An inductance L and a resistance R are first connected to a battery. After some time the battery is disconnected but L and R remain connected in a closed circuit. Then the current reduces to 37% of its initial value in time ?

(1) RL sec

(2) $\frac{R}{L}sec$

(3) $\frac{L}{R}sec$

(4) $\frac{1}{LR}sec$

In an LR-circuit, the time constant is that time in which current grows from zero to the value (where I₀ is the steady-state current) 

(1) 0.63 I₀

(2) 0.50 I₀

(3) 0.37 I₀

(4) I₀

In the figure magnetic energy stored in the coil is 

(1) Zero

(2) Infinite

(3) 25 joules

(4) None of the above

The core of a transformer is laminated to reduce energy losses due to  

(1) Eddy currents

(2) Hysteresis

(3) Resistance in winding

(4) None of these

A step-down transformer is connected to 2400 volts line and 80 amperes of current is found to flow in output load. The ratio of the turns in primary and secondary coil is 20 : 1. If transformer efficiency is 100%, then the current flowing in primary coil will be 

(1) 1600 A

(2) 20 A

(3) 4 A

(4) 1.5 A

A loss free transformer has 500 turns on its primary winding and 2500 in secondary. The meters of the secondary indicate 200 volts at 8 amperes under these conditions. The voltage and current in the primary is 

(1) 100 V, 16 A

(2) 40 V, 40 A

(3) 160 V, 10 A

(4) 80 V, 20 A

A transformer connected to 220 Volt line shows an output of 2 A at 11000 Volt. The efficiency is 100%. The current drawn from the line is:

1. 100 A

2. 200 A

3. 22 A

4. 11 A

A power transformer is used to step up an alternating e.m.f. of 220 V to 11 kV to transmit 4.4 kW of power. If the primary coil has 1000 turns, what is the current rating of the secondary ? Assume 100% efficiency for the transformer 

(1) 4 A

(2) 0.4 A

(3) 0.04 A

(4) 0.2 A

The primary winding of transformer has 500 turns whereas its secondary has 5000 turns. The primary is connected to an ac supply of 20 V, 50 Hz. The secondary will have an output of 

(1) 200 V, 50 Hz

(2) 2 V, 50 Hz

(3) 200 V, 500 Hz

(4) 2 V, 5 Hz

A step-down transformer is connected to main supply 200V to operate a 6V, 30W bulb. The current in primary is 

(1) 3 A

(2) 1.5 A

(3) 0.3 A

(4) 0.15 A