with these specifications:

Property	Value
Voltage	approximately 230 V
Frequency	50 Hz
Type	Alternating current (AC)

Key Points

• The mains supply is 230 V AC at 50 Hz. This means the voltage alternates 50 times per second.
• The mains voltage is dangerous — it is high enough to cause serious injury or death.
• Cells and batteries typically provide much lower voltages (1.5 V to 12 V) and are DC.

Exam Tip: You must remember these values: 230 V and 50 Hz for the UK mains supply. These are commonly tested.

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Comparing AC and DC

Property	DC	AC
Current direction	One direction only	Reverses repeatedly
Oscilloscope trace	Flat line	Sine wave
Examples	Batteries, cells	Mains supply, generators
Voltage	Constant	Continuously changing
UK mains	No	Yes (230 V, 50 Hz)

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Reading an Oscilloscope Trace

An oscilloscope displays voltage on the y-axis and time on the x-axis. The settings tell you the scale:

Setting	What It Controls
Volts per division (V/div)	The voltage represented by each vertical square
Time base (s/div or ms/div)	The time represented by each horizontal square

Worked Example 2

An AC trace on an oscilloscope reaches 3 divisions above the zero line. The setting is 2 V/div. What is the peak voltage?

$V_{\text{peak}} = 3 \times 2 = 6 \text{ V}$Vpeak​=3×2=6 V

Worked Example 3

The same trace completes one full cycle in 4 divisions. The time base is set to 5 ms/div. What is the frequency?

$T = 4 \times 5 = 20 \text{ ms} = 0.020 \text{ s}$T=4×5=20 ms=0.020 s

$f = \frac{1}{T} = \frac{1}{0.020} = 50 \text{ Hz}$f=T1​=0.0201​=50 Hz

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Why Does the UK Use AC?

• AC can be easily stepped up to very high voltages using a transformer for efficient long-distance transmission through the National Grid.
• At high voltage, the current is lower, which reduces energy lost as heat in the cables ($P = I^2R$P=I2R).
• AC can then be stepped down to safer voltages (230 V) for use in homes.

Exam Tip: If asked why AC is used for the mains supply, link it to the ability to change voltage using transformers for efficient transmission in the National Grid.

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Safety Considerations

Hazard	Detail
Electric shock	230 V can cause serious injury or death; current through the body can stop the heart
Burns	High current can cause severe burns
Fires	Overheating of cables can start fires

These hazards are why mains-powered appliances have safety features such as fuses, earth wires and circuit breakers (covered in the next lesson).

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Live, Neutral and Earth Wires

The UK mains supply uses a three-wire system:

Wire	Colour	Function	Voltage
Live	Brown	Carries the alternating voltage from the supply	Alternates between +325 V and −325 V (230 V rms)
Neutral	Blue	Completes the circuit and carries current back	Approximately 0 V
Earth	Green and yellow stripes	Safety wire — carries current to the ground if a fault occurs	0 V (connected to the ground)

Key Safety Points

• The live wire is always dangerous, even when a switch is off (if the switch is on the neutral wire).
• A switch should always be on the live wire so that the appliance is disconnected from the high voltage when the switch is open.
• The neutral wire is at approximately 0 V but still carries current during normal operation.

Exam Tip: The live wire is the dangerous one. Even when a device is switched off, the live wire may still be at 230 V if the switch is on the neutral side. This is a common exam question.

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Summary

• DC flows in one direction; AC reverses direction repeatedly.
• The UK mains supply is 230 V AC at 50 Hz.
• On an oscilloscope, DC gives a flat line; AC gives a sine wave.
• Frequency and time period are related by $f = \frac{1}{T}$f=T1​.
• AC is used for mains electricity because it can be transformed to different voltages for efficient transmission.
• The mains uses three wires: live (brown, dangerous), neutral (blue, ~0 V) and earth (green/yellow, safety).
• The live wire is always potentially dangerous — switches must be placed on the live wire.

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Extended Worked Examples

Worked Example 4 — Reading a Mains Trace