# Homework Help: Resistance of cylinder case a and case b

1. May 6, 2014

### delsoo

1. The problem statement, all variables and given/known data

what's the difference between case a (photo 1&2) and case b (photo 3 ) ? what cant i use the way of doing case b for case a? p/s : my own working for case a in photo 4

2. Relevant equations

3. The attempt at a solution

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2. May 6, 2014

### Simon Bridge

Do you have any reason to think they should be handled differently?

3. May 6, 2014

### delsoo

for case a , i do it in this way because since the length is already given , I sub l=0.05m (case b provide radius only, so i use 2pi r to find the total length in a coil) , area= uniform cross sectional area, 1.0mm^2

4. May 7, 2014

### Simon Bridge

For case (a) the length of the wire is given, not the length of the coil.

If you just put a current down the straight wire, you'd use $R=\rho L/A$ where L is the length of the wire and A is it's cross-sectional area.

But the problem writer is making a big deal out of the fact the wire is not straight and it isn't a regular wire.

i.e. It says the wire has a square cross-section - why is that important?

How would wrapping the wire into a coil make a difference?

Is it important that the wire is un-insulated?

Is it important that the sides of the wire touch each other?