Aperatus

How resistance is measured

The resistance of wire is calculated by measuring the current and the voltage.

Once I have the results I need to divide the voltage by the current which is V/I (v over I)

Ohms law

Ohms law states that the current through wire at a constant temperature is proportional to the potential difference. Therefore V over I is constant. This means that the resistance of a wire is constant along as the temperature remains constant.

Variables

- Length of wire
- Material of wire
- Diameter of wire
- Starting temperature of wire
- Voltage across the wire
- Current in circuit
- Increasing temperature of wire

All of the above must be taken into account in order for it to be a fair test as it is important that the results are accurate.

Predictions

The longer my wire is the greater the resistance, because the more wire there is then the more atoms there are and therefore there will be more collisions resulting in more energy being lost as heat.

Preliminary work

In a lesson previous to my experiment I set up a circuit which included a variable resistor. A variable resistor is used to shorten and lengthen the amount of wire used in a circuit. I found that when I shortened the wire with the resistor the resistance was 8 Ohms, when the resistor was in the middle the resistance was 12 Ohms and when the wire was at its longest the resistance was 20 Ohms. This helped me come to my hypothesis as it shows that the longer the wire the greater the resistance. Although the main reason for my preliminary work was to find out the best voltage to do the experiment with. I came to the conclusion that 6 volts was perfect as it was fairly high but not to high in order for the wire not to burn.

Results 1

Results 2

From my results I can see as predicted that as the length of the wire went up so did the resistance. As it is clear to see the resistance of the wire is directly proportional to the length of the wire. I know this because the line of best fit is a straight line through the origin showing that if the length of the wire increases then so does the resistance in proportion.

Now I know what resistance is, what causes it and that my hypothesis is true I can understand the following;

The length of the wire affects the resistance of the wire because the number of atoms in the wire increases or decreases as the length of the wire increases or decreases in proportion. The resistance of a wire depends on the number of collisions the electrons have with the atoms of the material, so if there is a larger amount of atoms then there will be a larger number of collisions that will increase the resistance of the wire.

resistance (Ohms) length (cm)

My results show that the greater the length of wire the greater the resistance, the resistance is directly proportional to the length of wire. This is clear as when the length was 10 cm the resistance read 0.69 & 0.67, and when the length was 20 cm the resistance read 1.54 & 1.51, and when the length was 30 cm the resistance was 2.4.

I can see from my results that in general the resistance was greater on my first set of recordings. I don’t know why this is because one would presume that the resistance would be greater on the second set as the wire would be hotter, but this is not the case. This shows that the experiment is not entirely accurate.

Although the experiment itself wasn’t as accurate as it could have been it was easy to conduct and my results are fairly accurate. Certain methods used in the experiment caused it not to be as accurate as it could of been for example:

- The end of a crocodile clip is about 3mm wide
- It was impossible with the equipment that was available to get the wire completely straight on the metre ruler.
- After a while the wire got very hot and therefore effected the resistance.

Improvements that could be made are:

- The wire to be properly stuck to the ruler stick
- Clips that make 100% contact with the wire used
- And a new peice of wire used for every recording so that it doesn’t get hot and affect the resistance.

The two graphs that show my results prove that the resistance is greater when the length of the wire is greater and that the resistance is directly proportional to the length of the wire. Therefore my two graphs fit my prediction and preliminary work.