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 :

1. 0.138H

2. 138.88H

3. 1.389H

4. 13.89H

1.  0.02 Wb

2.  0.06 Wb

3.  0.08 Wb

4.  0.01 Wb

A thin semicircular conducting ring (PQR) of radius 'r' is falling with its plane vertical in a horizontal magnetic field B, as shown in figure. The potential difference developed across the ring when its speed is v is: 

1. Zero

2. $Bv{\mathrm{\pi r}}^2/2$ and P is at the higher potential

3. $\mathrm{\pi rBv}$ and R is at the higher potential

4. 2BvR and R is at the higher potential

A coil of resistance 400 $\Omega$ is placed in a magnetic field. If the magnetic flux $\varphi$(Wb) linked with the coil varies with time t (sec) as $\varphi$= 50$t^2$+4. 
The current in the coil at t= 2s is:

1. 0.5 A

2. 0.1 A

3. 2 A

4. 1 A

The current i in a coil varies with time as shown in the figure. The variation of induced emf with time would be:

1.  

2.  

3.  

4.  

Two coils of self-inductance 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:

1. 10 mH

2. 6 mH

3. 4 mH

4. 16 mH

In which of the following devices, the eddy current effect is not used?

1. electric heater

2. induction furnace

3. magnetic braking in train

4. electromagnet

A 800 turn coil of effective area 0.05 $m^2$ is kept perpendicular to a magnetic field $5\times {10}^{-5} T$. When the plane of the coil is rotated by 90$°$ around any of its coplanar axis in 0.1 s, the emf induced in the coil will be:

1. 0.02V

2. 2 V

3. 0.2 V

4. $2\times {10}^{-3} V$