A current carrying rectangular loop PQRS is made of uniform wire. The length PR = QS = 5 cm and PQ = RS = 100 cm. If ammeter current reading changes from I to 2I, the ratio of magnetic forces per unit length on the wire PQ due to wire RS in the two cases respectively \(f^1_{PQ}:f^{21}_{PQ}\) is
(1) 1 : 2
(2) 1 : 4
(3) 1 : 5
(4) 1 : 3
