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

Current, resistance and potential difference

What is Ohm's law?

Ohm's law explains how three important electrical quantities are connected. It tells us how current, resistance, and potential difference work together in electrical circuits.

infoNote

Ohm's Law Definition

The law states that the current flowing through a component is directly proportional to the potential difference across it, as long as the temperature stays constant.

Understanding resistance

Resistance is a measure of how difficult it is for electricity to flow through a component. Think of it like water flowing through a pipe - a narrow pipe has more resistance than a wide pipe.

infoNote

Key Facts About Resistance

Resistance is measured in ohms (Ω)
Higher resistance means less current flows
Lower resistance means more current flows
The total resistance of a circuit depends on all the components in it

Types of resistors

Fixed resistors always have the same resistance value. Variable resistors can be changed using a slider or knob. This lets you control how much current flows in a circuit.

The Ohm's law equation

chatImportant

The Most Important Equation in Electricity

$V = I \times R$

Where:

V = potential difference (volts)
I = current (amps)
R = resistance (ohms)

Rearranging the equation

You can rearrange this equation to find any of the three values:

infoNote

Rearranged Forms of Ohm's Law

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

Ohmic and non-ohmic components

Ohmic components follow Ohm's law perfectly. When you plot their current against potential difference, you get a straight line through the origin.

Non-ohmic components don't follow Ohm's law. Their resistance changes as conditions change, so their graphs are not straight lines.

Circuit investigations

To investigate Ohm's law, you can:

Use a variable resistor to change the resistance
Measure current with an ammeter
Measure potential difference with a voltmeter
Plot graphs of current against potential difference

When components follow Ohm's law, the graph will be a straight line. The steeper the line, the lower the resistance.

lightbulbExample

Worked Example: Calculating Potential Difference

Let's say a resistor has a resistance of 50Ω and a current of 3A flows through it.

Step 1: Identify the known values

Resistance (R) = 50Ω
Current (I) = 3A

Step 2: Use Ohm's law equation $V = I \times R$

Step 3: Substitute the values $V = 3A \times 50Ω = 150V$

Answer: The potential difference is 150V

bookmarkSummary

Key Points to Remember

Resistance is measured in ohms (Ω) and shows how hard it is for current to flow
Ohm's law equation: $V = I \times R$
Ohmic components have straight-line graphs and constant resistance
Variable resistors let you control the current in a circuit
Higher resistance means less current flows through a component

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