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Resistance of Wire

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Abdullah Jafar Chowdhury                Resistance of Wire

26/07/2006                Science

Resistance of Wire



To investigate how the different lengths of a wire, between 70cm and 0cm, affects its electrical resistance.

Background Knowledge:

        Electrical resistance is a measurement of the degree to which an object opposes the flow of electric current.

The quantity of resistance in an electric circuit determines the amount of current flowing through it for any given voltage.




   R    Iimage14.pngimage02.png





R: Is the resistance of the object, usually measured in ohms (Ω).

V: Is the potential difference across the object, usually measured in volts (V).

I: Is the current passing through the object, usually measured in amperes (A).

This is how you measure the resistance of a wire.

Causes of resistance:

  • In metals:

A metal is made out of a layout of atoms, each with a shell of electrons. This can also be known as positive ionic layout. The outer electrons are free to separate from their parent atoms and travel through the layout, creating a 'sea' of electrons, making the metal a conductor. When an electrical potential difference (voltage) is applied across the metal, the electrons drift from one end of the conductor to the other under the pressure of the electric field. In a real material the atomic layout is never perfectly regular, so their deficiency alters the path of the electrons and cause resistance.

...read more.


1wooden board – the wires are placed on this piece of wood. Wood is used as it will not conduct the electricity1 Ammeter – to measure the current. A digital ammeter should be used for increased accuracy1 Voltmeter – to measure the voltage. If available, use a digital voltmeter, as this will increase the accuracy of the readings1 Ruler – used to measure the wires to different lengths, the whole purpose of this investigation

I have decided on using a power pack rather than the mains supply because unlike the power pack its voltage cannot be decreased or increased. The mains supply voltage is 240 volts whereas on the power pack the voltage can be controlled. I will do this only once. I know that the more I do the experiment the more reliable results I will get. However, due to time constraints I am, only able to do the experiment once.

Independent variables:

  • The length of the wire

Dependant variables:

  • The resistance of the wire

Controlled variables:

  • The voltage. If I did not control this variable I would have different readings every time I took a reading. By keeping it the same I know that the same amount of power will be going through the wires and it makes it a fair test. The different readings would come from the voltmeter and the ammeter, which in turn would give me unreliable results.

I will be recording the results showing the dependant variable. I will do this by putting it in a table form, comparing the resistance of various lengths. The difference in resistance could also be taken into consideration to check which wire is a better conductor. This would be done by inserting the resistance of the two different wires into a table form.

Fair Test:

To make sure that the results of the experiment are accurate, I need to make sure that certain variables are kept constant in order for a fair test. These are:

  • The lengths of the wires must be the same.
  • The current passing through the wires must be the same

These factors must be kept the same throughout the experiment so these are called constant variables; however I shall change the lengths of the wires so this will be the independent variable.

Safety Feature:

        I will have to be careful with this experiment as with all experiments. I will have to switch off the power supply after every reading to prevent the wires overheating. I will also have to tighten all the connections to get the proper results as well as to prevent electrocution. I will also have to check the polarities and make sure the wires are not touched with wet hands to prevent electrocution.

Measurements and Justification

        I will be calculating the electrical resistance of a wire by measuring its current and voltage; and how it changes with its length. This is because changing the length of the wire will affect the electrical resistance. I will do this by moving the crocodile clips up and down the wire which will change the length of the wire that the electrons will manage to flow through. I will do this seven times, from 70cm -60cm, from 60cm -50cm and so on and so forth. I will have to do this because this is a requirement for the experiment. This will show me clearly how the resistance in the wire changes with its length. I will then use an ammeter to record the current that is going through the wire. To measure the voltage I will be using a voltmeter. And lastly to calculate the resistance, I will be using the formula: R= V/1.



Note the width of the wire, what type of wire it is and the voltage passing through the wire.

  • Firstly assemble the circuit as shown in the diagram of apparatus using one type of wire at a time (start with any wire). Check to see if the wires have been connected the right way round. The positive side should connect to the negative side and vice versa. If this is not correct then the experiment will not work because the electrons will not flow.
  • When the circuit is complete; turn on the power supply to voltage setting 3.5. This voltage setting was chosen to prevent overheating which may cause the wire to melt as well as posing a risk from burns.
  • Let it run for about 5 seconds while the readings fluctuate then record the amps and volts. Always make sure that the power pack is not left on for a longer time period. This is to prevent overheating.
  • Turn off the battery pack and change the length of the wire that is being tested by moving one crocodile clip down by ten centimetres, which is the purpose of the experiment. Switch it back on and record the results
  • Repeat from the second point to the fourth point for both wires, with the exact same conditions.
...read more.


R=v/1 (Background Knowledge), I have calculated, electronically, the resistance; which is shown in my Analysis of results.

        The limitations of my experiment were the absence of adequate apparatus. In the laboratory I had to make use old voltmeters and ammeters with an analogue reading. With a digital voltmeter and ammeter, I could have more sensitive readings, as well as an easily legible display. With an analogue display, there is a possibility that I may have misread the readings.

        I received some anomalous results when I was working on the Nickel wire. This was because, whilst I was testing 20cm and 60cm of the wire, the readings on the ammeter and the voltmeter were constantly fluctuating. I then took an average reading in between the two extremities on the readings. These anomalous results are shown on the graph above.

        If I were to do this experiment again, I would check the circuits carefully before every test to make sure there are no faults, this would prevent any anomalous results.

        To make the experiment more reliable, I could have repeated the process, increased the accuracy of the variable and also look into the anomalies on the graph.



  • Booth, Graham.  McDuell, Bob. (2002) Modular Science for Edexcel (Modules 7-12) Oxford, Heinemann
  • Booth, Graham.  McDuell, Bob. (2002) Modular Science for Edexcel (Modules 1-6) Oxford, Heinemann


  • http://regentsprep.org/Regents/physics/phys03/bresist/default.htm
  • http://en.wikipedia.org/wiki/Electrical_resistance

...read more.

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