4.2.11Electricity and Magnetism
Investigating Resistance — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 15 of 31
Part 1 of 5Aim and variables
| Type | Quantity | Notes |
|---|---|---|
| Independent | Length of resistance wire, L (cm) | Changed in steps of 10 cm from 10 cm to 100 cm |
| Dependent | Resistance of the wire, R (Ω) | Calculated from the voltmeter and ammeter readings |
| Control | Voltage of the supply | Keep the power supply setting the same, for example 1.5 V |
| Control | Temperature of the wire | Use small currents and switch off between readings |
| Control | Material and thickness of the wire | Use one piece of thin resistance wire throughout |
Part 2 of 5Equipment and what each item does
Circuit parts
- Circuit parts A power supply or cell provides the voltage, and connecting wires join everything together.
- An ammeter in series measures the current, and a voltmeter in parallel across the wire measures the voltage.
Wire and measuring length
- Wire and measuring length A thin length of resistance wire is stretched alongside a metre rule so that its length can be read.
- Crocodile clips connect the wire into the circuit and can be slid along to choose the length.
Resolution
- Resolution The metre rule reads to 1 mm, the ammeter to 0.01 A and the voltmeter to 0.1 V;
- the smallest scale division limits how precise the results can be.
Part 3 of 5Method
1
Fix the resistance wire along the metre rule and set the power supply to 1.5 V.
2
Connect two crocodile clips 10 cm apart on the wire, and connect the clips into a series circuit with the ammeter, with the voltmeter in parallel across the wire between the clips.
3
Switch on and record the voltmeter and ammeter readings, then switch off.
4
Move the clip to increase the length by 10 cm and repeat the measurements.
5
Continue until the clips are 1 m apart, giving ten pairs of readings.
6
Repeat the experiment and calculate mean values for each length.
Part 4 of 5Analysis
1Calculate resistance
- Calculate resistance For every length use R = V ÷ I, where R is in ohms (Ω), V in volts (V) and I in amps (A).
2Plot a graph
- Plot a graph Draw resistance on the y-axis against length on the x-axis, and add a line of best fit.
3Describe the shape
- Describe the shape The graph is a straight line through the origin with a positive gradient.
4Conclude
- Conclude Resistance is directly proportional to the length of the wire: doubling the length doubles the resistance.
Part 5 of 5Evaluation and safety
Systematic errors
- Systematic errors Check that the first clip is at 0 on the rule and that both meters read zero before the circuit is switched on.
Random errors
- Random errors A hot wire has higher resistance, so keep the current low and switch off between readings to hold the temperature constant.
- Repeat readings and take means to reduce the effect of random variation.
Safety
- Safety A high current in a thin wire heats it, so never touch the wire while the circuit is on and switch off at once if you smell burning.
- Keep liquids away from the electrical equipment.
Key terms
| Term | Meaning |
|---|---|
| Resistance | A measure of how hard it is for current to flow through a component, measured in ohms (Ω). |
| Directly proportional | Two quantities where one doubles when the other doubles, giving a straight line through the origin. |
| Ammeter | A meter connected in series that measures current in amps. |
| Systematic error | An error that shifts all readings in the same direction, such as a zero error. |
| Line of best fit | A line drawn to pass as close as possible to all the plotted points. |
Avoid these mistakes
1
Leaving the current on between readings, so the wire warms up and its resistance rises.
2
Connecting the voltmeter in series or the ammeter in parallel.
3
Not using the zero point on the ruler, which gives a length that is wrong by a fixed amount.
4
Drawing a curved line through the points instead of a straight line of best fit through the origin.
5
Writing R = I ÷ V. The equation is R = V ÷ I.
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