Electrical charge and current Revision Notes for AQA GCSE Physics Combined Science

Electrical charge and current

What is electric current?

Electric current tells us how fast electric charge moves through a material.

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Think of it like water flowing through a pipe - current measures how much charge flows past a point every second. This analogy helps us understand that current is about the rate of flow, not just the amount of charge present.

In metals, electric current happens when tiny particles called electrons move through the material. These electrons carry the electric charge.

Understanding charge and current

Charge is measured in units called coulombs (C). This tells us how much electric charge we have.

Current is measured in units called amperes or amps (A). This tells us how much charge flows past a point each second.

Here's the key relationship: 1 amp = 1 coulomb per second

This means if 1 coulomb of charge flows past a point in 1 second, the current is 1 amp.

The charge equation

There's a simple equation that connects charge, current and time:

$Q = I \times t$

Where:

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

You can rearrange this equation to find any missing value:

To find current: $I = \frac{Q}{t}$
To find time: $t = \frac{Q}{I}$

Measuring current in circuits

To measure current, you need a device called an ammeter. Here's how it works:

The ammeter must be connected in series with the component you want to measure
This means the current flows through the ammeter as well as the component
The ammeter shows you the size of the current in amps

Important rule about series circuits

chatImportant

In a series circuit, the current is the same everywhere. This means:

All components have the same current flowing through them
If you measure 0.5A at one point, you'll get 0.5A at every other point too

Remember: Electric current only flows when there's a complete path (closed circuit) and a source of energy like a battery.

Working with the equation

Let's see how to use the equation $Q = I \times t$ with some practical examples:

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Worked Example 1: Calculating Charge

Problem: If a current of 2A flows for 5 minutes, how much charge flows?

Step 1: Identify the given values

Current ( $I$ ) = 2A
Time ( $t$ ) = 5 minutes = 300 seconds

Step 2: Apply the equation $Q = I \times t$

Step 3: Substitute and calculate $Q = 2 \times 300 = 600C$

Answer: 600 coulombs of charge flows

lightbulbExample

Worked Example 2: Calculating Current

Problem: If 1200C of charge flows in 4 minutes, what's the current?

Step 1: Identify the given values

Charge ( $Q$ ) = 1200C
Time ( $t$ ) = 4 minutes = 240 seconds

Step 2: Rearrange the equation to find current $I = \frac{Q}{t}$

Step 3: Substitute and calculate $I = \frac{1200}{240} = 5A$

Answer: The current is 5 amperes

Summary

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Key Points to Remember:

Electric current is the rate of flow of electric charge
Current is measured in amps (A), charge in coulombs (C)
Use the equation $Q = I \times t$ to solve problems
Ammeters measure current and must be connected in series
Current is the same at all points in a series circuit

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