Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Linked equation: V = IR
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Aim
To investigate how the length of a wire affects its resistance, and the resistance of resistors in series and in parallel.
Method (length of wire)
- Tape a length of wire to a metre ruler.
- Connect a crocodile clip at 0 cm and another at 10 cm. Connect an ammeter in series and a voltmeter in parallel across the wire between the clips.
- Use a low potential difference. Switch on, record the current and p.d., then switch off.
- Move the second clip in 10 cm steps up to 100 cm, recording V and I each time.
- Calculate resistance R = V ÷ I for each length and plot resistance against length.
- For combinations: measure V and I for resistors in series, then in parallel, and calculate the total resistance.
| Independent variable | Length of wire |
| Dependent variable | Resistance (calculated from V and I) |
| Control variables | Material and thickness of wire; temperature of wire; p.d. of supply |
Questions
Q1 (F) Name the independent and dependent variables.
Show answer
Independent: length of the wire. Dependent: resistance of the wire (calculated from the p.d. and current).
Q2 (F) How should the ammeter and voltmeter be connected?
Show answer
Ammeter in series with the wire; voltmeter in parallel across the length of wire being tested.
Q3 (F) Why should you use a low p.d. and switch off between readings?
Show answer
A current heats the wire. A hotter wire has a higher resistance, which would spoil the results – and a hot wire can burn you.
Q4 (F) The p.d. across a length of wire is 1.2 V and the current is 0.40 A. Calculate its resistance.
Show answer
F: V = I R
I: 1.2 = 0.40 × R
F: R = 1.2 ÷ 0.40
A: 3.0 Ω
Q5 (F/H) The graph of resistance against length is a straight line through the origin. What does this show?
Show answer
The resistance of the wire is directly proportional to its length – doubling the length doubles the resistance.
Q6 (F/H) 50 cm of wire has a resistance of 2.5 Ω. Predict the resistance of 80 cm of the same wire.
Show answer
Resistance per cm = 2.5 ÷ 50 = 0.05 Ω/cm
80 cm: 0.05 × 80 = 4.0 Ω
Q7 (F/H) Two 10 Ω resistors are connected first in series, then in parallel. What happens to the total resistance?
Show answer
In series the total is 10 + 10 = 20 Ω. In parallel the total is less than 10 Ω (the smallest single resistor) – for two equal 10 Ω resistors it is 5 Ω.
Q8 (H) A student’s straight-line graph does not pass through the origin. Suggest a reason.
Show answer
A systematic error (zero error): e.g. the resistance of the crocodile clips and connecting leads is included in every reading, or the first clip was not exactly at 0 cm.
