# Resistivity Question

1. Nov 30, 2009

### TheOne123

I was doing some questions from a book on resistivity and I came across this, I really can't work this out, I'm stumped.

2. Nov 30, 2009

### nasu

You can consider two half-rings connected in parallel. For each one use the formula for the resistance of a wire.

3. Nov 30, 2009

### Andrew Mason

Calculate the resistance of the ring in this circuit. Then work out the resistance of one half of the ring on the basis that the two halves of the ring amount to two half rings in parallel. Then work out the resistivity from that resistance.

AM

4. Nov 30, 2009

### TheOne123

Hmm ok. I'm still a bit cofused. Can someone start. Of te calculation for me please an I will go from there. Thanks in advance.

5. Nov 30, 2009

### ideasrule

As for practical difficulties, how many gold rings have you seen that have a perfectly rectangular cross-section?

6. Nov 30, 2009

### Andrew Mason

Use Ohm's law to determine the resistance.

Since this total resistance is from two sides of the ring in parallel, you can work out the resistance of each side of the ring:

$$\frac{1}{R} = \frac{1}{R_{half}} + \frac{1}{R_{half}}$$

Use $\rho L/A = R$ to find the resistivity, where L is the length of the conductor and A is the cross sectional area.

AM

7. Dec 1, 2009

### TheOne123

Ahh I still keep getting it wrong.

The answer should be 4.4 X 10-7 as it says in the back.

8. Dec 1, 2009

### nasu

Isn't 4.4 X 10-8?
This is more realistic.

9. Dec 1, 2009

### TheOne123

It says 4.4 X 10-7. How did you get 4.4 X 10-7?

10. Dec 1, 2009

### nasu

If you show your work I'll try to identify the mistake.
But I think you'll get about 4x10^(-8).