The resistance of a wire, R ohms , is directly proportional to its length , I meter , and inversely
proportional to the square of its radius , r meter.
(a) Express R in terms of l , r and a constant k.
(b) Maker the subject of the formula in (a).
(c) (i) 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 meters and a resistance of 60 ohms