I have chosen to investigate how the length of the wire affects the resistance.
Preliminary investigation
In Year 11, I studied resistance in module 4, and during this time I discovered good conductors have low resistance, and poor conductors have high resistance. I also found out, when experimenting, that current can cause heating in a wire, therefore I had to make sure the current was small enough, so the wire would not heat up.
Prediction
I predict that the longer the wire, the greater the resistance. The length is directly proportional to the resistance. As the length doubles the resistance doubles:
Electrons have further to travel in a long wire and therefore have more opportunities to collide with the positive ions. If the electrons collide with the ion they will be slowed down and they will lose some of their energy as some of the energy will be picked up by the ion.
Fair test and Variables
To make sure I am carrying out a fair test, I will keep the material of the conductor, the thickness of the metal, and the temperature the same. These are the control variables. I will change the length of the wire each time (independent variable), and I shall measure the voltage and current each time to calculate resistance (dependent variable). I shall therefore measure the resistance for each length, the gradient of my voltage/current graph, and in order to obtain a V/I graph I will measure the change in current and corresponding voltage across the wire.
Safety
- Current causes heating of wire, so I must make sure not to touch it otherwise I’ll be burnt.
- High currents may cause electric shock.
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Wet hands will make an electric shock even more fatal, so I must ensure that they are dry.
Apparatus
The following apparatus will be used:
- power pack, D.C
- Variable resistor
- Voltmeter
- Ammeter
- Wire
- Crocodile clips
Diagram
Method
- I set up the apparatus as in diagram
- I used a nichrome wire, and measured out different lengths; 50cm, 75cm, 100cm, 125cm, 150cm.
The wire must be held with the end of the crocodile clips, so they will grip the wire better, and electricity can flow more easily.
3. I used a variable resistor to change the current, and will record the voltage each time.
4. I switched off power pack and moved crocodile clip down
5. I repeated it and got an average
6. Used Ohm’s law V=IR as I have measured the current and voltage
Results
Experiment 1.
Experiment 2.
Averages
Conclusion
From both my graphs and results, I have noticed patterns.
- Resistance increases as length of wire increases.
- As the length of wire doubles, resistance doubles.
- As the length of wire triples, resistance triples.
These results have therefore proven my prediction to be correct. The resistance of a long wire is greater than the resistance of a short wire because as the length doubles, the resistance doubles. This is because the electrons have further to travel in a long wire and therefore have more opportunities to collide with the positive ions. If the electrons collide with the ion, they’ll be slowed down, and lose some energy, as it is picked up by the ion. So, as the length doubles, there will be twice as many collisions.
Ohm’s law: The current flowing in a wire is proportional to the voltage across it, providing the temperature is constant. V=IR
Evaluation
From looking at my graphs, current against voltage, I could see the current was proportional to the voltage - Ohm’s law. I found out that the steeper the graph, the lower the resistance, and the flatter the graph, the higher the resistance. A substance that gives a straight line graph is called an ohmic conductor.
I found a few anomalous results on my graphs. I think this is because while carrying out the experiment, it was difficult to situate them at the exact points apart from each other. Also, each time I carried out an experiment, it might have been better to let the wire cool down, or at least cut different pieces of wire.
Another experiment I could carry out for resistance against length would be to use a lamp, and note the brightness of the lamp, and see whether the resistance decreases or increases.
