2.1.4Electricity
Required Practical: Investigating Resistance
5 min read 7 parts Lesson 5 of 27 1 quiz
Section 1 of 7Aim and variables
Aim
- Aim Find how the resistance of a wire at constant temperature depends on its length.
Independent variable
- Independent variable Length of wire between the crocodile clips, L.
Dependent variable
- Dependent variable Resistance of that length of wire, R.
Control variables
- Control variables Potential difference of the supply, type and thickness of wire, and temperature of the wire.
Section 2 of 7Method
1
Connect a cell or low-voltage power supply (about 1.5 V), an ammeter in series, and a length of resistance wire taped to a metre ruler. Place a voltmeter in parallel across the wire section.
2
Attach the crocodile clips 10 cm apart on the wire.
3
Close the switch briefly and record the potential difference and current, then open the switch.
4
Move one clip to increase the length in 10 cm steps up to 100 cm, recording V and I each time.
5
Repeat the whole set and calculate mean values.
Section 3 of 7Processing results
1Calculate
- Calculate For each length, find R = V ÷ I.
2Plot
- Plot Graph resistance (y-axis) against length (x-axis) and draw a line of best fit.
3Interpret
- Interpret A straight line through the origin shows resistance is directly proportional to length.
4Explain
- Explain A longer wire gives electrons more ions to collide with, so resistance is greater.
Section 4 of 7Accuracy, safety and improvements
| Issue | Why it matters | What to do |
|---|---|---|
| Heating of the wire | Higher temperature raises resistance and ruins the pattern | Keep the current low; switch off between readings |
| Resolution of instruments | Ruler 1 mm, ammeter 0.01 A, voltmeter 0.1 V limit precision | Use the longest wire lengths for best precision |
| Hot wire or short circuit | Risk of burns | Do not touch the wire; use a low voltage supply |
| Poor contacts | Extra resistance at the clips | Press the clips firmly onto clean wire |
Section 5 of 7Series and parallel resistors (extension)
Combinations of resistors
In series
- In series Total resistance is the sum of the individual resistances, so it is larger than any one.
In parallel
- In parallel Total resistance is less than the smallest individual resistor because there are more paths for the current.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Independent variable | The variable you deliberately change. |
| Dependent variable | The variable you measure. |
| Directly proportional | Two quantities that increase in the same ratio, giving a straight line through the origin. |
| Resolution | The smallest change an instrument can detect. |
| Control variable | A variable kept constant so that it does not affect the result. |
Section 7 of 7Exam technique
1
Leaving the current on between readings, which heats the wire and increases its resistance.
2
Connecting the voltmeter in series or the ammeter in parallel.
3
Plotting length against resistance (axes swapped).
4
Claiming the graph proves proportionality even though the line does not pass through the origin.