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Key points

  • Resistance (shown as R) is a measure of how difficult it is for current to flow. Resistance is measured in units called ohms (惟).

  • The amount of flowing in a circuit is affected by the resistance of that circuit.

  • Each component in a circuit has a resistance.

  • Resistance can be calculated using the equation: Resistance = potential difference 梅 current

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Video - Resistance

Can you answer these questions based on the video?

  1. What happens to the current in a circuit when you add a piece of resistance wire?

  2. What is the unit of measurement for resistance?

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The effect of resistance on current

When a is connected a current flows - move through the and . As the electrons move, they collide with ions, which causes .

The more resistance there is in a circuit, the less current will flow.

The resistance increases when you add more components in . For example, the resistance of two lamps in series is greater than the resistance of one lamp. Less current will flow through them and they will not light up as brightly.

Two circuit diagrams. The left has a cell, switch, lamp and an ammeter labelled with 4 amps. The right has a cell, switch, two lamps and an ammeter labelled with 2 amps.
Figure caption,
The circuit on the right has two lamps in series. Having more lamps in series means that the resistance of the circuit is more. Less current flows so the bulbs are dimmer.

Resistance is measured in units called ohms, which is represented by the symbol 惟 (an uppercase Greek letter omega). For example, a resistance of 10 ohms can be written as 10 惟.

Three question marks

Did you know?

All wires and components have resistance, though usually the resistance of wires is small enough to ignore.

Three question marks
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Resistors

It can be useful to add resistance to a circuit. For example, increasing resistance can make a motor spin more slowly, or make a lamp light less bright. A component called a resistor can be added in series to a circuit to increase the total resistance.

The image shows different types of resistors.

A photo of five resistors which are different shapes and sizes
Image caption,
Components called resistors can be added to a circuit to change the total resistance. Resistors come in different shapes and sizes.

The circuit symbol for a resistor is a rectangle. These rectangles both represent resistors.

An image showing the symbol for a resistor, which is a thin rectangle. Image shows both horizontal and vertical symbols.
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Calculating resistance

Potential difference (p.d.) is the amount of energy transferred by each unit of charge passing between two points of a circuit. The unit for potential difference is the volt (V). For a given potential difference, the larger the resistance the smaller the current that can flow.

To find the of a component we need to know the potential difference (V) across it and the current (I) flowing through it. We can then use the formula to calculate the resistance:

\(Resistance = \frac{potential~difference}{current}\)

The equation can also be written using symbols: \(R = \frac{V}{I}\)

Example

A circuit contains a battery, a resistor and an ammeter. The battery has a potential difference of 12V, and the current flowing is 2A.

What is the resistance of the circuit?

A diagram showing a circuit with two cells, a resistor and an ammeter. The cells have a label showing 12 volts and the ammeter has a label showing 2 amps.

\(Resistance = \frac{potential~difference}{current}\)

\(Resistance = \frac{12~V}{2~A}\)

\(Resistance = 6~惟\)

A circuit contains a cell, a lamp, an ammeter and a voltmeter. The current flowing in the circuit is 3 A and the potential difference across the lamp is 6 V.

What is the resistance of the lamp?

A diagram showing a circuit with a cell, an ammeter, a lamp, and a separate branch containing a voltmeter. The ammeter has a label showing 3 amps and the voltmeter has a label showing 6 volts.

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Test your knowledge

Quiz

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Play the Atomic Labs game! game

Try out practical experiments in this KS3 science game.

Play the Atomic Labs game!
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