Charge & Current (Edexcel IGCSE Science (Double Award))

Revision Note

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Author

Ashika

Last updated

6 December 2024

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Current

Electric current is defined as
The rate of flow of electric charge
Current is measured in units of amperes or amps (A)
- 1 amp is equivalent to a charge of 1 coulomb flowing in 1 second, or 1 A = 1 C/s

This means the size of an electric current is the amount of charge passing through a component each second

Current flows
- when a circuit is formed e.g. when a wire connects the two oppositely charged terminals of a cell
- from the positive terminal to the negative terminal of a cell

Charge flows from the positive terminal to the negative terminal

Measuring current

Current can be measured using an ammeter
Ammeters must be connected in series with the component being measured

An ammeter can be used to measure the current around a circuit

Charge

The wires in an electric circuit are made of metal because it is a good conductor of electric current
In the wires, the current is a flow of negatively charged electrons

electrons-and-current, IGCSE & GCSE Physics revision notes

In metal wires, the current is a flow of negatively charged electrons. When a voltage is applied, electrons flow through the lattice of metal ions

Examiner Tips and Tricks

You should always consider current to be the flow of positive charge i.e. from the positive terminal to the negative terminal of a cell. This is known as conventional current.

This is in the opposite direction to electron flow, which is the flow of negatively charged electrons from the negative terminal to the positive terminal of a cell.

This is the convention we use because scientists defined conventional current before they discovered the electron

Calculating electric charge

Current, charge and time are related by the equation:

charge = current × time

$Q = I \times t$

Where:
- Q = charge, measured in coulombs (C)
- I = current, measured in amps (A)
- t = time, measured in seconds (s)

The current, charge and time equation can be rearranged with the help of the following formula triangle:

Current charge time formula triangle

Current charge time triangle, IGCSE & GCSE Physics revision notes

Formula triangle for the charge, current and time equation

Worked Example

When will 8 A of current pass through an electrical circuit?

A. When 8 J of energy is used by 1 C of charge

B. When a charge of 4 C passes in 0.5 s

C. When a charge of 8 C passes in 0.1 s

D. When a charge of 1 C passes in 8 s

ANSWER: B

The equation relating current, charge and time is:

$Q = I \times t$

Rearrange to make current I the subject of the equation:

$I = \frac{Q}{t}$

Consider option B, where Q = 4 C and t = 0.5 s:

$I = \frac{4}{0.5} = 8 A$

Therefore, the correct answer is B

A is incorrect as this is the definition of a voltage of 8 V between two points and does not describe current

C is incorrect as $I = \frac{8}{0.1} = 80 A$

D is incorrect as $I = \frac{1}{8} = 0.125 A$

Examiner Tips and Tricks

Electric currents in everyday circuits tend to be quite small, so it's common for examiners to throw in a unit prefix like 'm' next to quantities of current, e.g. 10 mA (10 milliamperes). Make sure you can convert these into standard units, e.g. 10 mA = 10 × 10^-3A.

Make sure to only use the triangle to help you rearrange the equation that links charge, current and time. Don't draw it if you are asked to write out the equation in full, such as Q = I × t, as you may lose marks for doing so.

Check out this revision note on speed, distance and time if you need a reminder on how to use formula triangles.

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