Factors Affecting the Resistance of a Wire.

FACTORS AFFECTING THE RESISTANCE OF A WIRE

Resistance is measured in Ohms (Ω)* and the amount of resistance determines how much current is allowed to flow by a component. 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 neighbouring 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. There are several factors that affect the resistance of a wire.

Length

If the length of the wire is doubled then there is more atoms obstructing the electron flow and so there is more resistance. In my experiment length is going to be a controlled variable.

Material

Some materials have more free electrons than others. This means that materials with lots of free electrons provide less resistance than a material with lots of fixed electrons because it allows electrons to flow more easily. In my experiment I am going to keep the wire material the same for all of the readings that I take. The wire that I am using is ‘32 SWG CONSTANTAN WIRE’.

Cross-sectional Area

A decrease in the cross-sectional area of a wire would result in a higher resistance because there is less inter-atomic space (space between atoms). A double in the cross-sectional area should halve the resistance. In my experiment this is also going to be a controlled variable.

Heat

To keep my experiment a fair test heat must be taken into account. If a wire is heated then the electrons vibrate. This, in turn, obstructs the electron flow. Heat MUST be kept constant. To do this I will only turn the power supply on in short bursts.

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Cross-sectional Area

Heat

To measure resistance you take both the Amps (A) and the voltage (V). You then divide the voltage by the current and find the resistance.

* Ohm's Law 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. Hence, the resistance of a metal increases as its temperature increases. This is because at higher temperatures, the particles of the conductor are moving around more quickly, thus increasing the likelihood of collisions with the free electrons.

I am going to investigate how the length of a wire affects its resistance. This is the circuit that I will use to measure both the current through and the voltage across the wire. For this I am using a Voltmeter and an Ammeter. I have performed a preliminary experiment (and set up this test circuit independently) in order to establish how many readings to take across a meter (m) and I have found that taking readings every five centimetres would be sufficient. Also taking three readings at different levels of power for every five centimetres would be beneficial because it gives an average figure of resistance. I will change the length of the wire each time and, for a fair test, keep everything else the same.

Prediction

I predict that if I increase the length of the wire the resistance will increase. This is because the amount of atoms obstructing the electron flow will increase. If the length of the wire is doubled then I would expect the resistance to double.

Changing the cross-sectional area will also alter the resistance of the wire. Doubling the cross-sectional area will result in a halving in the resistance. This is because the inter-atomic space is doubled.

Conclusions

The shape of my line is a straight line. My graph shows that as I increased the length of the wire, the resistance of the wire increased. My prediction was correct. I said that the length of the wire would affect the resistance. The line does not quite double but it does show a good reflection of how length affects resistance. There is more resistance as the line gets longer because there are more atoms obstructing the electron flow. Whilst extremely unlikely, it is conceivable that the power supply was providing a different voltage for some of the results. This is unlikely to be a problem in this investigation but it might have been an issue had we used batteries instead. For a particular result, one or more of the connections could have been faulty, causing extra resistance at the connections. A solution to this would be to, before each experiment, connect the connections together without the wire in place and measure the resistance then. If it is higher than it should be then the connections could be cleaned.

If you double the cross sectional area of the wire then the line still travels from the origin of the graph but at less of a steep gradient as a wire that is half a thin. The graph should reflect that the wire with double the cross-sectional area provides half of the resistance of the thinner wire.

There is one truly anomalous result and that was for a piece of wire of length 15cm. For this mistake I can only blame meter inaccuracies (if I had a more precise meter that went to ten decimal places instead of two then all of the results would have been more accurate), loss of detail through gaining averages and possibly human error in reading the meters or in the calculations. If I had used a more accurate meter that allowed me to measure current and Voltage to eight decimal places instead of two then all of my results would have been more accurate.

Evaluation

I feel that my results are reliable enough to support my predictions and conclusions. If I was to do this experiment again I would make would be to use pointers instead of crocodile clips , I would do this because pointers would be more accurate. The pointers would be more accurate because the tips have a much smaller area than the crocodile clips giving a more accurate measurement of the length of wire. The wire measurement was not correct. The solution to this is to measure the lengths more carefully and ensure that the wire is pulled tight against the metre rule.