Some metals are better conductors than others. The best conductors are those with a low resistance, like silver, copper, gold and aluminium. Copper and Aluminium also have the advantage of being relatively cheap and for normal everyday use, these are the metals employed. Those metals have a smaller amount of fixed atoms. A metal with a lot of atoms will have a greater resistance to one that hasn’t got a lot of atoms. Other metals, such as alloys of iron, nickel and chromium, have a high resistance and are poor conductors. The German physicist Georg Ohm studied the electrical resistances of various metals. The unit of resistance, the ohm , is named after him.
So, I need to find out about the different metals I am using to make a prediction as to whether they will have a big resistance. Nichrome is an alloy of nickel, iron and chromium and combines great strength with a high melting point. It is used for the wire heating elements in electric fires. So, I can predict that nichrome will have a high resistance as I discovered that alloys of nickel, iron and chromium as is nichrome do have a high resistance. Constantan is a resistance alloy with moderate resistance. Manganin is an alloy of manganese, copper and nickel. It is used in the form of wire for electrical measurements, because it’s electrical conductivity does not vary appreciably with temperature. I predict that Nichrome will provide the greatest resistance.
I will conduct two experiments. The first one will see how the length of wire and type of metal affects the resistance. The diameter will remain the same at 20swg. In the second experiment I shall vary the diameter and length of the wire whilst only using the metal nichrome.
Diagram
To carry out the investigations I will set up the circuit below.
Accuracy
I need to ensure that it is a fair experiment. So, I must put the results in the correct column and make sure the wires are the correct lengths and metals. I also need to take away 00.2 from every result, as this is the resistance of the leads. I must make sure I do this every time. Also, temperature affects the resistance in a wire, so I should conduct the experiment in the same laboratory and on the same day, to avoid a slight variation in temperature.
Safety
As I am dealing with electrical charge I must be careful every step of the way. I must do exactly as the teacher says.
Experiment 1
Apparatus
I will need to use the following:
●A multimeter (this measures both current and resistance)
● Assorted wires: - Constantan, nichrome and manganin, 20, 40, 60, 80 and 100 cms. All 20swg.
Plan
I shall try out 3 different metals and vary the length of each. I shall use Constantan, Nichrome and Manganin. I shall try lengths of 20,40, 60,80 and 100 cms. All of the wires will be 20swg, which is a diameter of 0.9mm. I shall record the results in a table. I shall repeat each experiment twice and then make an average of the two results. This should give me clear accurate results.
Method
- Collect the apparatus
- Connect the circuit shown on the previous page
- Make sure the correct wire is connected
- Record the resistance in a table
- Proceed to connect the next wire to the circuit
Experiment 2
Apparatus
I will need to use the following:
● A multimeter
● Nichrome wires of lengths 20, 40, 60, 80 and 100cms, with diameters of 20, 26 and 30swg.
Plan
I shall try out wires with three different diameters and different lengths. The wires will be 20, 40, 60, 80 and 100cms long. They will have diameters of 20swg, 26swg and 30swg. I shall record the results in a table and repeat each experiment twice. I will then make an average of the two results and to give me clear, accurate results.
Method
1. Collect the apparatus
- Connect the circuit shown on the previous page
- Make sure the correct wire is connected
- Record the resistance in a table
- Proceed to connect the next wire to the circuit
Results for both experiments
All results have had 00.2 taken away for the leads
Experiment 1
Diameter- 20swg
Experiment 2
Wire- NICHROME
20swg –0.9mm
26swg –0.45mm
30swg –0.3mm
Analysis of both experiments
I have presented my results in the form of a table and also in a graph. The red value in the table provides the resistance, while the length of the wire is noted alongside. I shall first analyse the results of my first experiment. The first thing that I notice is that Nichrome provided the most resistance out of the three metals for all three lengths. For example for a wire of length 20cms Nichrome gave 1.4Ω resistance while Manganin gave 1.1Ω and Constantan 0.6Ω. With this length of wire manganin provided more resistance than constantan. However for the rest of the lengths, constantan provided more resistance than manganin. Consequently I believe that there may have been a mistake in the reading of the resistance for manganin 20cms. So nichrome provides the most resistance, followed by constantan and then manganin.
Therefore my scientific knowledge and prediction were correct. I said that nichrome would provide the greatest resistance as I discovered that alloys of iron, nickel and chromium, have a high resistance. Nichrome is an alloy of iron, nickel and chromium. I also discovered that constantan is a resistance alloy with moderate resistance. Manganin is an alloy of manganese, copper and nickel. It only has one of the compounds that are known for making alloys with good resistance; nickel. Therefore it did not have a great resistance. My prediction that the resistance of a wire would increase with the length of the wire was also correct. This is because in a longer wire, the electrons would move slower because there is a greater distance for them to travel from – to + and so the resistance would be greater.
My second experiment’s results were presented in much the same way as the first. This time nichrome was used throughout and the diameter and length of the wire were varied. My results again show that resistance increases with length no matter the diameter of wire. This is because electrons move slower in a longer wire as they have further to go to get from – to +, causing greater resistance. As I predicted, the resistance also decreased as the diameter increased. They are inversely proportional to each other. For example, the resistance was at its greatest in the thinnest wire, 30swg or 0.3mm, with 2.6Ω of resistance in a wire of 20cms. The thickest wire -20swg or 0.9mm provided the least resistance, 0.3Ω of resistance in a wire of 20cms. This is because in a thicker wire there is obviously more room for the electrons to flow. They have more room to go around the fixed atoms, which obstruct the electrons and provide resistance. So in the thinner wire there is a greater resistance as there is less space for the electrons to flow around the fixed atoms.
So I can conclude from the results gathered in both of my experiments that resistance is affected by the length and thickness of the wire and the type of metal it is made from. I have learnt that resistance increases with the length of the wire. Alloys of nickel, iron and chromium, such as nichrome, have a high resistance and resistance increases as the diameter of a wire decreases.
Evaluation
As I pointed out before, I have an anomalous result. This is one that does not fit into the general pattern of my results. It is the result for manganin, length 20cms taken in the first experiment. The result was 1.1Ω of resistance, which is higher than it should have been. The result for a wire of 40cms of manganin was also 1.1Ω, this proves that something is wrong with the previous result. As I know that temperature can also affect the resistance of a wire, I would at first be inclined to think that this anomalous result was due to a fluctuation in temperature, i.e. if the reading was taken on a different day to the other readings. However, during my research I discovered that manganin is used in the form of wire for electrical measurements, because it’s electrical conductivity does not vary appreciably with temperature. Therefore it probably was not a change in temperature which produced this anomalous result. It is likely that the resistance value was misread or that there was a mistake in the calculations where 00.2 was removed from each value to account for the leads.
If I were to conduct the experiment again I would ensure that all readings were taken on the same day, as I do not think they were. Especially as I have learned that fluctuating temperatures can vary the resistance. I would also consider taking 3 readings instead of two and then getting an average. This would give me more accurate and reliable results. It would also be better if one person was assigned to taking the readings and one to deducting 00.2 from the readings. This would be better than people switching jobs, which might cause discrepancies in the results. Now that I have looked at how length, thickness and the metal of the wire affect resistance, I could also look at how temperature affects the resistance of a wire. I could also extend the experiment by taking into account these four factors all at once and looking at how the electrical conductivity varies.
