# Homework Help: Finding Current in a Wire

1. Dec 7, 2015

### david12445

1. The problem statement, all variables and given/known data
The terminals of a 0.70 Vwatch battery are connected by a 70.0-m-long gold wire with a diameter of 0.100 mm.
What is the current in the wire?

2. Relevant equations
V = IR
R = pL/a

3. The attempt at a solution

I try to find resistance first, resistivity of gold is 2.2 * 10 ^-8

Area of the wire is a = pi * r ^ 2
r = ( 0.1 * 10^-3 ) /2

So I found a = 7.85 * 10^-9

Then,
R = 2.2 * 10^-8 * 70 / (7.85 * 10^-9) = 196.1

I plug this in to V/R = I, .70/196.1 = .00357 = 3.57 mA, but this answer ends up being wrong. Any help on where I went wrong would be appreciated, thanks.

2. Dec 7, 2015

3. Dec 7, 2015

### david12445

I'm putting exactly 3.57 mA into MasteringPhysics, but it keeps saying my answer is incorrect. http://imgur.com/SZN22sv is an image of what the question is, did I type anything wrong? http://imgur.com/pl1MQbC here are the answers I have already tried.

4. Dec 7, 2015

### SammyS

Staff Emeritus
Where does that value for resistiviuty of gold come from?

5. Dec 7, 2015

### david12445

I searched online for the value of the resistivity of gold wire

6. Dec 7, 2015

### Staff: Mentor

Does Mastering Physics provide a reference table of constants to use? Perhaps they have a value for the the resistivity of gold that they expect you to use. The value that you found has just two significant figures and you're told to supply a result with three. This could make a difference in your least significant figure.

7. Dec 7, 2015

### david12445

No, I don't think there is one. I tried a value to three significant figures, 2.25 * 10^-8 but it still wasn't right. MasteringPhysics is usually pretty good about saying it's right as long as you are close, so that must mean my answer is a good bit off. Are there any numbers I put in wrong?

8. Dec 7, 2015

### Staff: Mentor

The value should be in the neighborhood of 3.5 mA. Using various values I've seen for the resistivity of gold I get a range from about 3.51 mA to 3.55 mA.

Could be the question is "broken" in Mastering Physics.

9. Dec 7, 2015

### Chandra Prayaga

Mastering Physics is a stickler for significant figures. Your textbook has a table of resistivities.