A coil has an inductance of 2.5 H and a resistance of 0.5 r. If the coil is suddenly connected across a 6.0 volt battery, then the time required for the current to rise 0.63 of its final value is 

(1) 3.5 sec

(2) 4.0 sec

(3) 4.5 sec

(4) 5.0 sec

Pure inductance of 3.0 H is connected as shown below. The equivalent inductance of the circuit is

(1) 1 H

(2) 2 H

(3) 3 H

(4) 9 H

If a current of 10 A flows in one second through a coil, and the induced e.m.f. is 10 V, then the self-inductance of the coil is 

(1) $\frac{2}{5}H$

(2) $\frac{4}{5}H$

(3) $\frac{5}{4}H$

(4) 1 H

The adjoining figure shows two bulbs B₁ and B₂ resistor R and an inductor L. When the switch S is turned off 

1. Both B₁ and B₂ die out promptly

2. Both B₁ and B₂ die out with some delay

3. B₁ dies out promptly but B₂ with some delay

4. B₂ dies out promptly but B₁ with some delay

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