The resistance of a wire.

The resistance of a wire

Planning

Introduction

Resistance occurs when the electrons travelling along the wire collide with the atoms of the wire.
These collisions slow down the flow of electrons causing resistance. Resistance is a measure of how
hard it is to move the electrons through the wire.

Longer wires have more resistance than shorter wires, as there are more atoms for the electrons to collide with.
Thinner wires have a larger resistance than thicker wires as it is harder to push through a thinner space.
Thicker wires have lower resistance than thinner wires and therefore produce less heat.
The hotter the wire the higher the resistance.
Twice the length = twice the resistance as there are twice as many atoms to collide with.
Twice the thickness = half the resistance as the electrons have twice as much room to move through the wire therefore halving the chance of collisions.

Factors

The following factors will have an effect on the resistance of the wire:

The thickness of the wire, the thicker the wire the lower the resistance.
The length of the wire, the longer the wire the higher the resistance.
The material used; e.g. copper will have a lower resistance than nickel.
The size of the current, the size will affect the resistance dramatically, the higher the current, the higher the resistance.

My chosen variable will be the length of the wire as there are many more possibilities and room for adjustment in this factor than there is in others, this will also be easier to measure than other factors such as the thickness of the wire. Firstly I was going to do the different types of material, but this factor would have been hard to make into a fair test. All materials are subject to wear and tear and therefore some materials may be affected more seriously by heat than others and the wire will degrade leading to unfair tests.

Prediction

Based on previous knowledge I predict that the resistance of the wire will increase in proportion to the length. Double the length the resistance will double, treble the length and the resistance will be three times as much as there will be three times as many fixed atoms for the moving electrons to bump into as they Pass through the wire.

Though my preliminary results say that this isn’t strictly true as 20 cm = 2.087ohms and 40 = 3.905, I believe that my preliminary test was an ...

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Prediction

Though my preliminary results say that this isn’t strictly true as 20 cm = 2.087ohms and 40 = 3.905, I believe that my preliminary test was an unfair test as it was affected by a number of unforeseen and unmeasured factors such as thickness of the wire. In a perfect experiment I believe these resistance’s’ would have been two and four.

Preliminary Experiment

At first I tried to do this with a 5cm, 10cm and 15cm length of wire, but they hot to quickly. As the wire got hotter the ions inside the wire began to vibrate increasing the chance of collisions and affecting the resistance. I will start my headings at 20cm because, clearly, short lengths heat up, influencing the resistance.

Equipment

Metre Ruler for measuring the length of the wire.

1 long piece of wire with medium thickness.

An ammeter to measure the current.

A voltmeter to measure the potential difference

A power pack to supply the current needed.

3 black wires and three yellow wires to connect the voltmeter wire power pack and ammeter together.

Method

Collect the equipment listed above.

Measure the wire to 100 cm and cut off any excess.
Link the circuit up so that the Ammeter and Voltmeter can read the current/PD, travelling through the wire and the power pack is supplying energy to the wire. There should be a resistor between the wire and the power pack.

Switch on the power pack and record the voltage and current.
Move the left/right crocodile clip down to 80cm and again record the results.
Continue decreasing the length by 20cm each time until you reach 0cm
Divide the voltage by the current on each column to find out the resistance.
Repeat this experiment and take averages.

During this experiment I have considered using safety goggles, but as no harmful liquids or objects are being used I do not think it is necessary. No gloves or other safety gear is needed, as there are no real risks that merit this sort of protection.

Obtaining Evidence

Analysis

The graph shows strong positive correlation, as the length increases so does the resistance. I predicted that based on previous knowledge I predict that the resistance of the wire will increase in proportion to the length. Double the length the resistance will double, treble the length and the resistance will be three times as much as there will be three times as many fixed atoms for the moving electrons to bump into as they Pass through the wire.

Though this prediction is not strictly true I believe that the errors were caused by unforeseen factors. At 20 cm the resistance was 2.3 ohms but at 40cm the resistance was 4.4 lower than the predicted 4.6 ohms. I believe this was down to factors that had not been taken into account or could not easily be controlled such as the temperature of the room. When the temperature was high the wire would be slightly hotter causing the fixed atoms to vibrate more and therefore resulting in more collisions coincidentally producing a higher resistance. My line of best bit on the graph strangely does not pass through 0, this is because there is a minimal but significant amount of resistance in the leads and crocodile clips leading to a higher resistance than 0 when there is 0cm of wire.

There are no anonymous results on my graph suggesting that the experiment was reasonably fair.

I have labelled three points in the graph 1,2 and 3. At point 1 the length is 20cm and the resistance is 2.1, this is 0.2 ohms below the line of best fit. At this point the resistance is 0.1 above my predicted resistance, this would lead me to conclude that the resistance through the leads and crocodile clips is 0.1 ohms.

At point 2 the length is 40 and the resistance is 4.6 this is 0.2 ohms above my line of best fit and 0.6 ohms above my predicted resistance (4 ohms). Either the temperature has risen causing a higher resistance or the resistance in the leads and clips has risen by 0.4 ohms due to unforeseen factors.

At point 3 the length is 80 and the resistance is 8.3 ohms this is below the line of best fit by 0.2 and above my predicted resistance for this point by 0.3. The temperature has either dropped now or the resistance in the leads and clips has dropped by 0.3 ohms.

Evaluation

Overall I believe I followed my method very closely while performing the experiment, however certain precautions could have been taken to insure this was a safer experiment. Firstly the wire could have been perfectly straightened before being measured to the millimetre rather than to the centimetre, a wire That was slightly bent in places would have led to the wire being longer than what is recorded in a table as the ruler is measuring extra length. This extra length would have caused extra resistance and there were several readings with extra resistance in my results. The current could have been more precisely measured using a digital power pack would have reduced human error such as seeing the amount of current from different points of view and changing it each time. The readings of my voltmeters were inaccurate due to this so in my real experiment I decided to use digital voltmeters. The temperature of the wire was a factor I could not fix as it would have been almost impossible to control the wire temperature due to the resistance heating it, the room temperature would also have been hard to fix. On a hot sunny day it is likely that my resistance readings would have been higher than my readings on a cold snowy day. The already present resistance in the crocodile clips could have been reduced by sanding off any dirt on the clips and cleaning them. Doing the same to the connectors at either end could have reduced the resistance in the leads. The wire’s thickness could have been measured at more points along the wire to make sure the wire’s thickness was the same all along. A thicker wire would lead to lower resistance and a lower resistance on a result would affect my calculated average.