Investigating the resistivity of constantan

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Fozia Saeed 6H        Physics Coursework        15/12/04

Investigating the resistivity of constantan

The aim of this experiment is to calculate the resistivity of constantan.

Resistivity of a material is a property of that type of material. It does not depend on the dimensions of the component/device, only from the materials from which it is made.

The equation for resistivity is:

Resistivity = Resistance x Cross-sectional area                p=RA

                        Length                                              l

R = Ohms (Ω)

p = Ohms x metre (Ωm)

l = Metres (m)

A = Metres x metres (m2)

There are 3 variables that can affect this experiment and these are as follows:

  • Length
  • Cross-sectional area
  • Resistance

Length can be controlled in this experiment and I have decided to vary the length of my wire. I will have 10 different lengths of wire. However, I have decided to keep the area constant by using the same thickness of wire for the 10 different lengths as it is only fair to change one thing at a time. Resistance is dependant on the voltage and current produced. Therefore, I will be changing voltage in the circuit using a variable resistor and recording the current for those voltage readings. Temperature is also a factor that can affect the resistivity of constantan but for this experiment we are assuming it is constant and therefore, will be ignoring it.

Apparatus

  • Wire (100cm long and 0.295cm thick)
  • Power supply (Direct current): Range 0V-6V
  • Connecting Wires
  • 2 Crocodile clips
  • Ammeter (Digital multimeter 20A)
  • Voltmeter (Digital multimeter 10V)
  • Meter ruler
  • Screw gauge
  • Cello tape
  • Variable resistor

Diagram

Method

  • Firstly, I will collect all the apparatus
  • I will then set it up as shown in the diagram above and I’ll make sure that there are no loose connections
  • I will then cut 100cm (and a bit extra) length of wire and place it on top of a meter ruler. I will cello tape it down and make sure the wire is as straight as possible
  • The tape will be stuck on away from the desired points where I’ll put my clips to take readings
  • I will begin with a length of 10cm, so I will place one crocodile clip just before the 0cm mark and the other crocodile clip will be placed just after the 10cm mark on the ruler

  • I will then measure the voltage and current. I will be changing the voltage using a variable resistor and I will take 10 different voltage and current readings
  • I will repeat the above two steps for another 9 different lengths of wire
  • I will record the readings and then plot V/I graphs
  • I will then plot resistance against length and interpret their relationship
  • I will then finally work out resistivity of each length and find the average.
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The 10 different lengths I will be using in this experiment are 10cm, 20cm, 30cm, 40cm, 50cm, 60cm, 70cm, 80cm, 90cm and 100cm. I will be measuring current at voltage readings 0.05V, 0.10V, 0.15V, 0.20V, 0.30V, 0.35V, 0.40V, 0.45Vand 0.50V. I have decided to take 10 different readings because I’ll have a range of values and it’s also manageable. I have decided to use a range of low voltages because this means I will get low current readings and therefore, the wire heats up less resulting in more accurate results.

Safety considerations

  • Safety goggles will be ...

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