Core Practical 2 - Determine the Electrical Resistivity of a Material

Question 1

What is the aim of this practical?

Answer 1

To Determine the Electrical Resistivity of a Material

Question 2

What equipment is required for this practical?

Answer 2

• test wire
• flying lead with a crocodile clip
• voltmeter
• ammeter
• metre rule
• micrometer
• power supply

Question 3

What is the first thing to do in this practical?

Answer 3

use a micrometer to measure the diameter of the test wire , this should be done at at least 3 points and an average should be taken

Question 4

What do we do when we have the diameter?

Answer 4

We half it to get the radius and use (pi)r^2 to get the cross-sectional area

Question 5

What is the procedure to obtain measurements from there?

Answer 5

Clamp the test wire to a ruler (one end should read 0 to avoid zero error).
Attach the flying lead to the test wire (we use a flying lead and a crocodile clip to attach this to any point along the wire).
Record the length, pd and current at that length.
Work out resistance by using V/I = R
Repeat for several different lengths at regular intervals

Question 6

What do we plot for this practical ?

Answer 6

Resistance against length (R = pL/A)

Question 7

What is the gradient of this graph?

Answer 7

p/A , so we multiply by area to get our resistivity

Question 8

What precautions can we take to increase accuracy of this practical?

Answer 8

• ensure wire is straight to make sure the reading of length is accurate (make it taut)
• avoid pressure on the wire, as deformation can change the reading of area and
• use an ohm-meter instead of an ammeter and voltmeter
• try to keep temperature constant (although we will always have a systematic error due to the heating effect of current going through the wire)
• ensure resistance is zero with no current (this is a systematic error and will shift the graph upwards)