RP05: Determination of resistivity of a wire Flashcards
State the equation used to calculate the resistivity of a wire.
resistivity = (RA)/l
How does the resistivity of a wire change when the cross sectional area decreases
The resistance of a wire is inversely proportional to the cross sectional area and so as the area decreases, the resistance will increase
How does the resistance of a wire change when the length is decreased?
The resistance of a wire is directly proportional to the length of the wire so as the length of the wire decreases its resistance also decreases.
How does the resistance of a wire change if the resistivity is increased?
The resistance of a wire is directly proportional to its resistivity, so as resistivity increases, the resistance also increases.
What is the unit of resistivity?
Ohm metres
How do you work out the cross sectional area of thin wire?
Using a micrometer, measure the wire’s diameter in at least three different places along the wire. Then input the average diameter into the circular area equation.
Suggest how the length of a conducting wire can be varied when carrying out this experiment.
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.
What device is used to measure the potential difference across the wire and how is it connected?
A voltmeter, which should be connected in parallel across the wire.
What device is used to measure the current flowing through the wire and how is it connected?
An ammeter, which should be connected in series with the wire.
Why should the power supply be switched off between readings?
The temperature of the wire should remain constant throughout the experiment. Switching the power supply off between readings will help ensure this, by mitigating the effect of heating the wire.
Why should the temperature of the wire remain constant throughout this experiment?
Temperature changes can affect the resistance of the wire. In this experiment, temperature is therefore a control variable.
Why does the resistance of a wire increase when its temperature increases?
As the temperature increases, the metal ions gain more kinetic energy and so vibrate more. These vibrating ions make it harder for charge to pass through the wire and so the wire’s resistance increases.
How can the resistivity of a wire be determined from a graph of resistance against length?
The gradient will be R/L and so by multiplying the gradient by the wire’s cross sectional area, you get resistivity.
Why should the current be kept low?
As current increases, the temperature of the wire increases. By keeping current low, the heating effect is kept to a minimum.
