Curent densitys relation to drift velocity

1. Feb 13, 2012

DODGEVIPER13

1. The problem statement, all variables and given/known data
A beam contains 4.0x10^8 doubly charged positive ions per cubic centimeter, all of which are moving north with a speed of 1.2x10^5 m/s.

(a) What is the magnitude of the current density ?
(b) What is its direction?
(c) What additional quantity or quantities are needed to calculate the total current in this ion beam?

2. Relevant equations
J=I/A=nqv (v = drift velocity)

3. The attempt at a solution
Well I tried this (4.0e8)(2)(1.6e-19)(1.2e5) which I thought would give what I wanted but apparently not. I was thinking by looking a dimensional analysis that it has something to do with the seconds in the velocity but im not sure?

2. Feb 14, 2012

ehild

Take care with the units of length. The ion density is given in particles/cubic centimeters, the velocity is given in m/s. The current density has to be in C/m2.

ehild

3. Feb 14, 2012

DODGEVIPER13

Ok I did that I converted (4e8) in particles/ cubic centimeter to 400 particles per cubic meter but it still did not give the right answer?

4. Feb 14, 2012

ehild

There are 400000000 particles in one cm3. How many cm3 makes one m3? (which one is bigger? 1 cm3 or 1 m3?) Where are there more particles? In a big volume or in the small volume?

ehild

5. Feb 14, 2012

DODGEVIPER13

Huh wait your confusing me I though 400 cubic meters was correct is it not?

6. Feb 14, 2012

ehild

What was 400 cubic meters?

ehild

7. Feb 15, 2012

DODGEVIPER13

im saying the conversion from cubic centimeters to cubic meters is 400

8. Feb 15, 2012

DODGEVIPER13

I got it thanks for all the help!!! WOOT