Practical: Resistivity
Analysis
Explain how you might change the apparatus to calculate your value for the resistivity with greater resolution
Instead of a multimeter (which reads to two significant figures), use a voltmeter and an ammeter (which both read to three significant figures)
Explain why plotting a graph improves your accuracy
Makes it easier to spot anomalies
Equipment
Wire
Ruler
2 leads and crocodile clip
Multimeter
Micrometer
Method
Attach one lead to a fixed end of the wire and the other lead with the crocodile clip to the wire approximately 10cm from the fixed end. The leads should be connected to the multimeter
Measure the distance between the two leads and record the resistance on the multimeter
Change the distance in intervals of 10cm and record the resistance each time
Measure the diameter of the wire using the micrometer
Calculation
Plot a graph of R against l where the gradient is p/A and A is known
Safety
The voltages and currents are low so they don't present a hazard
Disconnect the wire between readings so that it does not get hot
Explain why you need to use a wire to find the resistivity of a metal and explain what shape of sample would be suitable for a plastic
A metal has low resistivity and a wire has a small cross-sectional area and a great length so the resistance is becomes measureable
A plastic has high resistivity so a high cross-sectional area and small length would give a measureable resistance
Identify the sources of uncertainty in this experiment
Contact resistance between the wire and plug
Resistance between the crocodile clip and wire
The graph will not go through the origin because there is some resistance in the leads and between the leads and the wire
Explain why the current through the wire should be small
The wire must not heat up because resistivity is temperature dependent