Investigating the factors that affect resistance of a wire.

As this diagram demonstrates the longer wire has a smaller potential gradient than the smaller wire (at a potential slope of 10V). Therefore the flow of electrons is slower and the current lower.

The Length of the wire and the resistance of the wire are directly proportional.

How the Material of the wire affects the resistance of the wire

If valence electrons are loosely held then resistance will be low. Metals are good conductors and non-metals are insulators. Valence electrons are the electrons used in bonding (the electrons on the outer shell of the atom). For example chlorine goes around in pairs and share electrons to make 2 full outer shells, so it does not conduct electricity. Substances that have covalent electrons (share electrons) have all their electrons tied up so they are insulators. Metals have free valence electrons and therefore conduct electricity well.

Resistivity = ?

Conductivity = 1

?

The resistance of a material is directly proportional to the Resistivity.

How the cross sectional area affects the resistance of the wire

Increasing the cross sectional area is like adding resistors in parallel. Two wires side-by-side down a potential gradient means twice the number of charges moving down the "slope". Therefore the current is doubled and the resistance is halved.

Resistance is directly proportional to 1

Area

Therefore Resistance is inversely proportional to the cross sectional area of a wire.

Investigating how the length of the Wire affects the resistance of the wire

Fair Test

It is very important that this investigation is a fair test; therefore all the other variables apart from the length of the wire must be controlled. To control the temperature of the wire the current must be kept very low so that the temperature of the wire does not increase. If it did so then the resistance of the wire would increase. A current of 0.1 Amps was decided on, and this must be the same for every test I make. To keep the diameter and the material of the wire constant I will use the same piece of wire for each test. This means that the material and diameter remain the same. The diameter and material of the wire also affect its resistance so it is important that they are kept constant.

Experiment

Apparatus

I chose certain items to use as apparatus in order to get the best set of readings for my investigation possible. I chose to use Nichrome wire with a diameter of 0.38mm because it gave the best range of readings.

Diagram

Method

Set up the apparatus as shown in the diagram. At first make the length of the Nichrome wire 10cm. Keep the current constant at 0.1 A and record the amount of potential difference across the wire by taking the reading at the voltmeter. Adjust the accuracy of the reading by turning the dial on the multimeter between 20V, 2V and 20mV. Repeat the experiment 3 times for this particular length of wire. Then repeat but these times have 20cm of the Nichrome wire in the circuit. Again repeat the experiment 3 times. Repeat this procedure for lengths of wire going up by 10cm each time, 30, 40, 50, 60, 70, 80, 90cm and 1m.

I investigated a wide range of wire lengths so that I could make good use of the length of wire available and so that I could plots a good and correct graph, that follows the theory I have explained previously.

I repeated the experiment 3 times so that I could spot and correct any anomalies in my results.

How I chose my apparatus and how I used preliminary readings to refine my plan

Before I decided what apparatus to use in my investigation I completed a set of preliminary readings. I tested lots of different wires with different materials and diameters. I measured the potential difference across the wire at the minimum (10cm) and the maximum lengths of the wire (1m). This enabled me to see which type of wire gave the best range of readings so that they were easy to plot as a graph. I chose Nichrome wire with a diameter of 0.38mm because it gave the best range of results. It also enabled me to see what voltmeters and ammeters would be most suitable for my experiment. I chose an ammeter with a scale of 0 to 2.5 amps because it was most accurate for the current I have decided to use for all my experiments and shows 0.1A clearly. I chose to use the multimeter to record the potential difference across the wire because it had a scale of mV and is more accurate than the other voltmeters.

Predictions

I predict that as the length of the wire increases so does the resistance of the wire. This is because as the length of the potential slope increases the gradient of the slope decreases. This means that the current flowing through the wire is less and therefore the resistance of the wire has increased.

This is what I predict my final graph will look like. The length of the wire and the resistance of the wire are directly proportional.

Bibliography

www.sci-journal.com

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Physics for You by Keith Johnson- Stanley Thornes