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# Resistance of a Wire

Extracts from this document...

Introduction

Aidan Morrison 11C

Resistance of a Wire

To investigate how the resistance of a wire is affected by the length of the wire.

Theory

What is resistance?

Electricity is conducted through a conductor, in this case wire, by means of free electrons. The number of free electrons depends on the material and more free electrons means a better conductor, i.e. it has less resistance. For example, gold has more free electrons than iron and, as a result, it is a better conductor. The free electrons are given energy and as a result move and collide with neighboring free electrons. This happens across the length of the wire and thus electricity is conducted. Resistance is the result of energy loss as heat. It involves collisions between the free electrons and the fixed particles of the metal, other free electrons and impurities. These collisions convert some of the energy that the free electrons are carrying into heat.

How is it 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:

V = I ´ R                     where V = Voltage, I = Current and R = Resistance

This can be rearranged to:

R = V

I

Ohm’s Law

It is also relevant to know of Ohm’s Law, which states that the current through a metallic conductor (e.g. wire) at a constant temperature is proportional to the potential difference (voltage). Therefore V ¸ I is constant. This means that the resistance of a metallic conductor is constant providing that the temperature also remains constant.

Middle

http://www.regentsprep.org/Regents/physics/phys03/bresist/default.htm
The experiment was only carried out once

Since Ohm’s law isn’t accurate if the temperature isn’t constant the results found in the preliminary experiment are very inaccurate based on the limitations.

Evaluation

I would improve my experiment by making the measurement techniques more accurate, for instance I would measure the diameter of the putty exactly using a set of callipers, this would mean that I would know the exact diameter and thickness and keep it the same throughout the putty, I would also repeat the experiment at least three times in order to ensure a fair test. Another improvement would be to keep checking the temperature of the putty in order to ensure that the temperature at which the experiment is carried out is the same each time.

From the preliminary the following things have been learned and can be used to ensure that the actual experiment would be more accurate than it would be otherwise

1. The temperature of the wire must be kept the same everytime a reading is taken
2. The thickness of the wire should not change throughout  the experiment
3. The power sources should be fully charged.
4. there should be an equal distribution in the wire, e.g. no knots or coils
5. The material of wire used should be the same.

Second Experiment (Nichrome Wire)

This is experiment is to deduce whether or not there is a correlation between wire lengths and the resistance in a material. The material that is used to nichrome wire.

Conclusion

Measuring the lengths of the wire is also an inaccuracy as the rulers used are not exact, and it is difficult to get an accurate reading of length by eye, as the wire might not be completely straight, it may be of different thicknesses throughout the length. These would have contributed as well to the error. These results would be difficult to improve on as they are reasonably accurate, and there were no anomalous results. But if I were to do this experiment again, I would use newer, more accurate ammeters and voltmeters, a more accurate method of measurement, and take a much wider range of readings and more readings so that a more accurate average can be taken.

I would also investigate other factors, such as temperature, voltage and current, and see how these affect the resistance. I would also do the experiments under different conditions such as temperature and pressure to see if it makes any difference to resistance. As these results had a range of only 5 readings, from 0-100cm, and were only repeated three times, and that the results are not 100%, accurate due to the errors discussed earlier, then I would say that these results are not strong enough to base a firm conclusion on because there are so many sources of error, which are explained earlier.

If one were to assume that Ohm’s Law applies, then another possible explanation could be that at some points (more likely in the lower lengths), the wire was not allowed to cool completely so that the temperature was higher for that measurement. This would cause a higher resistance as explained previously. The wire was allowed to cool but not to a definite temperature, it was just based on how hot the wire felt, usually the cooling period was between 2-3 minutes.

This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.

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