If a wheel with $24$ metallic spokes each $40 \ cm$ long is rotated with a speed of $180 \ rev/min$ in a plane normal to the horizontal component of earth's magnetic field, the emf induced between the axle and the rim of the wheel is $E$. If the number of spokes is made $12$ and the wheel is rotated with a speed of $90 \ rev/min$ in the same field, the induced emf is

What is the cause of the force acting on a sliding conductor placed in a magnetic field?

$A$ coil of mean area $500 \ cm^2$ and having $1000$ turns is held with its plane perpendicular to a uniform magnetic field of $0.4 \ G$. If the coil is turned through $180^{\circ}$ in $\frac{1}{10} \ s$,then the average induced emf is $(1 \ G = 10^{-4} \ T)$. (in $V$)

Kamla peddles a stationary bicycle. The pedals of the bicycle are attached to a $100$ turn coil of area $0.10 \; m^2$. The coil rotates at half a revolution per second and it is placed in a uniform magnetic field of $0.01 \; T$ perpendicular to the axis of rotation of the coil. What is the maximum voltage (in $V$) generated in the coil?

$A$ rectangular loop with a sliding connector of length $l = 1.0 \, m$ is situated in a uniform magnetic field $B = 2 \, T$ perpendicular to the plane of the loop. The resistance of the connector is $r = 2 \, \Omega$. Two resistors of $6 \, \Omega$ and $3 \, \Omega$ are connected as shown in the figure. The external force required to keep the connector moving with a constant velocity $v = 2 \, m/s$ is ........ $N$.

$A$ horizontal straight wire $5 \ m$ long extending from east to west is falling freely at a right angle to the horizontal component of the earth's magnetic field $0.60 \times 10^{-4} \ Wb \ m^{-2}$. The instantaneous value of the emf induced in the wire when its velocity is $10 \ m \ s^{-1}$ is . . . . . . $\times 10^{-3} \ V$.