Question

Consider a block of copper that is a rectangular prism (a box) with sides 15 cm by 20 cm by 50 cm. The resistivity of copper is 1.68e-8. If you consider the resistances between the three sets of parallel faces, which has the middle value?

A. The resistance between the 15 x 20 cm faces.

B. The resistance between the 15 x 50 cm faces.

C. The resistance between the 20 x 50 cm faces.

So I know that resistance = (resistivity * length) / area

But what length should I use to calculate resistance for each answer choice? .15, .15, and .20 respectively?

And is area the area of each plane (i.e .15 * .20, .20 * .50, .15 * .50)?

Please tell me if these are the correct values to plug in.

Answer 1

pick any side to be the length. Then the area is the product of the other two sides. Now your resitiveiy units. You didn't list it, the SI unit is ohm-m, but many tables are in ohm-cm.
I will assume the units are ohm-m
example
a. length 50 cm. R=1.68e-8*.50 /(.15*.20)=2.8e-7
b. length 15cm. R=1.68e-8*.15/(.5*.2)=
2.5e-8

Answer 2

For calculating the resistance in choice A, would the length be .15? And then the resistance would be 8.4 * 10^-8?

Answer 3

Nope - made a mistake! I figured it out and got the problems right :)