3)Type of material Different materials have different resistances because the materials’ atomic structures are different so some metals have low resistances and some have high resistances. Therefore it is important to keep the material the same throughout the experiment unless a different material is used to check if the conclusion or theory works for all materials. If different materials are used throughout the investigation, it will affect the results. For example if sometimes copper is used and sometimes nichrome is used, the results where copper is used will be of a low resistance because of the material and not because of the diameter of the wire. Throughout the experiment Constantan and nichrome will be used. Constantan is used to make sure my conclusion works for all materials. The type of material will affect the amount of free electrons that are able to flow through the wire. The number of free electrons depends on the amount of electrons in the outer shell of the atoms, so if there are more or larger atoms then there must be more electrons available. If the material has a high number of atoms there will be high number of electrons causing a lower resistance because of the increase of the number of electrons. If the particles in the material are tightly packed together, the electrons will have more collisions and therefore more resistance.
4.Wire width If the wires width is increased the resistance will decrease. This is because of the
increase in the space for the electrons to travel through. Due to this increased space between
the atoms there should be less collisions.
All these factors must be kept constant to make the investigation fair.
In my test I am going to try and prove that the four above statements are correct.
The equipment I am going to use will be:
- One accurate ampere meter
- One accurate Volt meter
- Connecting Wires
- Battery pack- one cell battery
- Ruler
- Different metals
- Micrometer – a device that measures accurately the thickness of a wire.
The same apparatus must be used throughout the investigation.
This is what my circuit will look like:
I will place the varying materials of different length and cross sectional areas in the above circuit changing only one factor at a time measuring the current at the applied voltage and calculate the resistances to produce tables and graphs.
For each factor I’m going to repeat the reading twice and then take the averages to reduce human error. I’m going to use a suitable range of materials supplied for the test by the school, which are nichrome, copper and constantan. I will also use varying lengths of material ranging from 0cm to 60cm in intervals of ten cm, and different diameter wires, between 0.12sq.mm and 0.64sq.mm to give me varying cross sectional areas as it was felt that this would give us an acceptable conclusion.
To make this experiment safe I must make sure the wire is not connected for long periods of time. This makes the wire hot and destroys equipment, and disconnect the wires should the ammeter reading go too high.
Prediction
To prove my four factors that effect resistance is true.
- Material – If the material has many free electrons (such as metals) then these are good conductors and have a comparatively low resistance. If the material has few free electrons (such as plastics) these are good poor conductors and have a high resistance.
- Cross Sectional Area – The bigger the cross sectional area the lower the resistance, so resistance of a wire is inversely proportional to the cross sectional area. The thinner the wire, the higher the resistance. The thicker the wire, the lower the resistance. This is because the thinner the wire is, the less paths there are for electrons in the wire therefore the harder it is for current to flow. This results in the energy not being able to spread out as much, so the resistance will be higher.
- Length – The longer the wire the more potential difference is needed to push the same current through so the resistance must be proportional to the length.
- Temperature – Generally the resistance of a material will increase with temperature unless it has a negative temperature coefficient. No test will be carried out varying temperatures.
Results
My first results are for the material nichrome at a fixed length with varying thickness.
Nichrome
First reading:
Second reading:
Average resistance:
My second results are for different materials at a fixed length with the same thickness.
Different materials
First reading:
Second reading:
Average resistance:
My third results are for the material copper at a fixed length with varying thickness.
Copper
First reading:
Second reading:
Average resistance:
My final results are for the material nichrome at a variable length with fixed thickness.
Length
First reading:
Second reading:
Average resistance:
Analysing
My prediction indicated four factors that effect resistance, material, cross sectional area of the material and the length of the material, together with the material temperature. Analyses of the test results are listed below by these factors; however, a test on material resistance variation with temperature variation was not carried out.
Materials
The results show that different materials have different resistances.
The metal with the lowest resistance is copper at 0.15 ohms at the fixed length, that’s why it is used in everyday wiring because of its low resistance.
The next highest resistance material tested proved to be constantan at 0.49 ohms at the fixed length.
The highest out of the three was nichrome with a resistance value of 1.66 ohms at a fixed length.
Wire Length
The results show conclusively that the longer the wire the higher the resistance. The resistance of a length of wire is therefore proportional to the wire length because the potential difference has to push the electrons further so they have more atoms to collide into.
Cross Sectional Area
It can be seen from all the tables with varying wire diameter, and hence cross sectional area, that the resistance of the wire is inversely proportional to its cross sectional area and that the thicker the wire the lower its resistance. That’s why all heavy-duty appliances such as cookers and showers have thicker cables.
Note. It was found that some of the wires got rather hot during the tests indicating if the right kind of material was chosen with the appropriate length and cross section area heat producing appliances such as room heaters and kettles can be fabricated.
Also if you increase the voltage - then more current will flow, if the resistance is constant. If you increase the resistance - then less current will flow if voltage remains constant.
Evaluation
While the procedure used to obtain the evidence was adequate for this test and proved the predictions to be correct, The procedures could have been improved in various way as follows:
- Some of the tables showed anomalies, which could have been minimised, with many more repeats and a greater range giving more reliable data.
- More accurate measuring devices i.e. voltmeters and amp meters which would produce more accurate figures.
- If the test were carried put in a temperature-controlled environment less anomalies would be seen.
- We could have tried more materials to give a wider scope.
- Instead of connecting the voltmeter to the main circuit I would connect it to the wire, which is being tested. I would do this so that the voltmeter is measuring the voltage of just the wire being tested and not the wires of the main circuit as well
- I would use a digital voltmeter instead of an analogue meter. I would do this because a digital voltmeter is a lot more accurate than an analogue because if the needle in the analogue voltmeter is bent then the readings given off will be false whereas a digital voltmeter does not rely on a needle or any other manual movements.
- I could have done the test for the temperature of the wire but I would not have been able to carry out a fair test because it is extremely
difficult to produce and control the range of temperatures needed without the correct equipment.
- To make the experiments more reliable all apparatus should have been checked to see if it is functioning properly and is giving a true reading. This would then partly avoid systematic error.
Overall the predictions I made were proved to be correct. So proving the factors that affect the resistance of a wire is:
- Material
- Length
- Temperature
- Cross section area
