Red herring? (1 Viewer)

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qim

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I have a question:

A pencil "lead" is made from non-metallic material which has a resistivity, at room temperature, of 4.0 x 10^-3 m.

A piece of this meterial has a length of 0.15 m and a diameter of 1.40 x 10^-3 m.

Show that the resitance of this specimen is 390 Ohm.

-----


Well I though of using R = rho x l / A

But then I read earlier that " by examining the dimensions of the resistor (assuming it is made from resistance wire instead of carbon as some are) we can also work out its resistance"

Does that mean that the question is a red herring, as the pencil lead is made from non metallic material?

Help...

Thanks

qim
 

Njorl

Guy in a red jumpsuit
Science Advisor
221
8
Other than the units being wrong for resistivity (they should be ohmsxlength, not just length) there is no "red-herring". It is just a simple multiplication.

Njorl
 

qim

13
0
Thanks Njorl

But what am I to make of the statement that the equation should only work for resistors made of resistance wire?

That's the bit I could not understand.

Thanks

qim
 

enigma

Aerospace Engineer
Staff Emeritus
Science Advisor
Gold Member
1,732
8
I'm just guessing here:

Is it possible that they're covering themselves to take into account that typical resistors aren't uniform on the inside? IIRC, some are (deliberately) packed with small pockets of air.
 

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