Electromagnetic Induction

If the current 30A flowing in the primary coil is made zero in 0.1 sec, the emf induced in the secondary coil is 1.5 volt. The mutual inductance between the coils is

• 0.05 H

• 1.05 H

• 0.1 H

• 0.2 H

Two similar coils are kept mutually perpendicular such that their centres coincide. At the centre, find the ratio of the magnetic field due to one coil and the resultant magnetic field through both coils, if the same current is flown.

• $1:\sqrt{2}$

• 1:3

• 1:2

• $\sqrt{3}:1$

The primary and secondary coils of a transformer have 50 and 1500 turns respectively. If the magnetic flux  $\mathrm{\varphi }$ linked with the primary coil is given by  $\mathrm{\varphi } = {\mathrm{\varphi }}_{\mathrm{o}} + 4\mathrm{t}$ where $\mathrm{\varphi }$ is weber, t is time in second and ${\mathrm{\varphi }}_{\mathrm{o}}$ is a constant, the output vltage across the secondary coil is

• 90 v

• 120 V

• 220 V

• 30 V

A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 A, the efficiency of the transformer is approximately

• 30%

• 50%

• 90%

• 10%

A coil of inductance 300 mH and resistance 2 0 is connected to a source of voltage 2 V. The current reaches half of its steady state value in

• 0.05 s

• 0.1 s

• 0.15 s

• 0.3 s

When power is drawn from the secondary coil of the transformer, the dynamic resistance

• increases

• decreases

• remains unchanged

• changes erratically

The magnetic flux linked with a coil at any instant t is given by the equation $\mathrm{\varphi } = 5{\mathrm{t}}^3 - 100 \mathrm{t} + 300$. The magitude of emf induced in coil after 3 s is 

• 10 V

• 20 V

• 35 V

• 70 V

In 0.1 s, the current in a coil increases from 1 A to 1.5A. If inductance of coil is 60 mH, then induced current in external resistance of 32 will be

• 1 A

• 0.5 A

• 0.2 A

• 0.1 A

In 0.2 s, the current in a coil increases from 2.0 A to 3.0 A. If inductance of coil is 60 mH, then induced current in external resistance of 3 Ω will be

• 1A

• 0.5 A

• 0.2 A

• 0.1 A

A transformer having efficiency of 75 % is working on 220 V and 4.4 kW power supply. If the current in the secondary coil is 5 A. What will be the voltage across secondary coil and the current in primary coil?

• V_s = 220 V, i_p = 20A

• V_s = 660 V, i_p = 15 A

• V_s = 660 V, i_p = 20 A

• V_s = 220 V, i_p = 15 A