GCSEScienceAQACombined Science: TrilogyPhysicsRevision NotesElectricityCurrent, Potential Difference & ResistanceRequired Practical: Investigating Resistance

Required Practical: Investigating Resistance (AQA GCSE Combined Science: Trilogy: Physics): Revision Note

Exam code: 8464

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Written by: Ashika

Reviewed by: Caroline Carroll

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Required Practical 3: Investigating Resistance

Equipment List

Table of equipment and its purpose: a power supply, cell or battery provides the potential difference; wires connect the components; crocodile clips connect different lengths of resistance wire; an ammeter measures the current; a voltmeter measures the potential difference; two or more resistors and a thin resistance wire are the components being tested; a metre ruler measures the length of the wire.

  • Resolution of measuring equipment:

    • Metre ruler = 1 mm

    • Ammeter = 0.01 A

    • Voltmeter = 0.1 V

Resistance of the Length of a Wire at a Constant Temperature

  • The aim of this experiment is to investigate how the length of a wire at a constant temperature affects the resistance of electrical circuits

Variables

  • Independent variable = Length of resistance wire, L

  • Dependent variable = Resistance, R

  • Control variables:

    • Potential difference of the power supply

    • Temperature of the wire

Method

Circuit diagram of a 1.5 V cell connected in series with an ammeter and, through two crocodile clips, a length of thin resistance wire laid along a metre ruler. A voltmeter is connected in parallel across the section of wire between the clips.

Resistance of the length of wire apparatus

  1. Set up the apparatus by connecting two crocodile clips to the thin resistance wire a distance of 10 cm apart and setting the power supply to 1.5 V

  2. Connect the wire, using the clips, to the rest of the circuit

  3. Record the potential difference from the voltmeter and current from the ammeter

  4. Move the clips in 10 cm intervals further apart

  5. Take new measurements from the voltmeter and ammeter for each length reading

  6. Continue until the crocodile clips are a length of 1 m apart

  7. Repeat the whole set of readings and calculate a mean current and potential difference for each length

  • An example table of results might look like this:

Blank results table with columns for length in m, potential difference in V, current in A and resistance in Ω. The length column runs from 0.10 m to 1.00 m in steps of 0.10 m.

Analysis of Results

  • Calculate the resistance of each length of wire using the equation:

R=VI

  • Where:

    • R = resistance (Ω)

    • V = potential difference (V)

    • I = current (A)

  • Plot a graph of resistance (on the y-axis) against length (on the x-axis) and draw a line of best fit

  • An example graph might look like:

Graph of resistance in Ω on the y-axis against length in m on the x-axis. The plotted points lie close to a straight line of best fit that passes through the origin.

  • The graph should be a straight line through the origin with a positive correlation

    • This means that the longer the piece of wire, the higher the resistance

  • In other words, the resistance is directly proportional to the length of the wire

Examiner Tips and Tricks

This practical is notoriously difficult to obtain valid results from because it is so difficult to control for the heating effect in the wire. Temperature affects resistance, so when the wire heats up, the increased temperature affects the resistance as well as the length of the wire. When this happens, we call it a confounding variable. A variable other than the independent variable has impacted the results, and it is therefore impossible to tell the extent of the effect of the independent variable.

There are ways that you can minimise the impact of a confounding variable (scientists have to work around them all the time in real-life situations), and so this often comes up in exam questions.

Only have the circuit connected to take the reading, and then disconnect it straight away until you are ready to take the next reading. Remember that the heating effect is greatest with the shortest lengths of wire. With a fixed potential difference, a shorter wire has a lower resistance, so the current is larger, and the wire heats up more. A longer wire has a higher resistance, so the current is smaller, and there is less heating.

In an ideal scenario, you would wait until the wire had cooled back to room temperature before you took the next reading, but you simply don't have the time to do this in class. However, these are things you could talk about in an exam question if asked for suggestions on improving the investigation.

Make sure you connect the ammeter in series with the wire and the voltmeter in parallel across it. Drawing the voltmeter in series is the most common mistake, so check your circuit diagram before you move on.

Combinations of Resistors in Series and Parallel

  • The aim of this experiment is to investigate how combinations of resistors in series and parallel affect the total resistance in electrical circuits

Variables

  • Independent variable = Number of resistors

  • Dependent variable = Total resistance, R

  • Control variables:

    • Potential difference of the power supply

    • Temperature of the resistors

Method

Two series circuit diagrams, each with batteries, an ammeter and a switch. Figure 1 has a single resistor R1 with a voltmeter in parallel across it; Figure 2 has resistors R1 and R2 in series with the voltmeter in parallel across both.
Figure 3, a parallel circuit diagram: batteries, an ammeter and a switch in the main loop, with resistors R1 and R2 on separate parallel branches and a voltmeter in parallel across them.

Series and parallel resistor combinations apparatus

  1. Connect the circuit shown in Figure 1 to a 4 V battery, first with one resistor (R1), with the voltmeter connected in parallel and the ammeter in series

  2. Close the switch and record the reading on the voltmeter and ammeter

  3. Repeat steps 1 and 2 for just the second resistor (R2)

  4. Open the switch and connect both R1 and R2 in series as shown in Figure 2, connecting the voltmeter in parallel to both resistors

  5. Close the switch and record the new readings on the voltmeter and ammeter

  6. Open the switch and arrange R1 and R2 now in parallel, as shown in Figure 3

  7. Close the switch and record the readings on the voltmeter and ammeter

  • An example table of results might look like this:

Blank results table with columns for resistor, potential difference in V, current in A and resistance in Ω. The rows are for R1, R2, the series combination and the parallel combination.

Analysis of Results

  • Similar to the previous experiment, the resistance for each voltage and current reading is determined by the equation:

Equation: resistance R equals potential difference V divided by current I.

  • The results should show that in series:

    • The resistance of the combined resistors is equal to the sum of the two individual resistances

    • This is because the same current flows through each resistor, so the electrons must pass through the extra resistance one after the other

  • The results should show that in parallel:

    • The total resistance of the combined resistors is less than the resistance of the smallest individual resistor

    • This is because the current splits between the branches, so the total current is larger than for one resistor alone, while the potential difference across each resistor stays the same

Evaluating the Experiment

Improving the accuracy of the result

  • The first crocodile clip (connected to the circuit, not the wire) must start at 0 on the ruler

    • Otherwise, this could cause a zero error in your measurements of the length

  • Both the ammeter and voltmeter should be checked to start from 0

  • Only allow small currents to flow through the wire

    • This keeps the temperature of the wire constant, so it doesn't change its resistance

  • The current should be switched off between readings so its temperature doesn't change its resistance

Improving the experiment

  • Repeat each reading and calculate a mean

  • Remove any anomalous readings before calculating the mean

Extending the experiment

  • Add more resistors in series and parallel to calculate the effect on the combined resistance

Safety Considerations

  • When there is a high current flowing through a thin wire, the wire will become very hot

    • Make sure never to touch the wire directly when the circuit is switched on

  • Switch off the power supply right away if burning is smelled

  • Make sure there are no liquids close to the equipment, as this could damage the electrical equipment if spilled

  • The 1.5 V supply has a very low potential difference, so there is no risk of electric shock or electrocution. The main hazard is the wire getting hot, which could cause burns

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Ashika

Author: Ashika

Expertise: Physics Content Creator

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

Caroline Carroll

Reviewer: Caroline Carroll

Expertise: Head of Content Delivery

Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about delivering high-quality resources to help students achieve their full potential.