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I am conducting an experiment to find out how the length of wire affects resistance.

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Introduction

Science Coursework

Resistance

Introduction

I am conducting an experiment to find out how the length of wire affects resistance. The experiment I am conducting is passing an electric current through a wire made of constantan. I will take readings of the voltage and the current to find the resistance change when the length of the wire used is changed. I will be keeping the current and the voltage the same to obtain the results I want. The current will been kept unvaried by the use of a variable resistor. What I will be changing to find out about resistance is the length of the wire, 100cm – 10cm. I will work out the resistance of each experiment by use of ohms law, and I will repeat each length 3 times.

Method

To do the experiment I will first set up all the equipment (diagram below). The power pack will be set to 8volts and I will keep the current at a constant reading. To do this I will vary the variable resistor on each length change. I will connect all the equipment and turn on the electricity. I will then take a reading of the voltmeter. I will then change the length of the constantan wire by moving the crocodile clips down the wire to the length that is needed. I will then turn the power on to adjust the current flow.

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Middle

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The experiment will work by electricity passing through the constantan. The length of the constantan will change the amount of resistance there is. The longer and thinner the wire is the more resistance it has because of the amount positive ions the electrons will hit as they pass through the wire. Resistance is the property of any object or substance, resisting or opposing the flow of an electrical current. The amount of resistance in an electric circuit determines the amount of current flowing in the circuit at any voltage in the circuit. They can then show the amount of electrons passing through the wire.

The resistance is found by using ‘Ohms Law’. Ohms law, named after it discoverer, Georg Ohm (a German physicist). Ohms law states that the amount of current flowing through a conductor is directly proportional to the electromotive force applied between the ends of the conductor. Resistance is defined as the ratio of the electromotive force to current, this creates the formula R= V/I, where I is the current in amps, and V is the voltage. Ohms law works in both direct current (DC) and alternating current (AC), the circuit I am using though is DC.

image35.png

The higher the voltage the more resistance that will occur due to the amount of electrons hitting the positive ions, if there are enough collisions the energy from this can make the wire become extremely hot and glow.

Resistance in wires can be shown by the diagram beneath

image28.png

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Conclusion

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The factor in the resistance which takes into account the nature of the material is the resistivity . Although it is temperature dependent, it can be used at a given temperature to calculate the resistance of a wire of given geometry.

The inverse of resistivity is called conductivity. There are contexts where the use of conductivity is more convenient.

Electrical conductivity = σ = 1/ρ


Resistivity Calculation

The electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made (resistivity). Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as

Resistance = resistivity x length/area

image43.png

For a wire of length L = image44.pngm = image45.pngft
and area
A = image36.pngcm^2
corresponding to radius
r = image37.pngcm
and diameter
image38.pnginches for common wire gauge comparison
with resistivity =
= image39.pngx 10^image39.png ohm meters
will have resistance
R =image39.png ohms.

Enter data and then click on the quantity you wish to calculate in the active formula above. Unspecified parameters will default to values typical of 10 meters of #12 copper wire. Upon changes, the values will not be forced to be consistent until you click on the quantity you wish to calculate.

Standard wire gauges

Table of resistivities

The factor in the resistance which takes into account the nature of the material is the resistivity . Although it is temperature dependent, it can be used at a given temperature to calculate the resistance of a wire of given geometry.


www.hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html

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