PAG 03.1 Determination Of Resistivity

Question 1

State the equation used to calculate the resistivity of a wire

Answer 1

p=RA/L

Question 2

How does the resistance of a wire change when the cross-sectional area decreases?

Answer 2

The resistance of a wire is inversely proportional to the cross-sectional area and so as the area decreases, the resistance will increase

Question 3

How does the resistance of a wire change when the length is decreased?

Answer 3

The resistance of a wire is directly proportional to the length of a wire, so as the length decreases the resistance also decreases

Question 4

How does the resistance of a wire change if the resistivity is increased?

Answer 4

The resistance of a wire is directly proportional to the resistivity, so as the resistivity increases, the resistance also increases

Question 5

What is the unit of resistivity?

Answer 5

Ohm-metre

Question 6

How do you measure the cross-sectional area of a thin wire?

Answer 6

Using a micrometer, measure the wire’s diameter in at least three different places along the wire. Use the average diameter in the circular area equation

Question 7

Suggest how the length of conducting wire can be varied when carrying out this experiment

Answer 7

One end of the wire can be fixed and the other end can be connected to the circuit using a crocodile clip. The length of conducting wire can be changed by varying the position of the crocodile clip

Question 8

Describe how the length of the wire should be measured

Answer 8

By a metre ruler, with the wire held taught. This reduces the likelihood of an incorrect measurement due to kinks in the wire

Question 9

What device is used to measure the potential difference across the wire, and how is it connected?

Answer 9

A voltmeter should be connected in parallel across the wire

Question 10

What device is used to measure the current flowing through the wire, and how is it connected?

Answer 10

An ammeter should be connected in series with the wire

Question 11

Why should the power supply be switched off between readings?

Answer 11

The temperature of the wire should remain constant throughout the experiment. Switching the power supply off between readings will reduce the heating of the wire

Question 12

Why should the temperature of the wire remain constant throughout this experiment?

Answer 12

Temperature can affect the resistance of the wire. In this experiment temperature is a control variable

Question 13

Why does the resistance of a wire increase when it’s temperature increases?

Answer 13

As temperature increases, the metal ions gain more kinetic energy and so vibrate more. These vibrating ions make it harder for the charges to pass through the wire and so the wire’s resistance increases

Question 14

How can the resistivity of a wire be determined from a graph of resistance against length?

Answer 14

The gradient of the graph will be R/L and so by multiplying the gradient by the wires cross-sectional area, you will obtain the wire’s resistivity

Question 15

Why should the current used in this experiment be kept low?

Answer 15

As current increases, the temperature of the wire will increase. By keeping the current low, the heating effect is kept to a minimum

Question 16

Suggest how you could ensure that your length measurements are taken from the same position each time

Answer 16

A metre ruler could be taped in place below the fixed wire

Question 17

What factors lead to uncertainties in this experiment?

Answer 17

There will be resistance between the crocodile clips and wire as well as at the contact of the leads and the power supply. There may also be a zero error due to the positioning of the ruler and crocodile clip at the zero end

Question 18

What device could replace the voltmeter and ammeter in this experiment?

Answer 18

Instead of a voltmeter and ammeter, a multimeter could be used to measure current and potential difference
Note that this may lower the resolution of your data depending on the number of significant figures provided by the devices you have available

Question 19

How can the percentage difference in your experimental value and accepted value be calculated?

Answer 19

(Your value-accepted value)/accepted value x100

Question 20

Does the resistivity of a material change depending on dimensions?

Answer 20

Resistivity is a material property and so is fixed for a given material, regardless of dimensions