# Homework Help: Question 3

1. Feb 4, 2005

### kyang002

The electron drift speed in a gold wire is 3.0 * 10^-4.

1. What is the current density in the wire?

2. What is the current if the wire diameter is 0.50 mm?

I am unsure of which equation to use.
Q = I delta t ??

Can someone help me out?

2. Feb 4, 2005

### MathStudent

2. What is the current if the wire diameter is 0.50 mm?

I am unsure of which equation to use.
Q = I delta t ??

Can someone help me out?[/QUOTE]

Look in your book,, there should be some equations relating current to drift velocity and current density.

3. Feb 4, 2005

### kyang002

Q = I delta t is the equation that I found. It doesn't make sense to me. Obviously I already looked in the book and cannot figure it out, that is why I am posting on this forum. If you can please leave more helpful info on how to solve the problem instead of telling me to look in the book, that would be much better.

4. Feb 4, 2005

### dextercioby

Don't they give you the "n" for gold??You know what "n" stands for,right??U had to compute it at one of the other problems...

Daniel.

5. Feb 4, 2005

### MathStudent

I highly doubt that these equations are not given in your book, and it doesn't do you much good to just plug in numbers without knowing what these equations mean, but I'll list them here just in case there was a disaster at the publishing company, and they left these out. You should try to find these in your book later, and see where they come from .

$$I \ = \ nqv_dA$$

$$J \ = \ \frac{I}{A} \ = \ nqv_d$$

where $J$ is current density, $n$ is number of free moving charge particles per unit volume, $v_d$ is drift velocity, and $A$ is cross sectional area.

6. Feb 4, 2005

### kyang002

I never figured it out because I was stuck on how to figure out n and q.