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Factors which affect the resistance of a wire

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GCSE physics Coursework

Factors which affect the resistance of a wire

For my GCSE physics coursework I am going to investigate the factors that affect the resistance of a wire.

What is electrical resistance?

Electrical resistance is a measure of how a certain material opposes the flow of an electrical current flowing through it. Electricity is conducted through a conduct, in the case of my experiment the wire.

The electricity is conducted across the material through free electrons; this is why metals are good conductors because they have free electrons. The more free electrons that a certain material has the better a conductor it is.

When energy is passed through free electrons they start to vibrate and collide with other free electrons; this then happens across the whole of the wire and electricity is conducted across it, this is caused by a potential difference.

Resistance is the result of energy loss as heat.It involves collisions between other free electrons, metal and impurities in the metal; these collisions convert some of the energy that the free electrons possess to heat.

Resistance is a force that goes against the flow of a current in an electric circuit so that more energy is required to move the electrons around the circuit.

When energy is passed to free electrons the free electrons pass the energy on to the atoms in a conductor causing them too vibrate as well. This decreases the chances of an electron passing through the material without colliding into its atoms, this is what causes resistance.

How is resistance measured?

The resistance of a length of wire is calculated by measuring the current present in the circuit (in series) and the voltage across the wire (in parallel). These measurements are then applied to this formula:

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The factors that I have chosen to investigate are how the thickness and length affect the resistance of a wire.

As the length of a wire increases so does the resistance of the wire in the same proportion. Example, if the wire was doubled the resistance would be doubled.  This is because wire has naturally some impurities within it so as the length of the wire increases so does the amount of impurities within the wire. Therefore if there are more impurities in the wire then the electrons will need more energy to push across the impurities that oppose the flow of electrons.  

I predict that the resistance will double as the length of the wire doubles, this will be in a linear relationship and the graph will be a straight line x = y providing that the wire doesn’t heat up.

I predict that as the diameter of a wire is doubled the resistance is decreased by a quarter. This is because resistance is inversely proportional to the cross sectional area, in my experiment this would be the area of the wire. The area is proportional to the diameter squared. So if you double the diameter the area increases by a factor of four and the resistance decreases by a factor of four as the two relationships are inversely proportional to each other. This means that as one increases the other decreases by a constant (k). I predict that the graph will be curved but never touching the X axis as the equation of the line will be y=1/x

I will draw a diagram to illustrate this.

From the diagram I can see that as the radius is doubled the cross sectional area of the wire quadruples.

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In order to extend my experiment for more accurate results and a general reflection of how the resistance changes as the cross sectional area of a wire changes I could repeat my experiment with different wires that have different resistances and see if a common pattern emerges for all the different wires.

I could also repeat with experiment with a wider range of thicknesses for the wires to get a better impression of the shape of the graph.

The wires for the experiment would also have to be straightened out properly before cutting to ensure that it is cut correctly and that the length if right.

Temperature is a very hard factor to control that affects the outcome of my results but the wire could be put in a water bath as this keeps the temperature constant and the results would be more accurate.

In conclusion both of my predictions are right, as the length of the wire doubles so does the resistance. This is due to the fact that the electrons have to push past twice the amount of force that opposes their flow therefore the resistance is doubled.

My prediction as the diameter of the wire is doubled the resistance decreases by a quarter is true. The relationship between them is inversely proportional and this is proved by my graph. This is due to the fact that as the diameter is doubled the surface area increases by a factor of four, this means that the electrons within the wire has four times as much more space to move through the wire therefore there is less obstruction to their flow.

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