The UK mains supply

Definition

Mains supply

The alternating electricity supply delivered to UK homes through the national grid at about $V230\ \text{V}$ and $Hz50\ \text{Hz}$ .

The electricity that reaches the sockets and lights of a UK home through the national grid is called the mains supply or domestic supply.
Three facts describe it, and you need to recall all three together.
The supply is alternating current, written a.c., which means the current repeatedly reverses its direction.
Its frequency is $Hz50\ \text{Hz}$ .
Its potential difference is about $V230\ \text{V}$ .
Every home in the UK receives a supply with these values, so an appliance bought anywhere in the country works in any socket.
Other countries use different values, for example the USA supplies about $V120\ \text{V}$ at $Hz60\ \text{Hz}$ , which is why some appliances bought abroad will not work properly in the UK.

Frequency of the supply

Definition

Frequency

The number of complete cycles of an alternating supply that occur each second, measured in hertz ( $Hz\text{Hz}$ ).

Definition

Period

The time taken for an alternating supply to complete one full cycle, measured in seconds.

Frequency is measured in hertz ( $Hz\text{Hz}$ ), where $Hz1\ \text{Hz}$ means one complete cycle each second.
A frequency of $Hz50\ \text{Hz}$ means the mains supply completes $50$ full cycles every second.
In each cycle the current flows one way and then the other way, so it reverses direction $100$ times every second.
Each cycle lasts $s\dfrac{1}{50}\ \text{s} = 0.02\ \text{s}$ , which is the period of the mains supply.
The reversals are far too fast for you to notice, so a lamp on the mains appears to glow steadily.
The company that runs the grid keeps the frequency very close to $Hz50\ \text{Hz}$ by matching the output of power stations to the demand at every moment.
If demand suddenly rises faster than generators can respond, the frequency dips very slightly below $Hz50\ \text{Hz}$ , which signals that more power stations must raise their output.

Potential difference of the supply

The potential difference of the mains supply is about $V230\ \text{V}$ , which is roughly $150$ times larger than the $V1.5\ \text{V}$ of an AA cell.
The word about matters, because the actual value at a socket varies a little during the day as demand changes.
UK rules allow the supply to range from about $V216\ \text{V}$ to $V253\ \text{V}$ , so $V230\ \text{V}$ is the nominal value around which the supply is kept.
Before 1995 the UK quoted its mains supply as $V240\ \text{V}$ , so older books and appliances sometimes give that figure.
A potential difference this large lets mains appliances transfer energy quickly, which is why kettles, ovens and heaters run from the mains rather than from batteries.
The same potential difference is also large enough to drive a dangerous current through a person, which is why mains wiring is always insulated.

Example

Using the mains values

A kettle rated at $W2300\ \text{W}$ is plugged into the $V230\ \text{V}$ mains supply.
Rearrange $P = V I$ to find the current it draws.

\dfrac{P}{V} = \dfrac{2300}{230} = 10\ \text{A}

The supply has a frequency of $Hz50\ \text{Hz}$ , so the current through the kettle reverses direction $100$ times every second.
Each complete cycle of the current lasts $s0.02\ \text{s}$ .

Common Mistake

Do not swap the two numbers, because $50$ is the frequency in hertz and $230$ is the potential difference in volts.
Do not write units such as " $230$ amps" or " $50$ volts", since amps measure current and volts measure potential difference.
Do not say the current changes direction $50$ times a second, because each of the $50$ cycles contains two reversals.
Do not describe the mains supply as d.c., because cells and batteries give direct current while the mains gives alternating current.

Exam technique

Recalling the domestic supply

When a question asks you to describe the UK domestic supply, give all three facts: a.c., $Hz50\ \text{Hz}$ and about $V230\ \text{V}$ .
Always attach the correct unit to each number, because a value without its unit usually earns no mark.
If you are asked what $Hz50\ \text{Hz}$ means, write " $50$ complete cycles per second" rather than "it changes $50$ times".

Self review

Is the UK domestic supply a.c. or d.c.?
State the frequency of the UK mains supply and its unit.
State the potential difference of the UK mains supply.
What does a frequency of $Hz50\ \text{Hz}$ mean?
How many times each second does the mains current reverse its direction?

The UK mains supply

Definition

Mains supply

The alternating electricity supply delivered to UK homes through the national grid at about $V230\ \text{V}$ and $Hz50\ \text{Hz}$ .

The electricity that reaches the sockets and lights of a UK home through the national grid is called the mains supply or domestic supply.
Three facts describe it, and you need to recall all three together.
The supply is alternating current, written a.c., which means the current repeatedly reverses its direction.
Its frequency is $Hz50\ \text{Hz}$ .
Its potential difference is about $V230\ \text{V}$ .
Every home in the UK receives a supply with these values, so an appliance bought anywhere in the country works in any socket.
Other countries use different values, for example the USA supplies about $V120\ \text{V}$ at $Hz60\ \text{Hz}$ , which is why some appliances bought abroad will not work properly in the UK.

Frequency of the supply

Definition

Frequency

The number of complete cycles of an alternating supply that occur each second, measured in hertz ( $Hz\text{Hz}$ ).

Definition

Period

The time taken for an alternating supply to complete one full cycle, measured in seconds.

Frequency is measured in hertz ( $Hz\text{Hz}$ ), where $Hz1\ \text{Hz}$ means one complete cycle each second.
A frequency of $Hz50\ \text{Hz}$ means the mains supply completes $50$ full cycles every second.
In each cycle the current flows one way and then the other way, so it reverses direction $100$ times every second.
Each cycle lasts $s\dfrac{1}{50}\ \text{s} = 0.02\ \text{s}$ , which is the period of the mains supply.
The reversals are far too fast for you to notice, so a lamp on the mains appears to glow steadily.
The company that runs the grid keeps the frequency very close to $Hz50\ \text{Hz}$ by matching the output of power stations to the demand at every moment.
If demand suddenly rises faster than generators can respond, the frequency dips very slightly below $Hz50\ \text{Hz}$ , which signals that more power stations must raise their output.

Potential difference of the supply

The potential difference of the mains supply is about $V230\ \text{V}$ , which is roughly $150$ times larger than the $V1.5\ \text{V}$ of an AA cell.
The word about matters, because the actual value at a socket varies a little during the day as demand changes.
UK rules allow the supply to range from about $V216\ \text{V}$ to $V253\ \text{V}$ , so $V230\ \text{V}$ is the nominal value around which the supply is kept.
Before 1995 the UK quoted its mains supply as $V240\ \text{V}$ , so older books and appliances sometimes give that figure.
A potential difference this large lets mains appliances transfer energy quickly, which is why kettles, ovens and heaters run from the mains rather than from batteries.
The same potential difference is also large enough to drive a dangerous current through a person, which is why mains wiring is always insulated.

Example

Using the mains values

A kettle rated at $W2300\ \text{W}$ is plugged into the $V230\ \text{V}$ mains supply.
Rearrange $P = V I$ to find the current it draws.

\dfrac{P}{V} = \dfrac{2300}{230} = 10\ \text{A}

The supply has a frequency of $Hz50\ \text{Hz}$ , so the current through the kettle reverses direction $100$ times every second.
Each complete cycle of the current lasts $s0.02\ \text{s}$ .

Common Mistake

Do not swap the two numbers, because $50$ is the frequency in hertz and $230$ is the potential difference in volts.
Do not write units such as " $230$ amps" or " $50$ volts", since amps measure current and volts measure potential difference.
Do not say the current changes direction $50$ times a second, because each of the $50$ cycles contains two reversals.
Do not describe the mains supply as d.c., because cells and batteries give direct current while the mains gives alternating current.

Exam technique

Recalling the domestic supply

When a question asks you to describe the UK domestic supply, give all three facts: a.c., $Hz50\ \text{Hz}$ and about $V230\ \text{V}$ .
Always attach the correct unit to each number, because a value without its unit usually earns no mark.
If you are asked what $Hz50\ \text{Hz}$ means, write " $50$ complete cycles per second" rather than "it changes $50$ times".

Self review

Is the UK domestic supply a.c. or d.c.?
State the frequency of the UK mains supply and its unit.
State the potential difference of the UK mains supply.
What does a frequency of $Hz50\ \text{Hz}$ mean?
How many times each second does the mains current reverse its direction?

8.2.7 The UK domestic supply

The UK mains supply

Frequency of the supply

Potential difference of the supply

Using the mains values

Recalling the domestic supply

Review 8.2.7 The UK domestic supply by teaching Genie

8.2.7 The UK domestic supply

The UK mains supply

Frequency of the supply

Potential difference of the supply

Using the mains values

Recalling the domestic supply

Review 8.2.7 The UK domestic supply by teaching Genie

8.2.7 The UK domestic supply

The UK mains supply

Frequency of the supply

Potential difference of the supply

Using the mains values

Recalling the domestic supply

Review 8.2.7 The UK domestic supply by teaching Genie

8.2.7 The UK domestic supply Revision Guide

8.2.7 The UK domestic supply

The UK mains supply

Frequency of the supply

Potential difference of the supply

Using the mains values

Recalling the domestic supply

Review 8.2.7 The UK domestic supply by teaching Genie

8.2.7 The UK domestic supply Revision Guide