Direct and alternating potential difference

What you'll learn

What potential difference means and how it is linked to energy transfer.
The difference between direct potential difference and alternating potential difference.
What you need to remember about the UK mains supply.
How to describe AC and DC clearly in exam answers.

The starting point: potential difference

Electric circuits are all about moving charge. Charge is carried by particles such as electrons in metal wires.

A potential difference is what transfers energy to charges as they move around a circuit. You will often hear potential difference called voltage.

Definition

Potential difference

Potential difference, measured in volts (V), is the energy transferred per unit charge between two points in a circuit. A larger potential difference means more energy is transferred to each coulomb of charge.

The symbol for potential difference is usually $V$ . If energy $E$ is transferred to charge $Q$ , then:

\frac{E}{Q}

So 1 volt means 1 joule of energy transferred per coulomb of charge.

Example

Interpreting a voltage value

A 6 V battery transfers energy to 2 C of charge. How much energy is transferred?

Use the relationship between potential difference, energy and charge:
$\frac{E}{Q}$ , so $E = V Q$ .
Substitute the values:
$\text{ V} \times 2 \text{ C}$ .
Calculate the energy transferred:
$\text{ J}$ , so the battery transfers 12 joules of energy to the charge.

Direction matters: polarity

A supply, such as a cell or mains socket, has a polarity. This means one side is at a higher potential than the other at that moment.

In simple GCSE terms:

The positive terminal is at higher potential.
The negative terminal is at lower potential.
The potential difference between them can push charge around a complete circuit.

Definition

Polarity

Polarity describes which side of a supply is positive and which side is negative. If the polarity stays the same, the push on charge stays in the same direction. If the polarity reverses, the push reverses.

This idea is the key to understanding direct and alternating potential difference.

Direct potential difference

A direct potential difference has a constant direction. The polarity does not reverse.

The most common GCSE example is a cell or battery. One terminal stays positive and the other stays negative. If the battery is connected in a complete circuit, the current flows in one direction around the circuit.

Definition

Direct potential difference

A direct potential difference is a potential difference that acts in one direction only. It does not reverse polarity.

You may also see this linked to direct current, usually written as dc. Strictly, potential difference and current are different quantities, but they are linked:

A direct potential difference tends to produce a current in one direction.
A cell or battery supplies direct potential difference.
A graph of direct potential difference against time is usually shown as a horizontal line above or below zero.

Key Idea

Direct means no reversal

For GCSE, remember that direct means the potential difference keeps the same polarity, so the direction of the push on charge does not reverse.

Alternating potential difference

An alternating potential difference repeatedly changes direction. This means the polarity reverses again and again.

When the potential difference is positive, one terminal is at higher potential. When it becomes negative, the other terminal is at higher potential. If connected to a complete circuit, the current also repeatedly changes direction.

Definition

Alternating potential difference

An alternating potential difference is a potential difference that repeatedly reverses direction. It changes polarity over time.

A graph is one of the clearest ways to compare direct and alternating potential difference. Direct potential difference stays on one side of zero. Alternating potential difference crosses zero and changes sign.

Voltage-time graphs comparing direct potential difference and alternating potential difference

Example

Classifying a supply from its voltage-time behaviour

A supply has potential difference values that repeat like this: +12 V, +6 V, 0 V, -6 V, -12 V, -6 V, 0 V, +6 V. Is it direct or alternating?

Check whether the potential difference changes sign. Positive and negative values mean the polarity is changing.
The supply goes from positive values to negative values and then back again, so the direction of the potential difference reverses.
Because the polarity reverses repeatedly, this is an alternating potential difference.

Common Mistake

Do not just say “it changes”

For alternating potential difference, the important point is that it changes direction or reverses polarity. A direct potential difference could change size but still remain direct if it never reverses direction.

Frequency: how often AC repeats

Alternating potential difference repeats in a pattern. One complete repeat is called a cycle.

Definition

Frequency

Frequency is the number of complete cycles each second. It is measured in hertz (Hz), where 1 Hz means 1 cycle per second.

For example, a frequency of 50 Hz means 50 complete cycles every second.

A useful related idea is period. The period is the time taken for one complete cycle. You may see it on a potential difference-time graph.

\frac{1}{f}

where:

$T$ is the period in seconds (s)
$f$ is the frequency in hertz (Hz)

Example

Calculating the period of UK mains electricity

The UK mains supply has a frequency of 50 Hz. Calculate the time for one complete cycle.

Use the relationship between period and frequency:
$\frac{1}{f}$ .
Substitute $\text{ Hz}$ :
$\frac{1}{50}$ .
Calculate the period:
$\text{ s}$ , so one complete cycle takes 0.020 seconds.

Tip

Cycles and reversals

At 50 Hz, there are 50 complete cycles each second. Each cycle includes a positive half and a negative half, so the polarity reverses every half-cycle.

The UK domestic mains supply

Mains electricity is the electricity supplied to homes, schools and businesses through the National Grid and plug sockets.

For GCSE Physics, you need to know these facts:

Mains electricity is an alternating potential difference.
In the UK, the domestic mains supply has a frequency of 50 Hz.
In the UK, the domestic mains potential difference is about 230 V.

Key Idea

UK mains facts

The UK domestic mains supply is AC, has a frequency of 50 Hz, and has a potential difference of about 230 V.

The phrase “about 230 V” is important. In older resources you may see 240 V, but for the current GCSE specification you should use 230 V unless a question gives you a different value.

Also, 230 V is not saying the potential difference is fixed at 230 V at every instant. Because mains is alternating, the potential difference changes continuously. At GCSE, you do not need to calculate the peak value; just remember the quoted UK mains value is about 230 V.

Common Mistake

Mixing up the units

Do not write “230 Hz” or “50 V” for UK mains. The potential difference is about 230 V. The frequency is 50 Hz.

Comparing direct and alternating potential difference

Direct potential difference

Supplied by cells and batteries.
Polarity stays the same.
Current flows in one direction in a complete circuit.
A voltage-time graph stays on the same side of zero.

Alternating potential difference

Supplied by UK mains electricity.
Polarity reverses repeatedly.
Current changes direction repeatedly in a complete circuit.
A voltage-time graph crosses zero and changes sign.

Tip

Best exam wording

If asked to explain the difference, write: “A direct potential difference acts in one direction only, whereas an alternating potential difference repeatedly reverses direction.” That directly answers the physics.

Exam technique

In the exam

If the question asks for the difference between direct and alternating potential difference, focus on direction or polarity, not just examples like “battery” and “mains”.
Memorise the UK mains facts as a set: AC, 50 Hz, about 230 V.
On a graph, look for whether the potential difference crosses zero and changes sign. If it does, it is alternating; if it stays the same polarity, it is direct.

Self review

Check yourself

What does it mean for a potential difference to be direct?
How can you tell from a voltage-time graph that a supply is alternating?
What are the frequency and potential difference of the UK domestic mains supply?

Direct and alternating potential difference

What you'll learn

What potential difference means and how it is linked to energy transfer.
The difference between direct potential difference and alternating potential difference.
What you need to remember about the UK mains supply.
How to describe AC and DC clearly in exam answers.

The starting point: potential difference

Electric circuits are all about moving charge. Charge is carried by particles such as electrons in metal wires.

A potential difference is what transfers energy to charges as they move around a circuit. You will often hear potential difference called voltage.

Definition

Potential difference

The symbol for potential difference is usually $V$ . If energy $E$ is transferred to charge $Q$ , then:

\frac{E}{Q}

So 1 volt means 1 joule of energy transferred per coulomb of charge.

Example

Interpreting a voltage value

A 6 V battery transfers energy to 2 C of charge. How much energy is transferred?

Use the relationship between potential difference, energy and charge:
$\frac{E}{Q}$ , so $E = V Q$ .
Substitute the values:
$\text{ V} \times 2 \text{ C}$ .
Calculate the energy transferred:
$\text{ J}$ , so the battery transfers 12 joules of energy to the charge.

Direction matters: polarity

A supply, such as a cell or mains socket, has a polarity. This means one side is at a higher potential than the other at that moment.

In simple GCSE terms:

The positive terminal is at higher potential.
The negative terminal is at lower potential.
The potential difference between them can push charge around a complete circuit.

Definition

Polarity

This idea is the key to understanding direct and alternating potential difference.

Direct potential difference

A direct potential difference has a constant direction. The polarity does not reverse.

Definition

Direct potential difference

A direct potential difference is a potential difference that acts in one direction only. It does not reverse polarity.

You may also see this linked to direct current, usually written as dc. Strictly, potential difference and current are different quantities, but they are linked:

A direct potential difference tends to produce a current in one direction.
A cell or battery supplies direct potential difference.
A graph of direct potential difference against time is usually shown as a horizontal line above or below zero.

Key Idea

Direct means no reversal

For GCSE, remember that direct means the potential difference keeps the same polarity, so the direction of the push on charge does not reverse.

Alternating potential difference

An alternating potential difference repeatedly changes direction. This means the polarity reverses again and again.

Definition

Alternating potential difference

An alternating potential difference is a potential difference that repeatedly reverses direction. It changes polarity over time.

Voltage-time graphs comparing direct potential difference and alternating potential difference

Example

Classifying a supply from its voltage-time behaviour

A supply has potential difference values that repeat like this: +12 V, +6 V, 0 V, -6 V, -12 V, -6 V, 0 V, +6 V. Is it direct or alternating?

Check whether the potential difference changes sign. Positive and negative values mean the polarity is changing.
The supply goes from positive values to negative values and then back again, so the direction of the potential difference reverses.
Because the polarity reverses repeatedly, this is an alternating potential difference.

Common Mistake

Do not just say “it changes”

Frequency: how often AC repeats

Alternating potential difference repeats in a pattern. One complete repeat is called a cycle.

Definition

Frequency

Frequency is the number of complete cycles each second. It is measured in hertz (Hz), where 1 Hz means 1 cycle per second.

For example, a frequency of 50 Hz means 50 complete cycles every second.

A useful related idea is period. The period is the time taken for one complete cycle. You may see it on a potential difference-time graph.

\frac{1}{f}

where:

$T$ is the period in seconds (s)
$f$ is the frequency in hertz (Hz)

Example

Calculating the period of UK mains electricity

The UK mains supply has a frequency of 50 Hz. Calculate the time for one complete cycle.

Use the relationship between period and frequency:
$\frac{1}{f}$ .
Substitute $\text{ Hz}$ :
$\frac{1}{50}$ .
Calculate the period:
$\text{ s}$ , so one complete cycle takes 0.020 seconds.

Tip

Cycles and reversals

At 50 Hz, there are 50 complete cycles each second. Each cycle includes a positive half and a negative half, so the polarity reverses every half-cycle.

The UK domestic mains supply

Mains electricity is the electricity supplied to homes, schools and businesses through the National Grid and plug sockets.

For GCSE Physics, you need to know these facts:

Mains electricity is an alternating potential difference.
In the UK, the domestic mains supply has a frequency of 50 Hz.
In the UK, the domestic mains potential difference is about 230 V.

Key Idea

UK mains facts

The UK domestic mains supply is AC, has a frequency of 50 Hz, and has a potential difference of about 230 V.

The phrase “about 230 V” is important. In older resources you may see 240 V, but for the current GCSE specification you should use 230 V unless a question gives you a different value.

Common Mistake

Mixing up the units

Do not write “230 Hz” or “50 V” for UK mains. The potential difference is about 230 V. The frequency is 50 Hz.

Comparing direct and alternating potential difference

Direct potential difference

Supplied by cells and batteries.
Polarity stays the same.
Current flows in one direction in a complete circuit.
A voltage-time graph stays on the same side of zero.

Alternating potential difference

Supplied by UK mains electricity.
Polarity reverses repeatedly.
Current changes direction repeatedly in a complete circuit.
A voltage-time graph crosses zero and changes sign.

Tip

Best exam wording

Exam technique

In the exam

If the question asks for the difference between direct and alternating potential difference, focus on direction or polarity, not just examples like “battery” and “mains”.
Memorise the UK mains facts as a set: AC, 50 Hz, about 230 V.
On a graph, look for whether the potential difference crosses zero and changes sign. If it does, it is alternating; if it stays the same polarity, it is direct.

Self review

Check yourself

What does it mean for a potential difference to be direct?
How can you tell from a voltage-time graph that a supply is alternating?
What are the frequency and potential difference of the UK domestic mains supply?

Direct and alternating potential difference

What you'll learn

The starting point: potential difference

Direction matters: polarity

Direct potential difference

Alternating potential difference

Frequency: how often AC repeats

The UK domestic mains supply

Comparing direct and alternating potential difference

Direct potential difference

Alternating potential difference

You've reached the end

Direct and alternating potential difference

What you'll learn

The starting point: potential difference

Direction matters: polarity

Direct potential difference

Alternating potential difference

Frequency: how often AC repeats

The UK domestic mains supply

Comparing direct and alternating potential difference

Direct potential difference

Alternating potential difference

You've reached the end

Direct and alternating potential difference

What you'll learn

The starting point: potential difference

Direction matters: polarity

Direct potential difference

Alternating potential difference

Frequency: how often AC repeats

The UK domestic mains supply

Comparing direct and alternating potential difference

Direct potential difference

Alternating potential difference

You've reached the end

Direct and alternating potential difference Revision Guide

Direct and alternating potential difference

What you'll learn

The starting point: potential difference

Direction matters: polarity

Direct potential difference

Alternating potential difference

Frequency: how often AC repeats

The UK domestic mains supply

Comparing direct and alternating potential difference

Direct potential difference

Alternating potential difference

You've reached the end

Direct and alternating potential difference Revision Guide