How the Length of a Wire Affects its Resistance

Topics: Ohm's law, Electric current, Orders of magnitude Pages: 7 (1404 words) Published: July 25, 2005
How the Length of a Wire Affects Its Resistance

In my physics coursework I am going to investigate the effect of the length of a wire on its resistance. Resistance is the measure of how easy it is for current to flow through a wire. Current is the rate of flow of charge through a conductor, and it is measured in amps using an ammeter.

To help me plan my investigation I have completed a preliminary experiment in which I investigated the effect of the thickness of a wire on its resistance, using three constantan wires and one copper wire. I found that the resistance increased as the thickness of the wire decreased, however, when I did the test with the copper wire I found that it was so unresistant that there would not be a lot of point in conducting my experiment using a copper wire because I would not really be able to compare the results. I therefore decided to use a wire of a higher resistance; constantan, because it will allow me to get more reliable and comparable results.

Preliminary experiment results:

Wire type/diameter Resistance (W)

Copper 0.2mm 1.8 W

Constantan 0.2mm 9.1 W

Constantan 0.45mm 5 W

Constantan 0.9mm 1.8 W


The things that I will measure and observe are the voltage running through the wire and the current. This will allow me to calculate the resistance of the wire. To calculate the resistance of the wire I will divide the voltage (V) by the current (I), which will give me the resistance in ohms (W).


The equipment that I will use is:

- power pack

- ammeter

- voltmeter

- 30 SWG (diameter) constantan wire

- 1 metre ruler

- leads

- crocodile clips


This is exactly what I will do:

1) I will set up my experiment as shown in the diagram, with the ammeter connected in series and the voltmeter connected in parallel.

2) I will stretch the wire out along the metre ruler and place the crocodile clips exactly 100cm apart.

3) I will make sure that the voltmeter and ammeter are switched on, and I will then turn on the power pack at a voltage of 2 volts.

4) I will record the voltage and the current, and to calculate the resistance of the wire I will do the calculation voltage divided by current. This will give me the resistance of the wire in ohms.

5) I will repeat the original test for 100cm a further two times, giving me a set of three results. I will then find the average of these results by calculating the mean.

6) I will then repeat the experiment for wire lengths of 20cm, 40cm, 60cm, 80cm, 150cm, 200cm, 250cm and 300cm.

I will make my test fair by keeping some things the same and changing one thing at a time. The things that I will keep the same are the temperature of the wire, by switching off the power pack after collecting each set of results, the voltage setting of the power pack, 2 volts, and the equipment, including the wire, that I will use. The thing that I will change is the length of the section of wire to be tested.

To ensure that my equipment is safe I will switch off the power pack after collecting the necessary results for each length of wire, to prevent overheating of both the equipment that I will be using and the wire. I will also wear safety goggles incase of any sparks or anything else that could damage my eyes.


I predict that as the length of the wire increases, the resistance of the wire will also increase, because there will be more ions and electrons for the free electrons to collide with. I think that if you double the length of a wire, then the resistance will double as well, because it is twice the length of wire, therefore it is also twice as hard for the electrons to travel through the wire, as there are more ions and electrons contained within it. I therefore believe that the increase in resistance of the wire will be directly proportional to the increase in the length of the wire.


Length of wire (cm) Test no. Voltage (v) Current (a)...
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