practical 5 - resistivity of a wire

• equipment

  • 1m long constantan wire
  • voltmeter
  • ammeter
  • low voltage power supply
  • micrometer
  • metre ruler
• method

  • measure diameter of wire at various point using micrometer and record mean diameter
  • set up apparatus
  • adjust length to 0.1m using crocodile clips
  • record current on ammeter and voltage on voltmeter
  • calculate resistance
  • $R=$$\frac{V}{I}$
  • increase length by 0.1m and repeat increasing by 0.1m until 0.8m
  • repeat experiment twice more and record mean resistance for each length
• graphs and calculations

  • calculate cross-sectional area of the wire
  • $A=$$\frac{\pi d^2}{4}$
  • plot graph of mean resistance against length and draw line of best fit
  • resistivity = gradient x cross-sectional area
  • $\rho=$$\frac{RA}{l}=$$GA$
  • G is the gradient
• safety

  • wire can heat up and burn skin
  • wire can snap and injure

  precautions

  • disconnect crocodile clips inbetween measurements to avoid wire heating up
  • reduce voltage using variable power supply to ensure current doesn't rise too high
  • wear safety goggles
• improvements

  • wire heating up can cause change in resistance so disconnect between measurements or reduce voltage
  • wire should be free from kinks and held straight so length is accurate