Homework Help: Current and resistance help

1. Sep 28, 2006

tigert2004

Figure 26-24a gives the magnitude E(x) of the electric fields that have been set up by a battery along a resistive rod of length 9.00 mm . The rod consists of three sections of the same material and length but with different radii. The radius of section 3 is 5.00 mm.
E for section 1 is 2.5e3V/m, E for section 2 is 4e3V/m,
E for section 3 is 1.5e3V/m

I understand that p is the same for all of them put I am getting stuck after that

2. Sep 28, 2006

Andrew Mason

What is the question?

Check the length. 9 mm can't be right if it is a rod. It must be 9 m.

You can find the applied voltage and voltage drops across each section easily enough from the electric field and lengths. You also can determine the resistance of each section in proportion to the resistance of the other sections or the total resistance.

All we need to know is the question.

AM

3. Sep 28, 2006

tigert2004

I am sorry, The question is what is the radius of the first and second section. The length is 9 mm. I can find V by equaling it to LE.

4. Sep 28, 2006

Andrew Mason

Well the length doesn't really matter but if it is 9 mm, it is wider than it is long, hardly a rod.

Find the total applied voltage and voltage drops across each section.

With that, you can find the proportional resistance of each section. How is resistance related to resistivity, length and area? Use that information to find the relative areas of each section and use the area of section 3 to find the areas and radii of the other two.

AM