Question

You are given a copper bar of dimensions 3 cm × 5 cm × 8 cm and asked to attach leads to it in order to make a resistor. If you want to achieve the largest possibleresistance, you should attach the leads to the opposite faces that measure:

a) 3 cm × 8 cm.
b) 3 cm × 5 cm.
c) 5 cm × 8 cm.
d) Any pair of faces produces the same resistance.

Answer 1

we have the formula for resistance as
R = ?L/A -----------(1)
? = 1.68*10-8(Om)
if the opposite faces have 3 cm × 5 cm then
L= 0.08m
A = 0.03 m ×0.0 5 cm
=0.0015m2

so R = 1.68*10-8(Om)*0.08m /0.0015m2
=90*10-8 O
since the value of denominator of equation (1)for both the cases is larger than the first case so the value ofresistance is maximum for (b) compared to (a)&(c) so theanswer is (b).

Answer 2

b) 3 cm × 5 cm