Question: Q. 4. There are two coils $A$ and $B$ as shown in figure. A current starts flowing in $B$ as shown, when $A$ is moved towards $B$ and stops when $A$ stops moving. The current in $A$ is counter clockwise. $B$ is kept stationary when $A$ moves. We can infer that
(a) there is a constant current in the clockwise direction in $A$.
(b) there is a varying current in $A$.
(c) there is no current in $A$.
(d) there is a constant current in the counter clockwise direction in $A$. [NCERT Exemplar] Correct option : (d)
Explanation: When coil $A$ moves towards coil $B$ with constant velocity, so rate of change of magnetic flux due to coil $B$ in coil $A$ will be constant that gives constant current in coil $A$ in same direction as in coil $B$ by Lenz’s law.
Q. 5. Same as problem 4 except the coil $A$ is made to rotate about a vertical axis figure. No current flows in $B$ if $A$ is at rest. The current in coil $A$, when the current in $B($ at $t=0)$ is counter-clockwise and the coil $A$ is as shown at this instant, $t=0$, is
(a) constant current clockwise.
(b) varying current clockwise.
(c) varying current counter-clockwise.
(d) constant current counter-clockwise.
[NCERT Exemplar]
Show Answer
Solution:
Ans. Correct option : (a)
Explanation: In this case the direction of the induced electromotive force/induced current is determined by the Lenz’s law. According to the Lenz’s law the direction of induced emf or current in a circuit is such as to oppose the cause that produces it. This law is based upon law of conservation of energy.
When the current in coil $B($ at $t=0)$ is counterclockwise and the coil $A$ is considered above it. The counter-clockwise flow of the current in coil $B$ is equivalent to North Pole of magnet and magnetic field lines are emanating upward to coil $A$. When coil $A$ starts rotating at $t=0$, the current in coil $\mathrm{A}$ is constant along clockwise direction by Lenz’s rule.
[I] Q. 1. In the figure given, mark the polarity of plates $A$ and $B$ of a capacitor when the magnets are quickly moved towards the coil.
U] [O.D. Comptt. III 2017]
Ans. Accept both the answers :
[CBSE Marking Scheme 2017]
Detailed Answer :
From the figure, both magnets move towards the coil. Flux where induced current is in anticlockwise direction, if observed from left side and its direction clockwise as observed from right side. Hence, the direction of induced current is in clockwise direction which shows that plate $A$ of the capacitor is at higher potential as compared to plate $B$ of the capacitor, so plate $B$ will be negative while plate $A$ is positive.
