The small energy losses in transformers due to eddy currents can be reduced by

The radius of a coil of $N$ turns is $R$. If the plane of the coil is placed parallel to a uniform magnetic field $B$, then the flux linked with the coil is

An emf of 2.8 mV is induced in a rectangular loop of area $150 \mathrm{~cm}^2$ when the current in the loop changes from 3 A to 8 A in a time of 0.2 s . Then, the self-inductance of the loop is

A circular coil of area $3 \times 10^{-2} \mathrm{~m}^2, 900$ turns and a resistance of $1.8 \Omega$ is placed with its plane perpendicular to a uniform magnetic field of $3.5 \times 10^{-5} \mathrm{~T}$. The current induced in the coil when it is rotated through $180^{\circ}$ in half a second is

A coil of resistance $8 \Omega$, number of turns 250 and area $120 \mathrm{~cm}^2$ is placed in a uniform magnetic field of 2 T such that the plane of the coil makes and angle of $\frac{\pi}{6}$ with the direction of the magnetic field. In a time of 100 ms , the coil is rotated until its plane becomes parallel to the direction of the magnetic field. The current induced in the coil is

The plane of a circular coil of resistance $7.5 \Omega$ is placed perpendicular to a uniform magnetic field. The flux $\phi$ (in weber) through the coil varies with time $t$ (in second) as $\phi=2 t^2+3 t-2$. The induced power in the coil at time $t=3 \mathrm{~s}$ is

A conducting rod is moving towards right with a velocity $v$ in a uniform magnetic field $B$. If the direction of induced current $i$ is as shown in the figure, then the direction of $B$ is

Metal detector works on the principle of

A copper disc of radius 0.1 m rotates about an axis passing through its centre and perpendicular to its plane with 10 revolutions per second in a uniform transverse magnetic field of 0.1 T . The emf induced across the radius of the disc is

The self inductance of a coil depends on

A conducting circular coil is place in a uniform magnetic field with the magnetic field initially directed perpendicular to the plane of the coil. In step $A$, the coil is rotated from its initial position by $60^{\circ}$ about its diameter in time $t$. In step $B$, the coil is further rotated about the same axis in the same sense by another $120^{\circ}$ in time $2 t$. Ratio of emf induced in the coil in step $A$ to that in step $B$ is

An aeroplane is travelling horizontally towards west with a speed of $540 \mathrm{kmh}^{-1}$. The wing span of the plane is 20 m . If the horizontal component of the earth's magnetic field at the location is $2.5 \sqrt{3} \times 10^{-4} \mathrm{~T}$ and the dip angle is $30^{\circ}$, the potential difference developed between the ends of the wing is

If the vertical component of earth's magnetic field is $0.5 \times 10^{-4} \mathrm{~T}$ at a point. When an aeroplane of wing span 4 m is moving horizontally at this place at $360 \mathrm{kmh}^{-1}$, then the motional emf forced across the ends of the wings is

A boy is playing with the empty rim of a cycle wheel of radius 40 cm by rolling it along a horizontal road towards north with angular speed of $20 \mathrm{rad} \mathrm{s}^{-1}$. Considering the effect of magnetic field of earth, the e.m.f induced in the rim is

(Horizontal component of earth's magnetic field $=0.26 \mathrm{G}$ )

A wheel of 20 metallic spokes each 40 cm long is rotated with a speed of $180 \mathrm{rev} / \mathrm{min}$ in a plane normal to the horizontal component of earth's magnetic field $H_c$ at a place. If $H_c=0.4 \mathrm{G}$ (Gauss) at that place, the induced emf between the axle and the rim of the wheel is

A metal disc of radius 30 cm rotates with a constant angular velocity $\omega=100 \mathrm{rad} / \mathrm{s}$ about its axis. Find the magnitude of potential difference between the centre and the rim of the disc of the external uniform magnetic field on induction $B=4 \mathrm{mT}$ is directed perpendicular to the disc.

The magnetic flux through the triangular loop shown in the figure below

Where a uniform magnetic field of strength 2 T points perpendicularly into the plane of the triangle is

A wire loop of area $0.2 \mathrm{~m}^2$ has a resistance of $20 \Omega$. A magnetic field pointing normal to the loop has a magnitude of 0.25 T and is reduced to zero at a uniform rate in $10^{-4} \mathrm{~s}$. What is induced emf and resulting current?

A flat circular coil has 100 turns of wire of radius 10 cm . A uniform magnetic field exists in a direction perpendicular to the plane of the coil and it grows at a rate of $0.1 \mathrm{~T} / \mathrm{s}$. The induced emf in the coil is

A long solenoid has 20 turns per cm. A small loop of area $4 / \pi \mathrm{cm}^2$ is placed inside the solenoid normal to its axis. If the current carried by the solenoid changed steadily from 1.0 A to 3.0 A in 0.2 s , what is the magnitude of the induced emf in the loop while the current is changing?

Consider two solenoids $X$ and $Y$ such that the area and length of $Y$ are twice that of $X$ respectively and the magnetic energy stored in both the solenoids is same, then the ratio of magnitude of magnetic fields of the two solenoids $\frac{\left|\mathbf{B}_X\right|}{\left|\mathbf{B}_Y\right|}$ is

A varying current in a coil changes from 10 A to zero in 1.5 s . If the average emf induced in the coil is 200 V , the self-inductance of the coil is