The circuit should be set up as in the circuit diagram. It is important that the voltmeter is set up in parallel and the ammeter in series. The readings from the ammeter and voltmeter will be used to work out the resistance. This can be done using the formula:
R = V/I
Where V=voltage, I=current and R=resistance.
Fair Test:
To ensure that the investigation is carried out in a fair way and that the results will be accurate and reliable a number of things must be followed. The only variable in the test will be the length of the wire. The wire must be measured as accurately as possibly and the same circuit and power supply must be used throughout as different power supplies may have different voltages. The experiment should be repeated at least twice for each length of wire and an average taken to make sure that the results are reliable.
Diagram:
Dependant variable – The readings of the voltmeter and ammeter to calculate the resistance of the wire
Independent variable – The current, length of wire
Controlled variables – Type of power supply, type of wire, thickness of wire
Apparatus
*Power supply
*Connecting wires
*Resistance wire
*Crocodile clips
*Metre stick
*Ammeter
*Voltmeter
*Calculator
Method
*Set up the circuit as drawn on the previous page.
*Choose what length of resistance wire you will study first. Measure the wire with the metre stick and place the crocodile clips at the appropriate places on the wire to connect the resistance wire into the circuit.
*Switch the power supply on, and record the ammeter reading and the voltmeter reading in a table.
*Change the power supply voltage to get a second reading, and record the readings on the ammeter and voltmeter in a table.
*Use the formula R = V/I to get the resistance of the results. When both results have been calculated, get an average, and record this average in a table.
*Choose another length, and repeat the stages above.
Results :
I will now plot a line graph to illustrate my results.
Analysis:
The results from the graph give a clear indication of how the resistance compares to the wire length. There is a positive correlation. This means that when the length of the wire increases, the resistance also increases. The results are also directly proportional, meaning that when one increases, the other increases too.
I explained about resistance in my prediction. In metal wire, there are a number of atoms and free moving electrons. Electricity is the movement of these electrons through the wire. Resistance is caused when the free electrons moving through the wire collide with the atoms making their path through the wire more difficult. This means that if there are more atoms in the way to collide with the free electrons the resistance is increased. In a length of wire there will be a number of atoms, and in a wire twice the length, there will be twice the number of atoms. In turn this will lead to there being double the number of collisions between the electrons and the atoms increasing the resistance by 2.
The results support my prediction. This is because in the prediction I said that as the wire length increased, the resistance should increase. I also predicted that the results would be directly proportional, and the graph verifies this.
The line of best fit clearly show that the results followed the expected pattern very well.
