# Resistance, cross-sectional area of long coated wire

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1. Feb 27, 2017

### Cocoleia

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

2. Relevant equations

3. The attempt at a solution
I attempted all of the parts:

I think I did the right things for a-b-c-d, but I am pretty unsure about e & f. Can someone verify if my logic is right? Thanks.

2. Feb 27, 2017

### haruspex

You have d wrong. The formula you quoted is not quite right.

3. Feb 27, 2017

### Cocoleia

Ah I see it should be 1/Req. If I change that, will the following steps be right assuming I use that value ?

4. Feb 27, 2017

### haruspex

Your method in e) is right.
In f, you seem to have assumed all of I is in the inner wire when calculating the current density there.

5. Feb 27, 2017

### haruspex

If you read posts 2 and 3, you'll see that d) was wrong, and this led to the wrong expression in e.

6. Feb 27, 2017

### Cocoleia

How do I calculate it without assuming this? By the way, this isn't homework, I am just working on problems to prepare for an exam tomorrow.

7. Feb 27, 2017

### haruspex

One way would be to suppose the potential applied to the wire is V and compute what current would flow in each component.

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