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An Investigation into the effect of thelength of a wire on its resistance.

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Introduction

Heather Oakley

An Investigation into the effect of the length of a wire on its resistance

Resistance is the ability of a substance to resist, or oppose, an electric current in a conductor. This resistance makes the electrons of a current give up some of their electrical energy. This energy is converted to heat, light and other energies. Resistance can be calculated by dividing the potential difference (p.d.) across its ends by the current passing through it:

Resistance (R) =   p.d. (V)  h

                    Current (I)

Resistance is measured in Ohms (Ω). One ohm is the resistance of a conductor through which a current of 1 amp passes when the p.d. between its ends is 1 volt.

When calculating the resistance, the p.d. is measured in parallel by a voltmeter and the current is measured in series using an ammeter.

There are several variables which affect the resistance of a wire.

Temperature of wire

If a wire is hot then resistance is increased because electrons find it hard to make a route through the vibrating particles. This diagram shows this theory:

The red line represents the current travelling through the wireimage00.png

image01.png

To stop the temperature of the wire from influencing my results, I am going to put a protective thermistor in the circuit to stop it from getting too hot.

Wire Thickness

Thicker wires have less resistance than ones with a smaller cross-sectional area because the current finds it harder to make a route through.

...read more.

Middle

The protective resistor must not be touched as it will heat up significantly during the experiment.

Pilot Test

Before the main method was carried out, a preliminary experiment was carried out to determine whether a thick or thin wire should be used and to decide what the range of lengths should be for the nichrome wire.

After doing this preliminary experiment, it was decided that nichrome wire lengths fewer than 10cm should not be used as the wire overheated considerably and almost melted. Also, it was decided that the thicker of two wires should be used as this would present me with clearer results.

Prediction

As the length of the wire increases, the number of collisions between electrons and the molecules of the wire will increase because the current has further to travel. These collisions will result in an electrical energy loss (the electrical energy will become heat). The current will decrease because fewer electrons will be able to travel through as the wire gets longer. The voltage will increase because more push will be needed to get the current round the circuit. As the voltage increases and the current decreases, the resistance will increase.

...read more.

Conclusion

Another variable that could have altered was the temperature of the wire. As I never measured the temperature of the wire or of the room temperature, I cannot be sure that they stayed constant. This did not obviously affect the experiment severely because the results followed my prediction and scientific evidence very well.

More experiments could be done to investigate the other variables that affect the resistance of the wire:

  • Thickness of wire. Wires with different thicknesses could be tested to find the effect on the resistance. In this experiment, the length, wire material and temperature should all be kept constant. I predict that the resistance would decrease as the wire thickness increases showing that the resistance is inversely proportional to the diameter of the wire.
  • Material of wire. Different types of wires, such as nichrome, copper and aluminium could be used in place of the nichrome wire on the circuit I made. The wire length, thickness and temperature should all be kept constant. In this experiment, the resistivity of different materials will affect the resistance. Using the equation        = resistance x CSA/ length, the resistivity of each material of wire could be calculated. (CSA = cross sectional area)image03.png
  • Temperature of wire. Resistance could be tested in the same circuit at different temperatures. The length, width and material of the wire should all be kept the same. I predict that as the temperature increases, the resistance will decrease because current does not pass through hot wires as easily.

...read more.

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