The resistance of a wire, R ohms , is directly proportional to its length, I m, and inversely
proportional to the square of its radius, r m.
(a) Express R in terms of l, r and a constant k.
(b) Make r the subject of the formula in (a).
(c) (ii) Given that a wire with a length of 2 m and
a radius of 0.5 cm has a resistance of 40 ohms,
use your answer in (a) to find the value of k.
(ii) Hence, use your answer in (b) to find the
radius of a wire with a length of 4 m and a
resistance of 60 ohms.