# The formulae used to calculate - Sample Essay

Coursework: Investigate the factors which affect the resistance of a wire What is resistance? The dictionary defines resistance as: the property of any object or substance of resisting or opposing the flow of an electrical current. Put simply it is the ability of a material to restrain the amount of current going through it. There are many factors that can affect the resistance of a piece of wire. Length, thickness, material and temperature of the wire can all affect its resistance to current.

For example as we found out in class work in year 8, 20cm of copper wire has far less resistance than a 20cm length of nickel wire, simply because copper is a better conductor. Resistivity is the measure of how much a substance can restrain electrical current. This will also be calculated using my final results, comparing this to the official resistivity of the wire will also tell me how accurate my results were. I am going to investigate how the length of a piece of wire affects its resistance. For this experiment the wire we used was an alloy, rather than copper.

This is because copper is such a good conductor that you would have to use ridiculously long pieces of it to notice any difference in resistance between one length and another. I will investigate this by setting up a circuit to measure both voltage and current. Then I will put different lengths of wire into the circuit to get results. For every length of wire (going up in 10cm increments, all the way up to 100cm) I will measure the current at 6 different voltages and plot them on a graph of voltage to current for each length of wire.

The lengths of wire have to start at 20cm because, as we found out in previous experiments, anything less than that and you can’t obtain results for anything less than 3 volts because the wire gets red hot and melts (as we found in our preliminary experiments in previous class time. ) This is because of too much current flow. To make this a fair test all wire must be of the same material and thickness and must all be at the same temperature. All components in the circuit except for the variable must say the same.

Drawing the graph for each length of wire will ensure measurements are as reliable and accurate as possible by averaging out the results. Apparatus needed for this experiment: Power supply, Variable resistor, Wires (crocodile clips), Voltmeter, Ammeter, Metre ruler, Wire. Experimental set-up Safety problems When a lot of current flows through a wire it tends to get bit warm, sometimes even glowing red hot, touching wire when in this state could cause severe burns and if it touched work surfaces at this temperature it could be a fire hazard.

To deal with this we elevate the wire using crocodile clips and stand and clamp. This stops it touching all surfaces. Predictions My predictions are that by increasing the length of the wire we will also increase the resistance. This is because of the class work we did on resistors. This showed that two equal resistors, connected in series, is equal to one resistor with the resistance of both resistors added together. e. g.

This would also apply to wire. Effectively if you double the length of the wire, you double the resistance. e.g. I also predict that the thicker the wire, the less resistance e. g. Resisters in parallel (thickness of wire) Double the thickness = Halved resistance Triple the thickness = 1/3 resistance Quadruple the thickness = 1/4 resistance Preliminary Testing I did a preliminary investigation of 50cm of wire to re-enforce my predictions. The graph I produced also tested the formulae used to calculate the average resistance for the length of wire. P. D (Volts) Current (Amps)

The graph (next page)re-enforced my predictions and confirmed that the correct method for calculating the resistance was being used. At 50cm of wire I did an average current table to prove that the results did not vary enough to warrant doing it for every measurement. P. D Current 1 Current 2 Current 3 Average .