Electrical charge and current Revision Notes for AQA GCSE Physics

Electrical charge and current

What is electric current?

Electric current is the rate of flow of charge. Think of it like water flowing through a pipe - current tells us how much electrical charge flows past a point each second.

infoNote

The water pipe analogy is very helpful for understanding current. Just as water flow rate tells us how much water passes through a pipe per second, electric current tells us how much charge flows past a point each second.

In metals, electric current is caused by electrons moving through the material.

The relationship between charge, current and time

The amount of charge that flows depends on:

How big the current is
How long the current flows for

We can work this out using this formula:

Charge = Current × Time

$Q = I \times t$

Where:

Q = charge measured in coulombs (C)
I = current measured in amperes or amps (A)
t = time measured in seconds (s)

Using the triangle method

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Triangle Method for Formula Rearrangement

You can rearrange this formula using a triangle:

   Q
  ---
 I × t

To find Q: multiply I and t
To find I: divide Q by t
To find t: divide Q by I

This triangle method works for any formula with three variables where one equals the product of the other two.

Measuring current

To measure the size of current flowing through a component, we use an ammeter.

chatImportant

Important rules for ammeters:

Ammeters must be connected in series with the component
This means the current flows through the ammeter
Never connect an ammeter in parallel as this can damage the metre and create a dangerous short circuit

Current in series circuits

In a series circuit, the current is the same at all points. This means:

All ammeters in a series circuit will show the same reading
The current doesn't get "used up" as it flows around the circuit

infoNote

Example: If one ammeter reads 0.5A, all other ammeters in the same series circuit will also read 0.5A. This is because there's only one path for current to flow, so the same amount of charge must pass through every point.

Worked examples

lightbulbExample

Worked Example 1: Finding charge

A current of 1.5A flows for 2 minutes. How much charge flows?

Step 1: Convert time to seconds 2 minutes = 2 × 60 = 120s

Step 2: Use $Q = I \times t$ $Q = 1.5A \times 120s =$ 180C

lightbulbExample

Worked Example 2: Finding current

A charge of 1200C flows through a lamp for 4 minutes. What is the average current?

Step 1: Convert time to seconds 4 minutes = 4 × 60 = 240s

Step 2: Use $I = \frac{Q}{t}$ $I = \frac{1200C}{240s} =$ 5A

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Key Points to Remember:

Electric current = rate of flow of charge
Units: Current in amps (A), Charge in coulombs (C), Time in seconds (s)
Formula: $Q = I \times t$ (and rearrangements using the triangle method)
Ammeters measure current and must be connected in series
In series circuits, current is the same everywhere
Always convert minutes to seconds in calculations
Think of current like water flow in a pipe - it shows how fast charge is moving

Electrical charge and current (AQA GCSE Physics): Revision Notes