# Electron moles present in wire cross-section [SOLVED]

JoeyBob
Homework Statement:
See image
Relevant Equations:
I=C/t
So I know current is just coulombs/second. Electrons are also in the unit of coulombs, so I can get coulombs to cancel.

7.9C/s/1.602E-29C = 4.93133E29 1/s

Now I just need to get mol on top. There are 6.022E23 electron in a mol so 4.93133E29 1/s / 6.022E23 atoms/mol = 8.1889E5 mol/s.

Now my problem is that I don't know what to do with the diameter. My only hint is that the units also have m^2, which could imply that I need to multiply the above by area? So using A=pir^2 I would get an area of 0.5027 m^2. Obviously multiplying this value by the above value won't get me the right answer.

What am I missing here?

#### Attachments

• Wire.PNG
8.2 KB · Views: 66

Homework Helper
Gold Member
They want to know the number of moles per area per second. It should be obvious that you divide by the area.
You need to convert the centimeters to meters in computing the area. The number you give for the area is way off.

JoeyBob
They want to know the number of moles per area per second. It should be obvious that you divide by the area.
You need to convert the centimeters to meters in computing the area. The number you give for the area is way off.

Youre right, but even if I divide instead of multiply the area I still get the wrong answer. The answer is 1.63, but ill get 1629129.9. So its right but has too many digits it seems.

JoeyBob
Okay i forgot to convert the cm too, but when I convert it to m and divide by area I get an answer that's even bigger and also wrong (4.07 E11)

Homework Helper
Gold Member
The electron charge is 1.602E-19 .(not E-29).
Make sure you use the radius of the wire (rather than the diameter) in computing the area. =Edit=looks like you did. You then divide by the area. You should get the correct answer.

Last edited:
JoeyBob
The electron charge is 1.602E-19 .(not E-29).
Make sure you use the radius of the wire (rather than the diameter) in computing the area. You should get the correct answer.

So then I get 8.1991 E-5for mol/s. Dividing by area still doesn't work for me.

0.4cm = 0.0004 m. A=pir^2 = 0.000000502.

0.000081889/0.000000502 = 163, which is too large by a factor of 100.

Homework Helper
Gold Member
You would do better to use exponential notation. But the ## r=.004 ## m. I think everything then works.

• JoeyBob
JoeyBob
You would do better to use exponential notation. But the ## r=.004 ## m. I think everything then works.

Yeah I am dumb. cm is not mm.