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# Resistance of a Wire

Extracts from this document...

Introduction

Jatish Patel                                Physics Investigation                                 Mr. Griffith

Physics Investigation

Resistance of a Wire

Choosing a Variable

Before I begin the investigation, I must first decide which variable I should investigate.

Variables can be divided into 2 major groups: dependant variables and independent variables. In measuring the resistance of a wire the dependant variable is resistance. This is because the resistance will change when other variables are changed.

An independent variable is a variable which cannot be affected by other variables. There are many independent variables in measuring the resistance of a wire and I must first assess which one would be a suitable one to investigate:

• Temperature- I know that temperature affects resistance; as temperature increases so does the resistance. As the atoms vibrate they make it harder for the electrons to move through the material. This is because the electrons would be obstructed by and collide with the atoms as they move. This would be an unsuitable variable to investigate because it would be it would be difficult to create an environment with a constant temperature.
• Material- The material affects the resistance; each material has a certain amount of electrons which can flow through the material which is affected by the number of electrons in their outer-most shells. If there are more electrons in the outermost shells then the resistance is lower. This variable is not suitable to investigate because results can not be plotted on a graph.
• Density- The density affects the resistance because as the atoms are much more closely packed, the electrons will collide more frequently with them. Therefore as the density increases, the resistance does so as well. This variable is not suitable to investigate because results can not be plotted on a graph.
• Length- The length of wire affects its resistance because if the wire is longer, the electrons will have more frequent collisions with the atoms. This would be a good variable to investigate because it is a very simple experiment and the results can be plotted onto a graph easily.
• Diameter- As you increase the diameter of a wire you will decrease its resistance because the electrons have more space to flow through and will have fewer collisions with atoms. This is a good variable to investigate because it is a simple procedure and the results could be plotted on a graph but we may be limited by how many different diameters we can use.

For my investigation, I will investigate how the length of a wire affects its resistance. I have chosen this variable because it is a simple variable to investigate and the results could be plotted easily onto a graph. I have chosen length as opposed to diameter because with diameter, there is a limit to how many different diameters of a wire you can use, whereas with length you can use whatever length you want.

Aim

To investigate how the length of a wire affects its resistance.

Apparatus

Metre rule-this will be used to measure the length of the wire to 2 significant figures.

Leads- these will be used to connect the length of wire to the apparatus which will be used to measure the resistance.

Length of wire- this is what we will measure the resistance of.

Masking tape- this will be used to hold the length of wire down onto the metre rule.

Ohmmeter- this will be used to measure the resistance of the wire.

Ammeter- this will be used in conjunction with the voltmeter to measure the resistance of the wire.

Voltmeter- this will be used in conjunction with the ammeter to measure the resistance of the wire.

Diagram

Method

The apparatus will be set up as shown above: the length of wire (which is longer than 1m) will be placed on the metre rule and then folded over the ends of the metre rule. It will then be secured by masking tape.

Connect 2 leads to the ohmmeter. The other ends of the leads will be placed at a specific distance apart on the wire (l).The  distances I will use are 10cm, 20cm, 30cm, 40cm, 60cm, 80cm, 100cm.The resistance of each distance will be taken 3 times. 7 different lengths of wire will be used. Then a wire of a different gauge and different material will be used and this method will be repeated.

This procedure will then be repeated again with the first piece of wire but a different method will be used to measure the resistance: a voltmeter in parallel to the wire and an ammeter in series with the wire will be used.

Controlling the variables

I will try to control the other variables in the following way:

• Diameter- I will only use one gauge for each particular material of wire.
• Material- I will measure the resistance of one material at a time.
• Density- I will not need to control the density of the wire as it will be fixed.
• Temperature- I cannot control the temperature but this factor may not have a significant effect on my results.

Preliminary Experiment

There are many different types of wires. A wire’s type is defined by its material and its gauge. For my preliminary experiment I will test 5 different materials of wire:

• Copper
• Manganin
• Brass
• Constantan
• Nichrome

Middle

Results

These were the results I obtained when I tried to measure the resistance of a 38swg Nichrome wire using an ohmmeter.

 Length Resistance (Ω) (cm) 1 2 3 Average 10 5.8 5.8 5.8 5.8 20 11.5 11.5 11.5 11.5 30 17.3 17.3 17.3 17.3 40 23 23 23 23 60 34.6 34.6 34.6 34.6 80 46 46 46 46 100 57.7 57.6 57.8 57.7

These are the results I obtained when I tried to measure the resistance of a 26swg Manganin wire using an ohmmeter.

 Length Resistance (Ω) (cm) 1 2 3 Average 10 0.6 0.6 0.6 0.6 20 0.9 0.9 0.9 0.9 30 1.2 1.2 1.2 1.2 40 1.5 1.5 1.5 1.5 60 2.1 2.1 2.1 2.1 80 2.7 2.7 2.7 2.7 100 3.3 3.4 3.3 3.3

These are the results I obtained when I tried

Conclusion

• Make the wire straighter to make the readings for length accurate to 3 significant figures.
• Use more accurate instruments which could take the readings to a larger number of significant figures.
• Use a more accurate method to measure the length of the wire.
• Use leads and instruments with little or no resistance to make the results more accurate.
• Created an environment which controlled temperature.

I think that I carried out the experiment quite accurately and I followed all of the precautions but I encountered a slight problem when the instruments I used changed. Because the readings were not all taken in one session, I may have not used the same leads and instruments for one reading which I had used for another.

I think that my evidence was quite reliable with the exception of two results in the third experiment. My predicted graph was not exactly the same as my actual graph because on my actual graph the line of best fit did not go through the origin.

For further work we could use another method to measure the resistance of the wire: a method which is called the substitution method and is shown in the diagram below.

The resistances will be measured for the wire and the variable resistance and the resistance of the variable resistor will be changed until it shows the same resistance on the ammeter as the wire. The reading of the variable resistor will then be recorded.

This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.

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