A $$500 \mathrm{~W}$$ heating unit is designed to operate on a $$400 \mathrm{~V}$$ line. If line voltage drops to $$160 \mathrm{~V}$$, the percentage drop in heat output will be:
Current flows through uniform, square frames as shown in the figure. In which case is the magnetic field at the centre of the frame not zero?
A transformer which steps down $$330 \mathrm{~V}$$ to $$33 \mathrm{~V}$$ is to operate a device having impedance $$110 \Omega$$. The current drawn by the primary coil of the transformer is :
A cell of emf E and internal resistance r is connected to two external resistances $$\mathrm{R_1}$$ and $$\mathrm{R_2}$$ and a perfect ammeter. The current in the circuit is measured in four different situations:
(a) without any external resistance in the circuit.
(b) with resistance $$\mathrm{R_1}$$ only
(c) with $$\mathrm{R_1}$$ and $$\mathrm{R_2}$$ in series combination.
(d) with $$\mathrm{R_1}$$ and $$\mathrm{R_2}$$ in parallel combination.
The currents measured in the four cases in ascending order are