# Homework Help: Is the current density larger, smaller, or the same?

1. Oct 16, 2009

### tag16

1. The problem statement, all variables and given/known data
A section of a circular conductor of non uniform diameter carrying a current of 5.00 A. The radius of the cross-section A1 is 0.400cm.(a) What is the magnitude of the current density across A1?(b) The radius at A2 is larger than the radius at A1. Is the current at A2 larger, smaller, or the same? Is the current density larger, smaller, or the same? Assume one of these two quantities is different at A2 by a factor of 4 from its value at A1. Specify the current, current density, and radius at A2.

2. Relevant equations
J=I/A

3. The attempt at a solution
J=I/A= 5.00A/pi(.004)^2=99,472.8 A m^2

Is this right so far? I'm not sure since it's nonuniform. If it is then how do you find the current for A2? and if it's not then what do I need to do?

2. Oct 16, 2009

### Redbelly98

Staff Emeritus
Pretty much, except that
1. How many significant figures are justified in the answer?
2. The units are A / m^2, not A m^2, since you divided by the area.

Are you familiar with Kirchoff's Current Law?

3. Oct 17, 2009

### tag16

Is that the same thing as Kirchhoff's Rules? If not then, no I'm not.

4. Oct 18, 2009

### Redbelly98

Staff Emeritus
Yes, those. Kirchhoff's Current Law says, basically,

current out = current in​

5. Oct 18, 2009

### tag16

so for A2: J=I/A= 5.00A/pi(0.16)^2=62.1 A/m^2

and the to find the current I would use I=JA right?

6. Oct 19, 2009

### Redbelly98

Staff Emeritus
Not sure where you're getting the 0.16 radius from here.

The current does not change along a conductor.