RP05 - Determination of Resistivity of a Wire

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

  The resistance of a wire is inversely proportional to the cross-sectional 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 directly proportional to the length of the wire, so as the area 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
• How do you measure the cross sectional area of a 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 the 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 of 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 they vibrate more
  • These vibrating ions make it harder for charge to pass through the wire, causing the wire's resistance to increase
• How can the resistivity of a wire can be determined from a graph of resistance against length?

  • The gradient of the graph will be R/L
  • By multiplying the gradient by the wire's cross sectional area, you will get the wire's resistivity
• Why should the current, in this experiment, be kept low?

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

  • p = RA/L
  • R = pL/A

  where:

  R = y

  p/A = m

  L = x