Charge, current and potential difference

What you'll learn

How the structure of the atom links to electric charge.
How to draw and interpret simple circuit diagrams using GCSE symbols.
What current and potential difference mean, and how to calculate with $\times t$ and $\times V$ .
How resistance affects current, including series and parallel circuits.

1. Atoms and electric charge

Everything in a circuit is made from atoms. An atom has a tiny central nucleus, with electrons around the outside.

Diagram of atom structure showing protons, neutrons and electrons

Definition

Subatomic particles

An atom contains three main particles:

Protons: in the nucleus, positive charge, relative mass 1.
Neutrons: in the nucleus, no charge, relative mass 1.
Electrons: outside the nucleus, negative charge, very small relative mass.

Electricity happens because charge can move. In metal wires, the moving charged particles are electrons.

Key Idea

Charge in metals

In a metal circuit, current is caused by electrons moving through the metal. Electrons are negatively charged.

A neutral atom has equal numbers of protons and electrons, so the positive and negative charges balance overall.

2. Circuit diagrams and symbols

A circuit is a path that allows electric charge to flow. In GCSE Physics, we draw circuits using standard symbols, not pictures of real components.

A cell is a source of potential difference; the long line is the positive terminal and the short line is the negative terminal. A battery is two or more cells joined together.

GCSE circuit symbols for common components

Definition

Component

A component is any part of a circuit, such as a lamp, resistor, motor, diode, ammeter or voltmeter.

Important components include:

A switch, which opens or closes the circuit.
A resistor, which makes it harder for current to flow.
A variable resistor, whose resistance can be changed.
A lamp, which transfers electrical energy to light and thermal energy.
A motor, which transfers electrical energy to kinetic energy.
A diode, which allows current to flow easily in one direction only.
An LED, which is a light-emitting diode.
A thermistor, whose resistance changes with temperature.
An LDR, or light-dependent resistor, whose resistance changes with light intensity.

3. Closed circuits and current

For a current to flow, the circuit must be closed and must include a source of potential difference, such as a cell or battery.

Definition

Closed circuit

A closed circuit is a complete conducting loop with no gaps. If a switch is open, the circuit is broken and there is no current.

A source of potential difference gives charges energy. If there is a complete path, the charges move around the circuit.

Common Mistake

Open switch means no current

An open switch is a gap in the circuit. Even if a battery is connected, charge cannot keep flowing around the circuit, so the current is zero.

4. Current: the rate of flow of charge

Electric current tells you how quickly charge is flowing.

Definition

Current

Current is the rate of flow of charge. It is measured in amperes, often shortened to amps (A).

The equation is:

\times t

where:

$Q$ is charge in coulombs (C)
$I$ is current in amps (A)
$t$ is time in seconds (s)

You need to recall and use this equation.

Example

Calculating charge from current and time

A lamp has a current of 0.40 A through it for 30 s. Calculate the charge that flows.

Choose the correct equation because the question gives current and time: $\times t$ .
Substitute the values with units: $0.40\ \text{A} \times 30\ \text{s}$ .
Calculate the charge: $12\ \text{C}$ .

Tip

What one amp means

A current of 1 A means 1 coulomb of charge passes a point every second.

5. Potential difference: energy per charge

Potential difference is often called voltage. It tells you how much energy is transferred by each coulomb of charge.

Definition

Potential difference

Potential difference is the energy transferred per unit charge passed. It is measured in volts (V).

One volt means one joule per coulomb:

J/C1\ \text{V} = 1\ \text{J/C}

The equation is:

\times V

where:

$E$ is energy transferred in joules (J)
$Q$ is charge in coulombs (C)
$V$ is potential difference in volts (V)

You need to recall and use this equation.

Example

Calculating energy transferred by charge

A charge of 5.0 C passes through a motor with a potential difference of 12 V across it. Calculate the energy transferred.

Use the energy equation because the question gives charge and potential difference: $\times V$ .
Substitute the values: $5.0\ \text{C} \times 12\ \text{V}$ .
Calculate the energy transferred: $60\ \text{J}$ .

6. Ammeters and voltmeters

An ammeter measures current. It must be connected in series with the component, so the same current passes through the ammeter and the component.

A voltmeter measures potential difference. It must be connected in parallel across the component, so it compares the energy difference between the two ends of that component.

Series and parallel circuit diagrams showing ammeters and voltmeters

Example

Choosing where to place meters

You want to measure the current through a resistor and the potential difference across it.

For current, the ammeter must be part of the same path as the resistor, so place the ammeter in series with the resistor.
For potential difference, the voltmeter must compare the two sides of the resistor, so connect it in parallel across the resistor.
Check the circuit still has a complete loop through the cell, ammeter and resistor; the voltmeter is on a separate parallel connection across the resistor.

Common Mistake

Mixing up meters

Ammeters go in series. Voltmeters go in parallel. If you swap them, the circuit reading will be wrong and the circuit may not work as intended.

7. Series and parallel circuits

A series circuit has only one path for current. All components are on the same loop.

A parallel circuit has more than one path for current. Each separate path is called a branch.

Series circuits

In a series circuit:

The current is the same through every component.
The potential difference from the source is shared between components.
If one component breaks or a switch opens, the whole circuit stops.

Parallel circuits

In a parallel circuit:

The current splits between branches.
The current rejoins after the branches.
Components in different branches can still work if one branch is broken.
Each branch has the same potential difference as the supply, if connected directly across it.

Key Idea

Current at a junction

Current is conserved at a junction. The total current going into a junction equals the total current coming out.

Example

Using current conservation at a junction

A current of 3.0 A reaches a junction. One branch has a current of 1.2 A. Find the current in the other branch.

Apply conservation of current: total current into the junction equals total current out.
Set up the relationship: $A+I23.0\ \text{A} = 1.2\ \text{A} + I_2$ .
Rearrange and calculate: $AI_2 = 3.0\ \text{A} - 1.2\ \text{A} = 1.8\ \text{A}$ .

8. Resistance and current

Resistance is how much a component opposes the flow of current. It is measured in ohms (Ω).

Definition

Resistance

Resistance is the opposition to current in a circuit. A higher resistance makes it harder for charge to flow.

The equation linking potential difference, current and resistance is:

\times R

where:

$V$ is potential difference in volts (V)
$I$ is current in amps (A)
$R$ is resistance in ohms (Ω)

You need to recall and use this equation.

For a fixed potential difference, increasing the resistance decreases the current. Decreasing the resistance increases the current.

Example

Calculating current from potential difference and resistance

A resistor has a potential difference of 6.0 V across it and a resistance of 12 Ω. Calculate the current.

Rearrange $\times R$ to make current the subject: $\frac{V}{R}$ .
Substitute the values: $\frac{6.0\ \text{V}}{12\ \Omega}$ .
Calculate the current: $0.50\ \text{A}$ .

A variable resistor lets you change the resistance in a circuit. This changes the current. For example, increasing the resistance of a variable resistor reduces the current through a lamp, making the lamp dimmer.

Tip

Sanity check for resistance questions

If the same battery is used, a bigger resistance should give a smaller current. If your answer does the opposite, check your rearrangement.

9. Resistance in series and parallel

When resistors are in series, the current has to pass through each resistor one after another. Each resistor makes it harder for current to flow, so the total resistance increases.

When resistors are in parallel, there is more than one route for charge to take. This makes it easier for current to flow overall, so the total resistance decreases.

Key Idea

Resistors together

In series, adding another resistor increases the total resistance.
In parallel, adding another resistor decreases the total resistance because it adds another path for current.

Common Mistake

Thinking more resistors always means more resistance

More resistors only definitely means more total resistance when they are in series. In parallel, adding another branch can reduce the total resistance.

Exam technique

In the exam

Before using an equation, identify what the question is asking for: charge, current, time, energy, potential difference or resistance.
Draw or trace the circuit path: ammeters go in series, voltmeters go in parallel, and current is conserved at junctions.
Check your units carefully: time in seconds, charge in coulombs, current in amps, potential difference in volts, resistance in ohms, energy in joules.

Self review

Check yourself

Why must an ammeter be connected in series with the component it measures?
A current of 2.0 A flows for 15 s. What equation would you use to find the charge moved?
In a parallel circuit, what happens to the current at a junction?

Charge, current and potential difference

What you'll learn

How the structure of the atom links to electric charge.
How to draw and interpret simple circuit diagrams using GCSE symbols.
What current and potential difference mean, and how to calculate with $\times t$ and $\times V$ .
How resistance affects current, including series and parallel circuits.

1. Atoms and electric charge

Everything in a circuit is made from atoms. An atom has a tiny central nucleus, with electrons around the outside.

Diagram of atom structure showing protons, neutrons and electrons

Definition

Subatomic particles

An atom contains three main particles:

Protons: in the nucleus, positive charge, relative mass 1.
Neutrons: in the nucleus, no charge, relative mass 1.
Electrons: outside the nucleus, negative charge, very small relative mass.

Electricity happens because charge can move. In metal wires, the moving charged particles are electrons.

Key Idea

Charge in metals

In a metal circuit, current is caused by electrons moving through the metal. Electrons are negatively charged.

A neutral atom has equal numbers of protons and electrons, so the positive and negative charges balance overall.

2. Circuit diagrams and symbols

A circuit is a path that allows electric charge to flow. In GCSE Physics, we draw circuits using standard symbols, not pictures of real components.

A cell is a source of potential difference; the long line is the positive terminal and the short line is the negative terminal. A battery is two or more cells joined together.

GCSE circuit symbols for common components

Definition

Component

A component is any part of a circuit, such as a lamp, resistor, motor, diode, ammeter or voltmeter.

Important components include:

A switch, which opens or closes the circuit.
A resistor, which makes it harder for current to flow.
A variable resistor, whose resistance can be changed.
A lamp, which transfers electrical energy to light and thermal energy.
A motor, which transfers electrical energy to kinetic energy.
A diode, which allows current to flow easily in one direction only.
An LED, which is a light-emitting diode.
A thermistor, whose resistance changes with temperature.
An LDR, or light-dependent resistor, whose resistance changes with light intensity.

3. Closed circuits and current

For a current to flow, the circuit must be closed and must include a source of potential difference, such as a cell or battery.

Definition

Closed circuit

A closed circuit is a complete conducting loop with no gaps. If a switch is open, the circuit is broken and there is no current.

A source of potential difference gives charges energy. If there is a complete path, the charges move around the circuit.

Common Mistake

Open switch means no current

An open switch is a gap in the circuit. Even if a battery is connected, charge cannot keep flowing around the circuit, so the current is zero.

4. Current: the rate of flow of charge

Electric current tells you how quickly charge is flowing.

Definition

Current

Current is the rate of flow of charge. It is measured in amperes, often shortened to amps (A).

The equation is:

\times t

where:

$Q$ is charge in coulombs (C)
$I$ is current in amps (A)
$t$ is time in seconds (s)

You need to recall and use this equation.

Example

Calculating charge from current and time

A lamp has a current of 0.40 A through it for 30 s. Calculate the charge that flows.

Choose the correct equation because the question gives current and time: $\times t$ .
Substitute the values with units: $0.40\ \text{A} \times 30\ \text{s}$ .
Calculate the charge: $12\ \text{C}$ .

Tip

What one amp means

A current of 1 A means 1 coulomb of charge passes a point every second.

5. Potential difference: energy per charge

Potential difference is often called voltage. It tells you how much energy is transferred by each coulomb of charge.

Definition

Potential difference

Potential difference is the energy transferred per unit charge passed. It is measured in volts (V).

One volt means one joule per coulomb:

J/C1\ \text{V} = 1\ \text{J/C}

The equation is:

\times V

where:

$E$ is energy transferred in joules (J)
$Q$ is charge in coulombs (C)
$V$ is potential difference in volts (V)

You need to recall and use this equation.

Example

Calculating energy transferred by charge

A charge of 5.0 C passes through a motor with a potential difference of 12 V across it. Calculate the energy transferred.

Use the energy equation because the question gives charge and potential difference: $\times V$ .
Substitute the values: $5.0\ \text{C} \times 12\ \text{V}$ .
Calculate the energy transferred: $60\ \text{J}$ .

6. Ammeters and voltmeters

An ammeter measures current. It must be connected in series with the component, so the same current passes through the ammeter and the component.

A voltmeter measures potential difference. It must be connected in parallel across the component, so it compares the energy difference between the two ends of that component.

Series and parallel circuit diagrams showing ammeters and voltmeters

Example

Choosing where to place meters

You want to measure the current through a resistor and the potential difference across it.

For current, the ammeter must be part of the same path as the resistor, so place the ammeter in series with the resistor.
For potential difference, the voltmeter must compare the two sides of the resistor, so connect it in parallel across the resistor.
Check the circuit still has a complete loop through the cell, ammeter and resistor; the voltmeter is on a separate parallel connection across the resistor.

Common Mistake

Mixing up meters

Ammeters go in series. Voltmeters go in parallel. If you swap them, the circuit reading will be wrong and the circuit may not work as intended.

7. Series and parallel circuits

A series circuit has only one path for current. All components are on the same loop.

A parallel circuit has more than one path for current. Each separate path is called a branch.

Series circuits

In a series circuit:

The current is the same through every component.
The potential difference from the source is shared between components.
If one component breaks or a switch opens, the whole circuit stops.

Parallel circuits

In a parallel circuit:

The current splits between branches.
The current rejoins after the branches.
Components in different branches can still work if one branch is broken.
Each branch has the same potential difference as the supply, if connected directly across it.

Key Idea

Current at a junction

Current is conserved at a junction. The total current going into a junction equals the total current coming out.

Example

Using current conservation at a junction

A current of 3.0 A reaches a junction. One branch has a current of 1.2 A. Find the current in the other branch.

Apply conservation of current: total current into the junction equals total current out.
Set up the relationship: $A+I23.0\ \text{A} = 1.2\ \text{A} + I_2$ .
Rearrange and calculate: $AI_2 = 3.0\ \text{A} - 1.2\ \text{A} = 1.8\ \text{A}$ .

8. Resistance and current

Resistance is how much a component opposes the flow of current. It is measured in ohms (Ω).

Definition

Resistance

Resistance is the opposition to current in a circuit. A higher resistance makes it harder for charge to flow.

The equation linking potential difference, current and resistance is:

\times R

where:

$V$ is potential difference in volts (V)
$I$ is current in amps (A)
$R$ is resistance in ohms (Ω)

You need to recall and use this equation.

For a fixed potential difference, increasing the resistance decreases the current. Decreasing the resistance increases the current.

Example

Calculating current from potential difference and resistance

A resistor has a potential difference of 6.0 V across it and a resistance of 12 Ω. Calculate the current.

Rearrange $\times R$ to make current the subject: $\frac{V}{R}$ .
Substitute the values: $\frac{6.0\ \text{V}}{12\ \Omega}$ .
Calculate the current: $0.50\ \text{A}$ .

Tip

Sanity check for resistance questions

If the same battery is used, a bigger resistance should give a smaller current. If your answer does the opposite, check your rearrangement.

9. Resistance in series and parallel

When resistors are in series, the current has to pass through each resistor one after another. Each resistor makes it harder for current to flow, so the total resistance increases.

When resistors are in parallel, there is more than one route for charge to take. This makes it easier for current to flow overall, so the total resistance decreases.

Key Idea

Resistors together

In series, adding another resistor increases the total resistance.
In parallel, adding another resistor decreases the total resistance because it adds another path for current.

Common Mistake

Thinking more resistors always means more resistance

More resistors only definitely means more total resistance when they are in series. In parallel, adding another branch can reduce the total resistance.

Exam technique

In the exam

Before using an equation, identify what the question is asking for: charge, current, time, energy, potential difference or resistance.
Draw or trace the circuit path: ammeters go in series, voltmeters go in parallel, and current is conserved at junctions.
Check your units carefully: time in seconds, charge in coulombs, current in amps, potential difference in volts, resistance in ohms, energy in joules.

Self review

Check yourself

Why must an ammeter be connected in series with the component it measures?
A current of 2.0 A flows for 15 s. What equation would you use to find the charge moved?
In a parallel circuit, what happens to the current at a junction?

Charge, current and potential difference

What you'll learn

1. Atoms and electric charge

2. Circuit diagrams and symbols

3. Closed circuits and current

4. Current: the rate of flow of charge

5. Potential difference: energy per charge

6. Ammeters and voltmeters

7. Series and parallel circuits

Series circuits

Parallel circuits

8. Resistance and current

9. Resistance in series and parallel

Recap questions

You've reached the end

Charge, current and potential difference

What you'll learn

1. Atoms and electric charge

2. Circuit diagrams and symbols

3. Closed circuits and current

4. Current: the rate of flow of charge

5. Potential difference: energy per charge

6. Ammeters and voltmeters

7. Series and parallel circuits

Series circuits

Parallel circuits

8. Resistance and current

9. Resistance in series and parallel

Recap questions

You've reached the end

Charge, current and potential difference

What you'll learn

1. Atoms and electric charge

2. Circuit diagrams and symbols

3. Closed circuits and current

4. Current: the rate of flow of charge

5. Potential difference: energy per charge

6. Ammeters and voltmeters

7. Series and parallel circuits

Series circuits

Parallel circuits

8. Resistance and current

9. Resistance in series and parallel

Recap questions

You've reached the end

Charge, current and potential difference Revision Guide

Charge, current and potential difference

What you'll learn

1. Atoms and electric charge

2. Circuit diagrams and symbols

3. Closed circuits and current

4. Current: the rate of flow of charge

5. Potential difference: energy per charge

6. Ammeters and voltmeters

7. Series and parallel circuits

Series circuits

Parallel circuits

8. Resistance and current

9. Resistance in series and parallel

Recap questions

You've reached the end

Charge, current and potential difference Revision Guide