Current, resistance and potential difference Revision Notes for AQA GCSE Physics Combined Science

Current, resistance and potential difference

What is resistance?

Resistance tells us how hard it is for electricity to flow through something. Think of it like trying to push water through different pipes - some pipes let water flow easily, others make it harder.

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Understanding resistance is fundamental to electrical circuits:

Resistance is measured in ohms (Ω)
The higher the resistance, the harder it is for current to flow

How does resistance work in circuits?

The total resistance of a circuit depends on all the components in it. You can change the resistance by:

Adding different resistors to the circuit
Using a variable resistor (also called a rheostat)
A variable resistor has a slider or knob that changes its resistance

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Variable resistors are incredibly useful in circuits because they allow you to control the amount of current flowing through different parts of the circuit. This is how volume controls on radios work, or dimmer switches for lights!

Ohm's law

Ohm's law is one of the most important rules in electricity. It shows us how current, voltage, and resistance are connected.

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Ohm's law says: The current flowing through a component is directly proportional to the potential difference across it (at constant temperature).

This means:

Double the voltage → double the current
Half the voltage → half the current

The Ohm's law equation

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The fundamental equation:

$V = I \times R$

Where:

V = potential difference (voltage) in volts (V)
I = current in amps (A)
R = resistance in ohms (Ω)

You can rearrange this equation to find any missing value:

To find current: $I = \frac{V}{R}$
To find resistance: $R = \frac{V}{I}$

Ohmic and non-ohmic conductors

Ohmic conductors follow Ohm's law perfectly and maintain constant resistance at constant temperature. Non-ohmic conductors don't follow Ohm's law because their resistance changes.

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Examples of conductors:

Ohmic conductors:

Metal wires at constant temperature
Fixed resistors

Non-ohmic conductors:

Light bulbs (filaments get hot and change resistance)
Diodes

Using Ohm's law - worked example

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Worked Example: Finding Potential Difference

Question: A resistor has resistance 50Ω and current 3A flowing through it. What's the potential difference?

Solution: Step 1: Identify the known values

Resistance: $R = 50Ω$
Current: $I = 3A$
Unknown: Potential difference $V$

Step 2: Choose the correct equation Use $V = I \times R$

Step 3: Substitute and calculate $V = 3A \times 50Ω = 150V$

Answer: The potential difference is 150V

Understanding I-V graphs

When you plot current against voltage, you get different shaped graphs that tell us about the conductor's properties:

Ohmic conductors: Straight line through the origin (steeper line = lower resistance)
Non-ohmic conductors: Curved lines

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Reading I-V Graphs:

The gradient (steepness) of the line tells you about resistance:

Steeper line = lower resistance
Flatter line = higher resistance

This is because gradient = $\frac{\text{current}}{\text{voltage}} = \frac{I}{V} = \frac{1}{R}$

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Key Points to Remember:

Resistance measures how hard it is for current to flow - units are ohms (Ω)
Ohm's law equation: $V = I \times R$
You can rearrange this equation to find current ( $I = \frac{V}{R}$ ) or resistance ( $R = \frac{V}{I}$ )
Ohmic conductors give straight line graphs, non-ohmic give curved lines
Higher resistance = lower current (if voltage stays the same)

Current, resistance and potential difference (AQA GCSE Physics Combined Science): Revision Notes