Material
Different materials have different levels of resistance. Wires that have a greater number of ions in a set space will be more resistant. This is not a very good variable as there only three types of wire that can be used which is not a big enough range.
Prediction
For this investigation we shall use length as the variable. This is because it is the easiest to do. The wire we shall use will be Constantan. This is because it is not affected by heat as much as copper and nichrome.
I predict that as the length of the wire increases, the resistance increases, the current will decrease. The amount of current flowing in a circuit made up of pure resistances is directly proportional to the electromotive forces impressed on the circuit and inversely proportional to the total resistance of the circuit. A steady increase in voltage, in a circuit with constant resistance, produces a constant linear rise in current. If there is no resistance the current will be equal to the voltage. A steady increase in resistance, in a circuit with constant voltage, produces a progressively (a curve if graphed) weaker current. A steady increase in length, in a circuit with a constant voltage produces a steady increase in resistance.
This is because the greater the length of wire, the greater the number of ions which collide with the electrons being passed through. The resistance can be calculated by using the formula R = V/A. This gives you the potential difference between voltage and current. You could also take it further and calculate resistivity by multiplying resistance by area.
Safety:
The voltage will need to be kept low to ensure that there is not a too large of a current passing through the metal wire and overheating it.
Equipment:
To calculate the resistance we will need to know the voltage and the current. For this we will need an ammeter with a 100-milliamp shunt and a voltmeter with a 10th of a volt shunt. We will also need to measure the length of wire used.
100cm of Constantan
Voltmeter
Ammeter
Shunts
Power pack
Wires
Metre rule
Wire cutters
Sticky tape
Circuit Diagram
Apparatus Diagram
Preliminary:
After research and experimentation we decided the following factors:
- We have decided to use Constantan as it has an average resistance and is not affected by temperature as much as copper and nichrome.
- We are going to use 30swg (standard wire gauge) as it is not so thin that it will overheat when it is at its shortest length but it is not so thick that current will be very low when it is at its longest.
- We are going to use lengths of wire at 10cm intervals from 10cm to 50cm.
- We will keep the volts as close to 1.3v as possible.
- We will use an analogue ammeter as they give more accurate reading for small amounts.
Method:
We will start by gathering all of the equipment. We will then set up the circuit as shown in the circuit diagram. We will then clip one crocodile clip at one end of the wire and the other at 10cm further down. We will then turn on the power pack and slowly increase the voltage to approximately 1.3v. This is perfect voltage as the wire does not over heat or create too much resistance at any point. We will then note down the length, the voltage and the current and then turn it off again. We will then do this again for 20cm, 30cm, 40cm and 50cm. We will also take repeat readings to ensure accuracy. If there are any odd results we will repeat these again.
Fair Test:
To ensure that the test is fair the voltage will need to be kept the same. As we are working with such small volts it will be difficult to kept the voltage exactly the same but we will try to keep it as close as possible.
This table shows that the voltage was kept within 0.09v of its set amount. It also shows the current and the resistance decreasing and increasing at a steady rate.
The graph shows a straight line going from the origin right and upwards. This shows that if there was no wire the resistance would be 0. Whereas if the wire was 25cm long the resistance would be 2.4 ohms. My results agree with my prediction to an extent to which they actually proves my prediction. As the length increases, the resistance increases and the current drops at a steady rate. But there is not enough evidence for me to prove that this works for all metal wires. To see if it works on all metal wires we would have to repeat the experiment using different types of wire but this still wouldn’t prove that it is true for all metal wires.
Evaluation:
The results were all to a reasonable standard of accuracy. The repeat readings were similar to the original readings. All results were close to the line of best fit. There were no anomalous results during the experiment, this was due to the high level of care to ensure safety and accuracy. Extra care was taken as during he preliminary test the voltage was turned up too high and the wire overheated, singing a metre rule. This could have caused minor burns if anyone was to have touched the wire while it was at such a high temperature. If I were to redo the experiment I would take more care to ensure that the voltages were kept as similar as possible. I would also ensure that the voltages were kept low at all times, this could possibly be done by adding a fuse wire which would burn out and break the circuit if the current got too high. If I were to extend the investigation I could look at the resistances of other metals such as copper and nichrome. I could also look at wires with different thicknesses.
