What affects the resistance of a metal wire?

Physics for you, Keith Johnson, and Physics revision guide by Richard Parsons.

Plan

4 factors that affect resistance

· As the length of the wire increases, the resistance also increases.

· As the cross sectional area increases, the resistance decreases.

· Copper is a good conductor and is used for connecting wires, as different wires have different resistances this could be investigated.

· As the temperature increases, the resistance of a wire increases.

The factor that I am going to investigate is the length of the wire. I will constantly increase the length by 10cm starting at 10cm and ending at 100cm. Current is the flow of electrons around a circuit. For an electric current to flow, there must be no gaps in the circuit. Electrons flow from atom to atom round from the negative pole to the positive.

I predict that when I double the wire's length, the resistance will double. This means that when the electrons travel along the wire they are constantly hitting atoms, if the same amount of current is passed through a longer piece of wire there will be more collisions hence more resistance.

E.g. if through a piece of 10cm of wire the resistance is 2W.

If exactly the same thing is done again with a 20cm wire, I would expect the resistance to be 4W.

10cm

An amount of current will pass through

20cm

Same amount of current going in, the voltage

needs to be increased to keep the current the same,

but there is double the amount of atoms-increasing resistance.

To keep the current constant, the voltage will need increasing. As the voltage increases, it also increases the resistance as the same amount of current is passing through.

Required apparatus are ...

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E.g. if through a piece of 10cm of wire the resistance is 2W.

If exactly the same thing is done again with a 20cm wire, I would expect the resistance to be 4W.

10cm

An amount of current will pass through

20cm

Same amount of current going in, the voltage

needs to be increased to keep the current the same,

but there is double the amount of atoms-increasing resistance.

To keep the current constant, the voltage will need increasing. As the voltage increases, it also increases the resistance as the same amount of current is passing through.

Required apparatus are a power unit, ammeter, voltmeter, various connecting leads, and copper wire.

Method Power supply

Ruler Wire

Ammeter Voltmeter

Before I start the experiment the teacher will check it. I will pass a current (0.5A) through a piece of wire, the wire carries a resistance, and this causes the electrons to hit the atoms. The current will stay at 0.5A so I can test how the resistance changes in the different wire lengths, I will increase the voltage to ensure that the current is kept the same. I will not change anything apart from the length of the wire from 10cm to 100cm. The factors that will stay the same are

1. The cross sectional area of the wire, I will make sure that this stays the same by making sure the same wire is being used all the time.

2. The type of metal being used, I will make sure that only copper wire is being used.

3. The temperature of the wire-I will keep the current the same, this will ensure that the wire does not overheat. I will take 3 readings from each of the lengths and then use the average to put into my results table.. Resistance will be calculated by voltage/current. I will make sure that the wire or any other component gets too hot by keeping safe levels of current.

Results

Analysis

When I increased the wire length the resistance also increased. When the length of the wire had doubled, the resistance also doubled - this pattern is visible in the table.

E.g. (lengths 50 and 100 compared)

The resistance is proportional to the length. This took place, as there was the same amount of current passing through a longer wire, where there is more resistance than the shorter wire, so there are more collisions. The amount of current was kept the same by increasing the voltage e.g. when the length was 10cm, a voltage of 0.40V was required to give a current of 0.5Amps, but when the length was 100cm, the current was kept the same with a voltage of 3.40V. My results are reliable as I did the experiment three times. Every time the readings were similar proving that there was no mistake in collecting the results. My results could have been more accurate if better more accurate instruments were used. Unnecessary loss of heat within the connecting leads may have been a factor affecting accuracy. I think that my prediction has been justified.

Evaluation

The results that I got were very good, they were all accurate. I took 3 readings and used the average. 3 out of 10 points did not fit the line, this may have been prevented if better instruments were used, when attaching the crocodile clips to the wire this could have been carried out with more care.

My method was suitable as I got the results I wanted, the experiment could be repeated on a larger scale to make less room for error. The evidence I have for stating that the resistance is proportional to the length is that when the length is 20cm the resistance is 1.46 W and when the length was increased to 40cm, the resistance is 2.78 W. The resistance has roughly doubled, there are other examples of this as well. Length 50cm, resistance 3.52W and when the wire length is doubled the resistance is 6.8 W. I think that better equipment would have given more precise results.

To improve the experiment, the clips could be attached more carefully, a more accurate ammeter would make sure that the current is always exactly the same. A better voltmeter would have given better resistance results. The wires and clips used would also have an effect on the resistance, the wires could be insulated better to reduce heat loss. The results would then contain no anomalies. The above would definitely produce better events.