Required Practical: Investigating Resistivity

What is the indpendent variable of RP-Investigating Resistivity?

Length, L, of the wire (m)

What is the dependent variable of RP-Investigating Resistivity?

The current, I, through the wire (A)

What is the control variable of RP-Investigating Resistivity?

• Voltage through the wire

• The material the wire is made from

Steps for required practical - Investigating resistivity

• Measure wire diameter

  • Use a micrometer to take 5–10 diameter readings at random points along the constantan wire.

  • Calculate the mean diameter.

• Set up the apparatus

  • Tape or clamp the wire along a ruler so one end of the circuit connects at the 0 cm mark.

  • Connect the ammeter in series and the voltmeter in parallel across the wire.

• Initial measurement

  • Attach the flying lead to the wire at 0.25 m.

  • Set the power supply to 6.0 V, and check this voltage on the voltmeter.

• Take current readings

  • Switch on the supply, read the current, then switch off immediately to prevent heating.

  • Repeat this at least 3 times for each length and calculate an average current.

• Vary the length

  • Move the flying lead in 0.25 m intervals (or another interval giving 8–10 total lengths) up to the full 2.0 m wire.

  • Record repeated current readings for each length.

• Calculate resistance

  • For each length, use the average current with the measured voltage to calculate the average resistance:

What is the first step of RP - investigating resistivity?

Measure wire diameter

• Use a micrometer to take 5–10 diameter readings at random points along the constantan wire.

• Calculate the mean diameter.

What is the second step of RP - investigating resistivity?

Set up the apparatus

• Tape or clamp the wire along a ruler so one end of the circuit connects at the 0 cm mark.

• Connect the ammeter in series and the voltmeter in parallel across the wire.

What is the third step of RP - investigating resistivity?

Initial Measurement

• Attach the flying lead to the wire at 0.25 m.

• Set the power supply to 6.0 V, and check this voltage on the voltmeter.

What is the fourth step of RP - investigating resistivity?

Take current readings

• Switch on the supply, read the current, then switch off immediately to prevent heating.

• Repeat this at least 3 times for each length and calculate an average current.

What is the fifth step of RP - investigating resistivity?

Vary the length

• Move the flying lead in 0.25 m intervals (or another interval giving 8–10 total lengths) up to the full 2.0 m wire.

• Record repeated current readings for each length.

What is the sixth step of RP - investigating resistivity?

Calculate resistance

• For each length, use the average current with the measured voltage to calculate the average resistance:

• R = V/I

How do you calculate cross sectional area of wire?

pi(d)²/4

What are sources of systematic errors in RP?

• The end of the wire that is attached to the circuit (not the flying lead) must start at 0 on the ruler

  • Otherwise, this could cause a zero error in your measurements of the length

What are sources of random errors in RP?

• Only allow small currents to flow through the wire

  • The resistivity of a material depends on its temperature. The current flowing through the wire will cause its temperature to increase and affect its resistance and resistivity. Therefore the temperature is kept constant and low by small currents

• The current should be switched off between readings so its temperature doesn't change its resistance

• Make at least 5-10 measurements of the diameter of the wire with the micrometer screw gauge and calculate an average diameter to reduce random errors in the reading

What are the safety considerations of RP?

• When there is a high current, and a thin wire, the wire will become very hot.

  • Make sure never to touch the wire directly when the circuit is switched on

• Switch off the power supply right away if you smell burning

• Make sure there are no liquids close to the equipment, as this could damage the electrical equipment