# Help on Resistivity of Material Type of Question

1. Jul 15, 2007

### The Vin

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

You have a 25 mm long cylindrical carbon resistor, with a cross-section of radius 15·10-6 m.

What is its resistance?

2. Relevant equations

Resistance = pL/A where p is the resistivity of the material, L is the length, and A is the area(?).

Also, given is that the p for a carbon material is 3500 x 10^-8 ohm meters. And 25 mm = .025 m.

3. The attempt at a solution

I'm stuck on this problem because I can't figure out what the A in the equation works out to be or refers to. I know it's an area, but what area? The area of a cylinder? The surface area? I'm not quite sure. I assumed A referred to the surface area of the cylinder, and given the length and cross sectional radius, I computed the surface area. But when I plugged in the formula (3500 x 10^-8) (.025 m)/ surface area of cylinder, the resistance value I got was incorrect.

I know this is a really nit picky question, but I'm going insane trying to figure out what the area refers to in this particular problem. Any help would be appreciated.

2. Jul 15, 2007

### cristo

Staff Emeritus
A is the cross sectional area of the wire. See here for more details.

3. Jul 15, 2007

### The Vin

How do you compute that area though? The problem gives the radius and length of the wire, as well as the shape, but I'm not sure how to use that information to compute an area.

4. Jul 15, 2007

### cristo

Staff Emeritus
You're told that the resistor is cylindrical, and so the cross section of this resistor is a circle with radius as given in the OP. How do you find the area of a circle?