For each test I am going to use 1 volt.
The variable that I am going to change each time is the length of the constantan wire.
To ensure a fair test I will measure the length of the wire accurately each time.
To ensure that the experiment is safe I will avoid touching the constantan wire because it will be hot. Make sure that I turn off the power supply to the circuit when I am altering the distance of the crocodile clips so that there will be no chance of getting an electric shock. I have also included a bulb n the circuit to restrict the current.
I will start by measuring 1 metre accurately with the ruler. I will then attach the crocodile clips 1 metre apart and then record the reading on the ammeter. I will repeat this process 10 times by moving one of the crocodile clips 10 centimetres down the wire.
I will also ensure that no magnetic materials are placed near to the experiment as this may affect the resistance.
Prediction
Metals are good conductors of electricity because they have free electrons to carry the current. I think that the longer the piece of wire is the higher the resistance will be. This is because the resistance in a metal conductor happens because as the electrons move through the material (when a voltage has been applied) they collide with the atoms in the material and as a result loose energy.
Electrical Resistance is the ratio of voltage to current.
To find the resistance you use the equation:
RESISTANCE (R) = VOLTAGE (V) ÷ CURRENT (I)
The temperature of the material will also affect the resistance. A hot metal has a larger resistance than a cooler one. I will not have to worry about the temperature though because I am testing Constantan wire and its resistance is not affected by heat.
Resistance is simply how difficult it is for the electrons to move through a material. The longer the material is, the harder it will be for the electrons to move through the material.
I think that the resistance will be proportional to the length as shown in the graph below.
The same principal applies to the diameter of the wire.
This means that the resistance of a wire is:
-
Proportional to its length L
-
Proportional to its cross sectional area A
So we can write the resistance of a wire in terms of a constant of proportionality called the resistivity of the material.
R=ρ x L
A
R=ρ x L
A
Where R=Resistance
L=Length
A=Area of cross section
ρ=Resistivity of the material
The unit of resistivity is the ohm metre (Ωm)
Using this formula I can work out what my results should be.
From the ‘Table of Physical and Chemical Constants’ I found out that the resistivity of constantan wire is:
49x10 Ωm
The diameter of the wire is 0.3mm
To find the cross sectional area of the constantan wire I used the formula
Cross Sectional Area= πD
4
=π x 0.3
4
=0.07065 x 10
By using the formula R=ρ x L
A
I calculated what my results should be.
EVALUATION
The experiment was a success. It gave me a go set of results with no anomalous results. The graph and the results table that compares both of the resistivity tests that I did, shows that the results are very similar and the difference is very small. The graph comparing my results with the results that I obtained by using the formula shows that they are very similar which means that my experiment was accurate and successful. From the results I can see that the length of the wire is proportional to the resistance because as the length of the wire increases the resistivity increases. This can be explained because resistance in a metal conductor happens because as the electrons move through the material when a current is passed through it, they collide with the atoms in the material. Therefore if you increase the length the chances of the electrons colliding with the atoms and losing energy is increased.
