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# Investigate the effect of resistance on the wire with the length and width.

Extracts from this document...

Introduction

Planning

Aim: To investigate the effect of resistance on the wire with the length and width.

List of equipment

• Piece of constantan wire (different length and different thickness, cross    sectional  area)
• Ammeter
• Voltmeter
• Variable register
• Power pack
• Crocodile clips

The resistance R of a conductor can be found by measuring the current (I) through it when a p.d V is applied across it and then using R = V. This is called the ammeter-voltmeter method.                                                        I

Set up the circuit of the experiment in which the unknown resistance (R) is 30cm of  SWG 34 constantan wire. Altering the rheostat changes both the p.d.  V and the current (I). Record in a table, with three columns, five values of I (e.g., 0.10, 0.15, 0.20, 0.25, 0.30 A        )

The current flowing through a metal conductor is directly proportional to the p.d. across the conductor, provided that its temperature remains constant. If we divide the voltage by the current, we get a number which gives us a measure of resistance. In fact, this is how resistance is defined:

Resistance of a conductor =   p.d. across the conductor____

Current through the conductor

ohms   R = V  volts

I  ← amperes

The result  R = V   is called Ohm’s law. However it is really the definition of resistance.

I

Middle

0.4

0.2

0.24

10

0.2

1.2

0.2

0.24

15

0.1

0.4

0.3

0.24

15

0.3

1.2

0.2

0.28

5

0.1

0.9

0.2

0.28

5

0.3

1.3

0.2

0.28

10

0.1

0.4

0.4

0.28

10

0.5

1.1

0.4

0.28

15

0.2

0.4

0.6

0.28

15

0.6

1.0

0.6

.

From this preliminary experiment I will increase the length of the wire up to 30cm in order to get the reliable results.  The reason why I chose the thin wire is because to get a high resistance and therefore it is easy to measure. I have used three different widths because I want to find out width gives the best results. To get precise measurement I will measure the value of the current and voltage up to two decimal places.

Obtaining evidence

This is how the set up of my apparatus looks like. First I join everything together and check if everything is in the right place. Then I adjust the 30cm length of wire to the length of 5cm, 10cm, 15cm, 20cm and 25cm, with the help of the crocodile clip.

Conclusion

For the variation of the resistant, I could not draw one line on one graph and I have drawn 5 lines on one graph.  I could not draw my conclusion on the graph because the variations of the width are not great enough to produce significant variation in the resistance. Only 2 points fell on the line and the third one slipped out.  If the straight line passes through 3 points on the graph we can depend on the reliability (length against resistance graph). In contrast in the width against resistance graph, only 2 points falls on the straight line. Therefore I cannot depend on the reliability. This anomaly is due to the fact that the wires which I have chosen are not considerably vary in thickness (width). Another way of doing that is to take reading using the constantan wire and then the second reading connecting 2 wires as shown in the prediction and keep on doing this for 3, 4 wires.

The anomalous points on the graph (2 points) may be due to the crocodile clips which were not properly attached. There may be lack of specification in observing the ammeter and voltmeter with the eye.  There may be fluctuation in the external and in the internal temperature of the conductor.

I can use a conductor made of nichrome which is an alloy of 60% nickel, 24% iron and 16% chromium. Accurate laboratory standard are made from a coil of resistance wire inside a protective plastic case.

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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