A current balance is a device to measure magnetic forces. It is constructed from two parallel coils, each with an average radius of 12.5 cm. The lower coil rests on a balance; it has 20 turns and carries a constant current of 4.0 A. The upper coil, suspended 0.314 cm above the lower coil, has 50 turns and a current that can be varied. The reading of the balance changes as the magnetic force on the lower coil changes. What current is needed in the upper coil to exert a force of 1.0 N on the bottom coil?

Ammeter Variable dc power supply Wire wrapped around disk circumference 50 tums 20 turns 0.314 cm Plate on which dc power supply Stand with clamp lower coil rests Triple beam balance to support upper coil to weigh lower coil