GCSE Physics revision

Electrical charge and current

Current, potential difference and resistance

AQA 4.2.1.2

Your specification

AQA student objectives

Learning pathway

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ALL 🎯

For electrical charge to flow through a closed circuit the circuit must include a source of potential difference.

MOST 🎯🎯

Apply the specified scientific knowledge of electrical charge and current to a relevant example.

SOME 🎯🎯🎯

Analyse an unfamiliar example of electrical charge and current using the specified scientific ideas.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What Is Electric Charge?

Charge is a measure of the total current that has flowed over a certain period of time.
Charge is represented by the letter Q and is measured in coulombs (C).
Using the water analogy, if current is the rate of water flowing in a river, charge is the total amount of water that has flowed past a point in a given time.

1. What Is Electric Charge?AQA

Notes

Charge is a measure of the total current that has flowed over a certain period of time.
Charge is represented by the letter Q and is measured in coulombs (C).
Using the water analogy, if current is the rate of water flowing in a river, charge is the total amount of water that has flowed past a point in a given time.

The Charge Equation

The equation for charge is Q = I × t, where Q is charge in coulombs, I is current in amps (A), and t is time in seconds (s).
Q
To find current, rearrange the equation to I = t .
Q
To find time, rearrange the equation to t = I .

Worked Example 1: Finding Charge

A kettle draws a current of 12 A and takes 50 seconds to boil — to find charge, substitute directly into Q = I × t.
Calculating 12 × 50 = 600 C, so 600 coulombs of charge passed through the wire.

Worked Example 2: Finding Current with Unit Conversion

A phone charger transfers 43.2 kC over 2 hours — before calculating, units must be converted to coulombs and seconds.
43.2 kC is converted by multiplying by 1,000 to give 43,200 C, and 2 hours is converted to 7,200 seconds.
Substituting into I = t gives 43200
Q
7200 = 6 A, so a current of 6 amps flowed through the wire.

4. Worked Example 2: Finding Current with Unit ConversionAQA

Notes

A phone charger transfers 43.2 kC over 2 hours — before calculating, units must be converted to coulombs and seconds.
43.2 kC is converted by multiplying by 1,000 to give 43,200 C, and 2 hours is converted to 7,200 seconds.
Substituting into I = t gives 43200
Q
7200 = 6 A, so a current of 6 amps flowed through the wire.

Basic Circuit Components

A simple circuit can include components such as a cell, a lamp, an ammeter, and a voltmeter, each serving a specific purpose.
An ammeter measures the rate of flow of charge (current) through a circuit, whilst a voltmeter measures potential difference across a component.
What is Electric Current?
Electric current is the flow of free electrons through a conductor such as a wire.
Free electrons are able to move through a metal wire and conduct electricity because they are not bound to a single atom.
Each electron carries a tiny negative charge, which is far too small to be measured individually.

5. Basic Circuit ComponentsAQA

Notes

A simple circuit can include components such as a cell, a lamp, an ammeter, and a voltmeter, each serving a specific purpose.
An ammeter measures the rate of flow of charge (current) through a circuit, whilst a voltmeter measures potential difference across a component.
What is Electric Current?
Electric current is the flow of free electrons through a conductor such as a wire.
Free electrons are able to move through a metal wire and conduct electricity because they are not bound to a single atom.
Each electron carries a tiny negative charge, which is far too small to be measured individually.

Why We Measure Many Electrons

Because a single electron's charge is immeasurably small, we measure the combined charge of a very large number of electrons.
The specific number of electrons used is 6.24 × 10^{18}, which together carry exactly one unit of charge.
This quantity of charge — carried by 6.24 × 10^{18} electrons — is defined as one Coulomb.

6. Why We Measure Many ElectronsAQA

Notes

Because a single electron's charge is immeasurably small, we measure the combined charge of a very large number of electrons.
The specific number of electrons used is 6.24 × 10^{18}, which together carry exactly one unit of charge.
This quantity of charge — carried by 6.24 × 10^{18} electrons — is defined as one Coulomb.

The Coulomb: Unit of Charge

The Coulomb is the SI unit of electric charge, represented by the symbol Q.
The symbol for the unit Coulomb itself is a capital C, so a charge of one Coulomb is written as 1 C.
An ammeter measures the number of Coulombs of charge passing a point in a circuit per second.

The Charge, Current, and Time Equation

The key equation linking charge, current, and time is: Q = I × t In this equation, Q is charge (measured in Coulombs, C), I is current (measured in amperes, A), and t is time (measured in seconds, s).
A formula triangle can be used to rearrange the equation: cover the quantity you want to find to see whether to multiply or divide the other two.

8. The Charge, Current, and Time EquationAQA

Notes

The key equation linking charge, current, and time is: Q = I × t In this equation, Q is charge (measured in Coulombs, C), I is current (measured in amperes, A), and t is time (measured in seconds, s).
A formula triangle can be used to rearrange the equation: cover the quantity you want to find to see whether to multiply or divide the other two.

Rearranging the Equation

To find current, rearrange the formula to I = Q / t To find time, rearrange the formula to t = Q / I Always check your units before substituting values — current must be in amps and time must be in seconds.

Worked Example 1: Finding Charge

A current of 0.5 A flows for 12 seconds; using Q = I × t, the charge is Q = 0.5 × 12 = 6 C.
This type of question is a straightforward substitution where both current and time are given directly.

Worked Example 2: Finding Current

A charge of 2.5 C flows for 5 seconds; rearranging gives I = Q / t = 2.5 / 5 = 0.5 A.
When the question gives charge and time but asks for current, you must rearrange Q = It before substituting values.

Worked Example 3: Using an Ammeter Reading

If an ammeter reads 3.2 A and the time is 4 seconds, the charge is Q = 3.2 × 4 = 12.8 C.
Remember that an ammeter reading directly gives you the current (I) in amperes, which you then use in the equation.

Key Takeaways: Charge and Current

Current is defined as the rate of flow of charge, measured in Coulombs per second (which is equivalent to amperes).
Charge is carried by electrons, but we measure it in bulk — one Coulomb equals the charge on 6.24 × 10^{18} electrons.
The equation Q = It is essential to memorise for GCSE Physics and can be applied to a wide range of exam questions.

13. Key Takeaways: Charge and CurrentAQA

Notes

Current is defined as the rate of flow of charge, measured in Coulombs per second (which is equivalent to amperes).
Charge is carried by electrons, but we measure it in bulk — one Coulomb equals the charge on 6.24 × 10^{18} electrons.
The equation Q = It is essential to memorise for GCSE Physics and can be applied to a wide range of exam questions.