Required practical - Investigating resistance Revision Notes for AQA GCSE Physics Combined Science

Required practical - Investigating resistance

What this practical is about

This practical helps you understand how electrical resistance works in different situations. You'll investigate two main things:

How resistors behave when connected in series and parallel circuits
How the length of a wire affects its resistance

infoNote

This investigation combines two important electrical concepts that will help you understand how circuits behave in real-world applications. The skills you develop here will be essential for understanding more complex electrical systems.

Equipment you need

To carry out this investigation, you'll need:

Battery or power supply
Ammeter (measures current)
Voltmeter (measures potential difference)
Switch
Crocodile clips
Two resistors of the same value
Resistance wire
Metre ruler
Variable resistor
Connecting leads

infoNote

Make sure all your equipment is in good working order before starting. Check that your metres are properly zeroed and your connections are secure to get accurate results.

Method 1: Series and parallel circuits

Setting up the circuits

You need to build two different circuits to compare how resistors behave:

Series circuit: Connect two identical resistors one after the other in a single loop with the battery, ammeter, and voltmeter.

Parallel circuit: Connect two identical resistors on separate branches that both connect back to the main circuit with the battery, ammeter, and voltmeter.

chatImportant

Always check your circuit connections before switching on the power supply. Incorrect connections can damage equipment or give misleading results.

What happens in each circuit

In series circuits: The potential difference (voltage) gets split between the resistors. Each resistor gets a share of the total voltage from the power supply.

In parallel circuits: Each resistor gets the same potential difference as the power supply. The voltage across each branch equals the battery voltage.

Taking measurements

For both circuits, you need to:

Record the potential difference across the resistors
Record the current flowing through the circuit
Calculate the total resistance using the equation: $R = \frac{V}{I}$

lightbulbExample

Worked Example: Calculating Resistance

If you measure:

Potential difference (V) = 6.0 V
Current (I) = 0.5 A

Then: $R = \frac{V}{I} = \frac{6.0}{0.5} = 12 \text{ Ω}$

Method 2: Investigating wire resistance

Setting up the circuit

Connect a length of resistance wire in a circuit with:

Battery
Ammeter (to measure current)
Voltmeter (to measure potential difference across the wire)
Switch
Variable resistor (to control current)

Changing the wire length

Use crocodile clips or a jockey key to change the length of wire in the circuit. Test different lengths and record your measurements each time.

chatImportant

Keep the current constant throughout your investigation by adjusting the variable resistor. This ensures that any changes in resistance are due to wire length, not current variations.

Taking measurements

For each length of wire:

Measure the potential difference across the wire
Measure the current through the wire
Calculate the resistance using $R = \frac{V}{I}$
Take at least two readings for each length to check your results

infoNote

Taking repeat readings helps identify any anomalous results and makes your data more reliable. If readings differ significantly, investigate possible causes like loose connections or equipment issues.

Recording your results

Create a table to record all your measurements:

Length of wire (in cm) OR type of circuit arrangement
Potential difference (in volts)
Current (in amps)
Calculated resistance (in ohms)

For the wire investigation, plot a graph with:

Length of wire on the x-axis
Resistance on the y-axis

infoNote

A well-drawn graph will help you identify patterns in your data and spot any anomalous results. Use a ruler for straight lines and plot points carefully.

Key findings

Series vs parallel circuits:

Total resistance is higher when resistors are in series
Total resistance is lower when resistors are in parallel
This happens because of how the current can flow through the different paths

Wire length and resistance:

Longer wires have higher resistance
The relationship is directly proportional - if you double the length, you double the resistance
Your graph should show a straight line through the origin

chatImportant

If your graph doesn't show a straight line through the origin, check your measurements and calculations. Common causes include systematic errors in measurement or incorrect circuit setup.

Why this matters

Understanding resistance helps explain:

How electrical circuits work in everyday devices
Why some wires get hot when current flows through them
How to design circuits that work safely and efficiently

bookmarkSummary

Key Points to Remember:

Series circuits split voltage between components, parallel circuits give equal voltage to each branch
Longer wires have more resistance than shorter ones
Use the equation $R = \frac{V}{I}$ to calculate resistance from your measurements
Always take repeat readings to make your results more reliable
Plot graphs to see patterns in your data clearly

Required practical - Investigating resistance (AQA GCSE Physics Combined Science): Revision Notes