A Five Wire Junction

1. Feb 1, 2008

cse63146

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

The information about the magnitudes of the current density and the diameters for wires 1, 2, 3, and 4 is given in the table. Some of the values are unknown.
Wire Current density (A/mm^2) Diameter (mm) Total Current (A)
1-------- 1.6 -------- 2.0 -------- ???
2 -------- ??? -------- 3.0 -------- 2.0
3 -------- 3.0 -------- 1.1 -------- ???
4 -------- 0.8 -------- ??? -------- 4.0

What is the current at the 5th wire?

2. Relevant equations

I know you need to use the conservation of current, but I'm just not sure what formula to use.

3. The attempt at a solution

Last edited: Feb 1, 2008
2. Feb 1, 2008

usman27

study kirchhoff's junction law...
J = I/pi*r^2 ; pi = 3.14...
Hope this helps... good luck.

3. Feb 1, 2008

cse63146

would I need to convert the current density and diameter to metres before calculations?

4. Feb 2, 2008

ravi1611

it I5 is going to be-9.84

5. Feb 2, 2008

cse63146

Yep, that's the answer I got, just wasnt sure if it was right. Thanks.

Last edited: Feb 2, 2008
6. Feb 2, 2008

usman27

yes... always go meters for length unless otherwise specified!
and yess the answer is -9.9

7. Feb 2, 2008

cse63146

in this case, I didnt convert; I just left it as it

8. Feb 2, 2008

usman27

That is because the diameter is in (mm) so is the current density :)

9. Feb 2, 2008

cse63146

Ah, good to know. I probably would have done this on the test without noticing. You probably saved me a few marks, Thanks.