# Red herring? (1 Viewer)

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#### qim

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
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

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
Gold Member
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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