When a charged particle moving with velocity  is subjected to a magnetic field of induction , the force on it is non-zero. This implies that

• angle between  is necessarily 

• angle between  can have any value other than 

• angle between  can have any value othe than zero and 

• angle between  can have any value othe than zero and 

Two coils of self-inductances 2 mH and 8 mH are placed so close together that the effective flux in one coil is completely linked with the other. The mutual inductance between these coils is

• 10 mH

• 6 mH

• 4 mH

• 4 mH

The magnetic potential energy stored in a certain inductor is 25 mJ, when the current in the inductor is 60 mA. This inductor is of inductance

• 0.138 H

• 138.88 H

• 13.89

• 1.389 H

If the flux associated with a coil varies at the rate of 1wb/min the induced emf is

• zero

• 6oV

• $\frac{1}{60}\mathrm{V}$

• 1V

A circular coil of mean radius of 7 cm and having 4000 turns is rotated at the rate of 1800 revolutions per minute in the earth's magnetic field (B=0.5 gauss), the emf induced in coil will be

• 1.158 V

• 0.58 V

• 0.29 V

• 5.8 V

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

• $\sqrt{3}:1$

78.

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