Question

Two metallic wires made from copper have same length but the radius of wire 1 is half of that of wire 2. The resistance of wire 1 is R. If both the wires are joined together in series, the total resistance becomes
A) 2R B) R/2 C) 5R/4 D) 3R/4

Answer 1

Using equation R = rho ×l / A

Where R = resistance; l = length; A = cross sectional area, rho = resistivity.

Assume the wires are circular cross section.

Wire 1 has resistance R.

Wire 2 is the same length, but twice the radius, so 4 times the cross sectional area. A=πr²

So wire 2 has a resistance of R/4.

So total resistance of 2 in series

= R + R/4

= 1.25 * R