5.1 Alternating Current

A free GCSE Physics lesson for AQA (8463) from OBA School: direct and alternating potential difference, the UK mains supply, oscilloscope traces and the National Grid. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the difference between direct current (d.c.) and alternating current (a.c.)
• State the frequency and potential difference of the UK mains supply and describe the live, neutral and earth wires
• Use an oscilloscope trace to find the peak potential difference, the time period and the frequency of a supply
• Describe how the National Grid uses step-up and step-down transformers and explain why it is an efficient system

Key words

• Direct current (d.c.): A current that always flows in the same direction, for example from a cell or battery.
• Alternating current (a.c.): A current that keeps reversing its direction. It is produced by an alternating potential difference.
• Frequency: The number of complete cycles every second, measured in hertz (Hz).
• Time period: The time taken for one complete cycle, in seconds: T = 1 / f.
• Peak pd: The largest potential difference reached, measured from zero (the centre line) to the top of a wave.
• Live wire: The wire whose potential alternates between large positive and negative values. It is the dangerous wire.
• Neutral wire: The wire that completes the circuit. It stays at or close to 0 V because it is earthed at the substation.
• National Grid: The nationwide network of transformers and cables that links power stations to homes and businesses.

Direct and alternating current

A cell or battery pushes charge round a circuit in one direction only. Its potential difference (pd) has a fixed polarity, so the current is direct current (d.c.).

The mains supply is different. Its pd keeps reversing direction, many times every second, so the current through a mains appliance flows one way, then the other way. This is alternating current (a.c.). For a heater or a filament lamp the direction does not matter: it heats up just as well.

Key fact: d.c. = one direction only (cells, batteries, solar cells). a.c. = repeatedly reverses direction (mains supply, generators).

The UK mains supply

In the UK the mains supply is a.c. with a frequency of 50 Hz and a pd of about 230 V. The 230 V is the steady d.c. value that would give the same heating effect. Because the pd alternates, the live wire actually swings to a peak of about +325 V and then to about −325 V in every cycle.

A mains cable usually contains three wires:

• Live; Potential alternates, peak about ±325 V (230 V supply); Job carries the alternating pd from the supply
• Neutral; Potential at or close to 0 V; Job completes the circuit back to the supply; earthed at the local substation
• Earth; Potential 0 V; Job safety wire: only carries a current if there is a fault

The time for one cycle is called the time period, T. It is linked to the frequency f by:

time period, T (s) = 1 ÷ frequency, f (Hz) so f = 1 ÷ T

For the UK mains: T = 1 ÷ 50 = 0.02 s (20 ms). Remember 1 ms = 0.001 s.

Worked example: time period of a different supply

Some countries use a mains frequency of 60 Hz. Calculate the time for one cycle in milliseconds.
T = 1 ÷ f = 1 ÷ 60 = 0.0167 s
0.0167 s × 1000 = 16.7 ms

Answer: T = 16.7 ms (3 s.f.)

Using an oscilloscope

An oscilloscope draws a graph of pd against time on a screen marked in squares called divisions. Two controls set the scales:

• Y-gain = volts per division on the vertical axis. Turning it up makes the trace taller.
• Time base = time per division on the horizontal axis (often ms per division). Changing it changes how many cycles fit across the screen.

Worked example: reading the trace above

Peak pd = peak height in divisions × Y-gain = 3 div × 2 V/div = 6 V
Time period = divisions for one cycle × time base = 4 div × 2 ms/div = 8 ms = 0.008 s
Frequency f = 1 ÷ T = 1 ÷ 0.008 s = 125 Hz

Answer: peak pd 6 V, frequency 125 Hz

• If the frequency of the supply goes up, each cycle takes less time, so more cycles fit on the screen (the waves look squashed together). The height does not change.
• If the peak pd goes up, the waves get taller. The number of cycles does not change.
• A battery gives a flat horizontal line above the centre line. Reversing the battery moves the line the same distance below the centre line.

Exam tip: Measure the peak from the centre line, not from the bottom of the trough. Measure several cycles and divide by the number of cycles to get a more accurate time period.

The National Grid

Electricity reaches homes from power stations through the National Grid, a network of cables and transformers. A transformer changes the size of an alternating pd.

• Generators in a power station produce an alternating pd of about 25 kV.
• Step-up transformers at the power station increase the pd to hundreds of thousands of volts (for example 275 kV or 400 kV) for the transmission cables.
• Step-down transformers near towns reduce the pd in stages to safer levels: 230 V for homes, higher values such as 11 kV or 33 kV for large factories.

Why use such a high pd? For the same power, P = V I, so a higher pd means a smaller current. The cables have resistance, and the power wasted heating them is P = I² R. A smaller current means much less energy is wasted, so the grid is an efficient way to transfer energy.

Worked example: current in the cables

A power station sends out 10 MW (10 000 000 W).
At 25 kV: I = P ÷ V = 10 000 000 ÷ 25 000 = 400 A
At 400 kV: I = 10 000 000 ÷ 400 000 = 25 A
The current is 16 times smaller, so the heating loss (∝ I²) is 16² = 256 times smaller.

Answer: stepping up the pd cuts the current from 400 A to 25 A

Common mistakes

• Saying the live wire is at a steady 230 V. Its pd alternates and reaches a peak of about 325 V in each direction.
• Measuring the peak pd from the bottom of a trough to the top of a crest. That is twice the peak value.
• Using a time period in milliseconds in f = 1 ÷ T without changing it to seconds first.
• Saying a step-up transformer increases the power or the current. It increases the pd and decreases the current.
• Calling the neutral wire the safety wire. The earth wire is the safety wire; the neutral wire completes the circuit.

Revision questions

Try these, then check your answers below each one.

Question 1. What is the frequency of the UK mains electricity supply? (Choose from: 50 Hz, 60 Hz, 230 Hz, 325 Hz.)

Answer: 50 Hz

Working: The UK mains supply is a.c. at 50 Hz (50 cycles per second) with a pd of about 230 V.

Question 2. True or false? A battery produces an alternating current.

Answer: False

Working: False. A battery gives a direct current: the pd has a fixed polarity, so the current flows in one direction only.

Question 3. Calculate the time period of the UK mains supply (50 Hz).

Answer: 0.02 s

Working: T = 1 ÷ f = 1 ÷ 50 = 0.02 s (20 ms).

Related lessons

• Previous lesson: 4.9 Required Practical: Resistors in Series and Parallel
• Next lesson: 5.2 Cables and Plugs
• In this chapter: 5.3 Electrical Power and Potential Difference
• In this chapter: 5.4 Electrical Currents and Energy Transfer
• In this chapter: 5.5 Appliances and Efficiency

Continue in the full course

This lesson is section 5.1 (AGCPH0501) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.