A copper rod AB of length L, pivoted at one end A, rotates at constant angular velocity ω, at right angles to a uniform magnetic field of induction B. The e.m.f developed between the mid point C of the rod and end B is :
A conducting rod is moved with a constant velocity v in a magnetic field. A potential difference appears across the two ends
An aeroplane in which the distance between the tips of the wings is \(50\text{ m}\) is flying horizontally with a speed of \(360\text{ km/hr}\) over a place where the vertical component of earth’s magnetic field is \(2 \times 10^{-4}\text{ Wbm}^{-2}\). The potential difference between the tips of the wings would be:
Induced EMF is \(e = B_v l v\). Converting speed: \(v = 360\text{ km/h} = 100\text{ m/s}\). Thus, \(e = (2 \times 10^{-4}) \times 50 \times 100 = 1.0\text{ V}\).
Assertion (A): The probability of burn out of a dc motor is maximum, when the motor is just switched on.
Reason (R): No back emf is developed in the armature of dc motor, when it is just switched on.
Assertion (A) is true: When a DC motor starts, its speed is zero, thus the back EMF (\(epsilon_b\)) is zero. This leads to the maximum current (\(I = \frac{V - \epsilon_b}{R_a}\)) drawn from the supply, which can cause burnout. Reason (R) is true: Back EMF is proportional to the motor's angular speed (\(epsilon_b = k\Phi\omega\)), so it is zero at startup (\(omega = 0\)). Reason (R) correctly explains Assertion (A).