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Resistance of a wire.

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Physics AT1, Resistance of a wire.



Electric current, the flow of electric charge, usually in the form of electrons. In a circuit, it is measured using an ammeter, and the SI unit of measurement is the ampere (A). It may be direct current (D.C.), in which case the charge flow is one way, as from a battery. Or it may be alternating current (a.c.), as from a mains supply. Here, the charge flows alternately backwards then forwards in a circuit many times every second. The equation for current is I=Q/T; A=C/S; current= charge/time.


Resistance, the ratio of the potential difference (p.d. or voltage) across a conductor to the electrical current which flows through it as a result. The unit of measurement is the ohm (O), this being the resistance of a conductor requiring a potential difference of 1 volt across its ends to produce a current of 1 ampere. For a given metal conductor at constant temperature the value is the same whatever the current (Ohm's law), but rises if the temperature rises the equation for Ohms law is V=IR; Voltage (p.d.) =current x resistance. Any conductor possessing resistance gives off heat when a current flows through it.

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Voltage (V)

Resistance (Ω)



Length (cm)

Average Current (A)

Average Voltage (V)

Average Resistance (Ω)










  • 100 cm of constantan wire
  • Voltmeter
  • Ammeter
  • Crocodile clips
  • Leads
  • Power pack
  • Meter rule



  1. I will measure out 100 cm of the constantan wire
  2. I will set up the apparatus as in the diagram
  3. I will take three readings from the ammeter and the voltmeter and the voltage of the power pack will be at 5 volts
  4. Repeat at
  • 90 cm
  • 80 cm
  • 70 cm
  • 60 cm
  • 50 cm
  • 40 cm
  • 30 cm
  • 20 cm and
  • 10 cm


Before starting the experiment I will test all the equipment to make sure that all the equipment is not faulty. I will also do the experiment at a safe voltage. During the experiment I will turn the power pack on then quickly take a reading then switch it off.

Fair testing:

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The results that I have obtained support my original prediction. This is because in the prediction I said that as the wire length increased, the resistance should increase. I also said that the link should be directly proportional. The results have shown that this is true.

The line of best fit clearly shows that the results followed the expected pattern very well. The points are very close if not touching the line. This shows how the results were directly proportional through out, as the gradient remained the same.


I think that my investigation was performed to a fairly high standard and good quality. I think that if I were to investigate another factor as well as the length, this investigation would have been improved by quite a lot. Also I feel that if I were to investigate the different types wires and their resistance in conjunction with the length, this piece of coursework would have been of an extremely high standard and of an excellent quality.  

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