# Resistance & Current in a wire

1. Jun 6, 2014

### victorializ

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

The resistivity of gold is 2.44 x 10-8 Ω m at a temperature of 20°C. A gold wire, 0.5 mm in diameter and 44 cm long, carries a current of 380 ma. The number of electrons per second passing a given cross section of the wire, is closest to:

A) 2.4 × 10^18
B) 1.2 × 10^22
C) 2.8 × 10^14
D) 2.4 × 10^17
E) 6.3 × 10^15

2. Relevant equations

R=ρL/A

3. The attempt at a solution

R=(2.44 x 10^-8) (.44)/∏(.00025^2)
R= .0547
then i multiplied the resistance by the current (.38) to get .021 for a voltage.
then i divided that by (1.6 x 10^-19) and got 1.3 x 10^17 which isn't right but is close to the answer which is A

2. Jun 6, 2014

### Staff: Mentor

Why did you divide a voltage by the electron charge? Do you expect the right unit "per second" as result of that calculation?

You are given the current. What does a current of 1 A (example value) mean?

3. Jun 6, 2014

### rude man

Something's off by a factor of 18.5 and you call that 'close'?

Learn that people are going to give you problems with a lot of irrelevant information!

4. Jun 6, 2014

### victorializ

so i need to use the fact that 6.241 x 10^18 electrons/sec pass through a circuit which is 1 A

when i use that number though i still have it wrong so i'm not really sure where else my calculations are messed up.

5. Jun 6, 2014

### dauto

But the current isn't 1 A.

6. Jun 6, 2014

### victorializ

oh my goodness so most of the information in this problem is irrelevant? all i need is to do a proportion between 1A and the 38 mA in the problem?

7. Jun 7, 2014

### Staff: Mentor

If that gets you closer to the right answer, go for it!

380 mA would be even better ...

8. Jun 7, 2014

### Staff: Mentor

Yes (apart from the typo (?) NascentOxygen pointed out).