Resistance

[ATB_roo]

In an experiment in science I had to investigate how the length of wire affected resistance. To calculate the resistance of the wire I used the formula resistance = voltage / current. When I was doing the experiment I found that if pulled the piece of wire tighter, the current would increase. Obviously, this would mean a difference in my resistance results. I asked my teacher but she didn't know. Does any one here know why the tautness of the wire would affect its resistance?

[mibn2cweus]

Without seeing your set-up I can only guess that pulling on the wire effected (improved) your electrical connection between the wire and the rest of the circuit?

[97026]

Resistivity (p) - greek symbol rho (not uncapitalised P)

The electrical resistivity of a material is defined by
R = (pL) / A

R = resistance of some conductor in ohms
L = length of conductor in metres
A = Area os cross-section of conductor in square metres
p = The resistivity of the material (look in text book or data sheet) of which the conductor is made

So you can play around with the area of cross-section seeing the effect on the resistance. Obviously, even if you stretch the material the cross section decreases. The "tautness" stretches the material a little. some of it permenately if you pull enough. Further reading "Hookes law"!

[ugly_bob]

97026 -
Did you mean the electrical resistance of an object is defined by
R=(pL)/A
?

[rojash]

However, 97026, what you describe would seem to produce exactly the opposite effect to that experienced by ATB_roo

[ugly_bob]

Good point rojash. Didn't notice that. I'm going with mibn2cweus then - must have made a better connection somehow.

[Hamish]

can you tell us the voltage used, approx diam of the wire, approx length of the wire and the observed change in resistance.

[Hamish]

Oh and approximately how much force you applied. Ta

[97026]

RoI_I_ox! Too busy with the equation. Sending PhD back!
Must be making a better connection or more surface area.
<< :(

[Big Al1st]

First, the total length of the wires will effect the amount of resistance. The longer the wire, the more resistance that there will be. There is a direct relationship between the amount of resistance encountered by charge and the length of wire it must traverse. After all, if resistance occurs as the result of collisions between charge carriers and the atoms of the wire, then there is likely to be more collisions in a longer wire. More collisions means more resistance.

Second, the cross-sectional area of the wires will effect the amount of resistance. Wider wires have a greater cross-sectional area. Water will flow through a wider pipe at a higher rate than it will flow through a narrow pipe; this can be attributed to the lower amount of resistance which is present in the wider pipe. In the same manner, the wider the wire, the less resistance that there will be to the flow of electric charge. When all other variables are the same, charge will flow at higher rates through wider wires with greater cross-sectional areas than through thinner wires.

Hope this helps

[ATB_roo]

Wow, thanks for the replies everyone. Very helpful indeed. I asked my teacher again the other day and she said it was because when I pulled the wire tighter I made a better connection with the crocadile clips and the wire.
This was an investigation for my GCSE coursework by the way, I got an A* :) Thanks again.