Resistance required practical

Aim: To investigate how the length of a wire affects its resistance, and how the total resistance of resistors changes when they are combined in series and in parallel.

Variables

• Independent: Length of the wire (part 1); how the resistors are connected, series or parallel (part 2)
• Dependent: Resistance, calculated from current and potential difference
• Control: Material of the wire; Thickness (diameter) of the wire; Temperature of the wire (keep the current low); Potential difference of the supply

Equipment

• Constantan or nichrome wire fixed along a metre rule
• Crocodile clips
• Ammeter
• Voltmeter
• Battery or low-voltage power supply
• Switch
• Connecting leads
• Two or more fixed resistors of known value

Method

1. Tape the wire along a metre rule and connect a crocodile clip at the zero mark.
2. Build a circuit with the ammeter in series with the wire and the voltmeter connected across the length being tested.
3. Attach the second crocodile clip at 10 cm from the first.
4. Close the switch, record the current and potential difference, then open the switch straight away.
5. Move the clip in 10 cm steps up to 100 cm and repeat the readings each time.
6. Calculate the resistance for each length using R = V ÷ I.
7. For part 2, connect two resistors in series, measure V and I across the pair and calculate the total resistance.
8. Repeat with the same resistors connected in parallel, then compare both totals with the single resistor values.

Safety

• The wire can get hot when current flows: switch off between readings and do not touch the wire while it is on.
• Keep the potential difference low to avoid overheating and burns.
• Check leads for damaged insulation before starting.

Results and calculations

Record length, current, potential difference and resistance in a table, using R = V ÷ I (from V = I R). Plot resistance against length; a straight line through the origin shows resistance is directly proportional to length. For resistors, series total R_total = R₁ + R₂, while the parallel total is less than the smallest single resistor.

Common mistakes and improvements

• The wire heats up, which increases its resistance: use a small current and switch off between readings.
• Measuring length from the wrong point, or crocodile clips that are wide: always measure from the inside edge of the first clip to the inside edge of the second.
• The line may not pass through the origin because of contact resistance at the clips: this is a systematic error, so make sure clips are tight and clean.
• Wires that are kinked or not straight give an inaccurate length: pull the wire taut along the ruler.

Exam tips

• Describe the result clearly: resistance is directly proportional to length, because electrons collide with more ions in a longer wire.
• Be able to draw the circuit with correct symbols, especially the voltmeter in parallel across the wire.
• Explain why the parallel total is lower: adding a branch gives the current an extra path, so the total current increases for the same potential difference.
• Expect questions about why a graph does not go through zero; mention a zero error or contact resistance.
• State that keeping the current low keeps the temperature constant, which is a control variable.