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/sci/ - Science & Math

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3944603 No.3944603 [Reply] [Original]

I'm having trouble understanding resistivity.

Resistivity is ohms times meters, so an ohm can be thought of as resistivity per meter.

That would mean that a long wire would have less resistance than a short one, if resistivity is constant. Right? How does that make sense.

>> No.3944630

bump

>> No.3944639

Resistivity = ohm/meter
ohm = resistivity*meter
not
ohm = resistivity/meter

>> No.3944648

resistivity is the inherent electrical resistance in a material. the longer the material, the more resistance.

>> No.3944655

Newtons Second law
A force always has an opposite force

When current has resistance act upon it (4 ohms for example ) the current automaticly pushes back at 4 ohms to cause the current to equal

So even though you have 4 ohms constantly for 3 meters, and you have 12 ohms in a total resistance series. your wire pushes back to have 12 ohms of resistance in total to connect it

>> No.3944678
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3944678

>>3944655

>> No.3944679

if you keep resistivity over the course of the wire then obviously the resistivity per meter will drop

I don't see how to explain it any more than that

>> No.3944692

>>3944655
WTF?!!

>> No.3944832

R=Pd/Area, not R=P/d

the units are the same, but the formulae are different.

>> No.3944905

think of resistivity as resistance density

>> No.3946503

The resistance of a conductor (like a wire) depends on its length, its cross section and some factor (rho) describing the resistivity of the material the current flows through. Resistivity means how 'bad' the stuff conducts while conductivity describes how 'good' it conducts.

If you want to describe the resistance of some length of said wire you get R=rho*l/A where A is the cross section of the wire. The longer the wire the higher the resistance and the thicker the wire the lower its resistance.

If you express this in terms of the units you get Ohm=(rho)*meter/meter^2. Solving this for (rho) gets you the unit for the resistivity: [rho]=Ohm*meter^2/meter or (reduced) [rho]=Ohm*meter. This is one of the cases where the origin of the unit is somewhat hidden in its reduced form.

In practice you will find that resistivity is often given in a more convenient unit like Ω*mm^2/m because wire diameter (-> cross section) is mostly given in millimeter. Copper e.g. has a value of about 0.018 Ω*mm^2/m. To convert this value into the (Ω*m) form you'll have to replace 1mm by 10^-3m and then you get 1Ω*mm^2/m = 10^-6Ω*m or 1µΩm. The value for copper then would be 0.018µΩm or 18nΩm.