Question 2 — Resistance of a wire (slide-wire potentiometer)
Slide a contact along ~105 cm of taped-down constantan wire, reading current and the p.d. across increasing lengths BC, to find how resistance depends on length.
RUN IT ON THE BENCH
Your readings feed straight into the answer table below as you record them.
ON YOUR BENCH — FROM THE CONFIDENTIAL INSTRUCTIONS
Question 2 — resistance-of-a-wire apparatus
- power supply(1)
- switch(1)
- ammeter(1)
- voltmeter(1)
- constantan (Eureka) resistance wire(1)
- terminals B and C(2)
- sliding contact S(1)
- connecting leads
THE BENCH — SLIDE-WIRE: TERMINAL B, THE WIRE, SLIDING CONTACT S, TERMINAL C
YOUR R–d GRAPH
A straight line through the origin means R ∝ d — resistance is proportional to length for a uniform wire. The gradient is the wire's resistance per centimetre.
Why doesn't the current change when I slide S?
The ammeter sits in the main loop — supply, switch, ammeter, the whole wire from B to C, back to the supply — so it reads the current flowing through the entire wire, not just the B-to-S section. Moving S only changes where the voltmeter taps in; it doesn't open or close any part of the main loop, so I stays essentially constant throughout.
Why is the voltmeter connected between B and S, not B and C?
You're investigating how resistance depends on length, so you need the p.d. across exactly the length you're measuring — from B to wherever S touches the wire. Tapping at C instead would always give you the p.d. across the full 100 cm, whatever length you thought you were testing.
Why must the wire be uniform, and only taped down at its ends?
Resistance depends on both length and cross-sectional area (R = ρL/A). Taping it down along its whole length risks stretching or kinking it unevenly, changing its area from point to point. Leaving it free between the two anchors keeps it uniform, so length is the only thing changing between readings — exactly what R ∝ d requires.
YOUR TASKS · 0/3
- ✓Close the switch and record the steady current I
- ✓Record V and R at all five sliding-contact positions (20–100 cm)
- ✓Read the wire's resistance-per-cm off your graph's gradient
TABLE 2.1 — YOUR READINGS
Close the switch, record I, then slide S to a marked position and record a reading.
YOUR ANSWERS
Close the switch. Measure the current I in the circuit. Include the unit.
Place the sliding contact at d = 20.0 cm from B and measure the p.d. V across BC; record d and V in Table 2.1, open the switch, then calculate R = V / I and record it [2]. Close the switch and repeat for d = 40.0, 60.0, 80.0 and 100.0 cm [3].
| d /cm | V /V | R /Ω | |
|---|---|---|---|
| d = 20.0 cm | |||
| d = 40.0 cm | |||
| d = 60.0 cm | |||
| d = 80.0 cm | |||
| d = 100.0 cm |
Complete the column headings in Table 2.1.
Plot a graph of resistance R/Ω (y-axis) against length d/cm (x-axis). Draw a best-fit line.