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To investigate resistance in a wire when its length is varied.

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Investigation into electrical resistance of a metal wire

Aim- To investigate resistance in a wire when its length is varied

Preliminary information-

In a metal the electrons are free to move around in what is sometimes called “a sea of electrons.” All materials resist the flow of electricity through them. This is because when electrons move through a conductor such as a metal they collide with the atomic lattice of the conductor material because the protons are also moving slightly due to their thermal energy.

When an electron collides with the protons in the atomic lattice it will lose some of the kinetic energy of its motion along the conductor. This energy loss is the cause of electrical resistance.

The more resistivity a material has, the more insulating its effect is and if a material has little resistivity it is a good conductor.  Resistance is defined by the following equation:         

resistance R=        potential difference V

                                current I

This is Ohm’s law. It means that the amount of steady current through a resistor is directly proportional to the potential difference across the resistor. Therefore if the voltage between two ends of a wire quadruples the current going through the wire will quadruple and the ratio V to I will stay the same.  Ohm’s law does not work for every material but for most of them.

The unit of resistance must therefore be the volt per ampere but this is given the name ohm, which is represented by the Greek letter omega: Ω.

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I will keep all these factors the same except length, which I am investigating, in order to keep it a fair test. I will also use the same equipment throughout the experiment including the same piece of wire.    

Apparatus        -A piece of constantan (resistivity at 25˚C=49 x 10   Ω/m) wire 1.1m                     long to use to measure resistance at different lengths. I will cut it at 1.1m and wind it round the safety pins so that it is kept firmly in the desk

-A metre rule to measure how far down the wire to clip the crocodile clips for each reading

                -Two drawing pins to keep the wire fastened to the desk

                -A multimeter set to ohmmeter setting to measure the resistance

                -Two leads to attach the multimeter to the constantan wire

                -Two crocodile clips to attach the leads to the wire

                -Wire cutters to cut the wire at the beginning of the experiment

Prediction-        I predict that as the length of my wire increases so will my resistance and it will do this in proportion to the length. I predict that my resistance at 1m will be double that at 0.5m because there will be double the amount of atoms for the electrons to collide with so there should be twice the number of collisions. I also predict that my resistance at 0.9m will be three times my resistance at 0.3m. I know this because R ∝  l.

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I have a lot of confidence in my results because I have repeated my experiment a few times and every time I have repeated it I have come up with the same results. To make my experiment even more reliable I could repeat it again but I think that to repeat it three times is sufficient.

To further this experiment I could investigate longer lengths of wire or shorter lengths of wire. I would carry out the experiment very similarly to how I carried out this one. I would collect a range of values from 1.1m to 2.0m in 10cm intervals. I would keep other things constant as in original experiment. I would carry out the method as following:

-Set up electrical circuit as shown in diagram including cutting the piece of constantan wire to different lengths with wire cutters

-Record resistance for different lengths of wire, clipping different wires in between the two crocodile clips  

-Repeat experiment three times to improve reliability in case some results were incorrect and take an average

  I could also investigate length using different materials such as                 . I would use the method I used in the original experiment to do this and I would just change the material to        .  

Bibliography        - Key Science Physics by Jim Breithaupt

  • www.physics.gla.ac.uk

- University of Bath science: Physics by Robert Hutchings

Imogen Hagarty                

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