# Homework Help: Combined resistance of a cell and an ammeter

1. Nov 11, 2007

### fickle

[SOLVED] Combined resistance of a cell and an ammeter

1. The problem statement, all variables and given/known data

I had to do a lab where a 105cm 28 swg wire was used to find the combined resistance of a cell and an ammeter. The only apparatus was a switch, 1.5v cell, 0-1A ammeter, 105cm & 20cm wires and connecting wires.

Switch, cell and ammeter are connected in series, and the 105cm wire is connected at the ends of the switch and ammeter. For 6 different lengths of the wire, the current readings are recorded. After that, the 20cm wire is inserted between the switch and the cell in the circuit, and the current readings are recorded for the same lengths of the 105cm once more.

Results:

For lengths increasing from 15cm to 90cm:

Current without 20cm in circuit/A:
0.36, 0.30, 0.26, 0.22, 0.20, 0.16

Resistance without 20cm in circuit/Ohms:
4.17, 5.00, 5.77, 6.82, 7.50, 9.38

Current with 20cm in circuit/A:
0.24, 0.22, 0.20, 0.18, 0.16, 0.12

Resistance with 20cm in circuit/Ohms:
6.25, 6.82, 7.50, 8.33, 9.38, 12.50

2. Relevant equations

Two graphs were plotted of the equation R = k(1/S) - r

Where R is the resistance of the length of the 105cm wire, k is a constant, S is the current reading and r is the resistance of the other components in the circuit, assuming the connecting wires have negligible resistance.

3. The attempt at a solution

Well, I've plotted both graphs of R against 1/S; finding R from V/I for the observations. k is the gradient, and it's the same value on both graphs, 1.5. It was advised to extend the y-axis (R) to -2 ohms. -r, the y intercept and the resistance of the cell and ammeter that I am supposed to find is zero on both graphs. Is this possible? Can it really be zero?

I'm wondering if my teacher set up the lab to give this value by changing the suggested wires to be used in the experiment, to test a skill we're supposed to be assessed by in our labs. I just want to know if it's realistic to get a dead zero y-intercept for a graph of this equation, or if I measured something incorrectly during the lab.

Any help would be appreciated! Thanks!