The back emf induced in a coil, when current changes from \(1\) ampere to zero in one milli-second, is \(4\) volts. The self-inductance of the coil is:
1. \(1~\text{H}\)
2. \(4~\text{H}\)
3. \(10^{-3}~\text{H}\)
4. \(4\times10^{-3}~\text{H}\)

Average energy stored in a pure inductance L when a current i flows through it, is

1.  ${\mathrm{Li}}^2$                           

2.  $2{\mathrm{Li}}^2$

3.  ${\mathrm{Li}}^2/4$                       

4.  ${\mathrm{Li}}^2/2$

A small magnet is along the axis of a coil and its distance from the coil is 80 cm. In this position the flux linked with the coil are $4 \times {10}^{–5}$ weber turns. If the coil is displaced 40 cm towards the magnet in 0.08 second, then the induced emf produced in the coil will be -

1.  0.5 mV                       

2.  1 mV 

3.  7 mV                           

4.  3.5 mV

A wooden stick of length $3\mathcal{l}$ is rotated about an end with constant angular velocity $\mathrm{\omega }$ in a uniform magnetic field B perpendicular to the plane of motion. If the upper one third of its length is coated with copper, the potential difference across the whole length of the stick is –

1. $\frac{9\mathrm{B\omega }{\mathcal{l}}^2}{2}$                           

2. $\frac{4\mathrm{B\omega }{\mathcal{l}}^2}{2}$

3. $\frac{5\mathrm{B\omega }{\mathcal{l}}^2}{2}$                           

4. $\frac{\mathrm{B\omega }{\mathcal{l}}^2}{2}$

A train is moving at a rate of 72 km/hr on a horizontal plane. If the earth's horizontal component of magnetic field is 0.345 A/m and the angle of dip is 30°, then the potential difference across the two ends of a compartment of length 1.7 m will be-

1.  $1.7 \mathrm{V}$                             

2.  $85\sqrt{3}\times {10}^{-4} \mathrm{V}$

3.  $85\times {10}^{-4} \mathrm{V}$                 

4.  $850 \mathrm{\mu } \mathrm{V}$

An air-plane with 20m wing spread is flying at $250 {\mathrm{ms}}^{-1}$ straight south parallel to the earth’s surface. The earth’s magnetic field has a horizontal component of $2 \times {10}^{–5}$ $\mathrm{Wb} {\mathrm{m}}^{–2}$ and the dip angle is 60º. Calculate the induced emf between the plane tips is:

1.  0.174 V                 

2.  0.173 V 

3.  1.173 V                 

4.  0.163 V

A wire of fixed lengths is wound on a solenoid of length $\mathcal{l}$ and radius r. Its self inductance is found to be L. Now if same wire is wound on a solenoid of length $\mathcal{l}/2$ and radius r/2, then the self inductance will be –

1.  $2\mathrm{L}$                     

2.  $\mathrm{L}$ 

3.  $4\mathrm{L}$                     

4.  $8\mathrm{L}$

A wire in the form of a circular loop of radius 10 cm lies in a plane normal to a magnetic field of 100 T. If this wire is pulled to take a square shape in the same plane in 0.1 s, average induced emf in the loop is: 

1.  6.70 volt                   

2.  5.80 volt 

3.  6.75 volt                   

4.  5.75 volt

A superconducting loop of radius R has self inductance L. A uniform and constant magnetic field B is applied perpendicular to the plane of the loop. Initially current in this loop is zero. The loop is rotated by 180°. The current in the loop after rotation is equal to –

1. zero                                 

2. $\frac{{\mathrm{B\pi R}}^2}{\mathrm{L}}$

3. $\frac{2{\mathrm{B\pi R}}^2}{\mathrm{L}}$                           

4. $\frac{{\mathrm{B\pi R}}^2}{2\mathrm{L}}$

A coil of inductance 8.4 mH and resistance 6$\mathrm{\Omega }$ is connected to a 12 V battery. The current in the coil is 1.0 A at approximately the time. 

1.  500 ms                 

2.  20 ms 

3.  35 ms                   

4.  1 ms