Question: Q. 2. A rectangular frame of wire is placed in a uniform magnetic field directed outwards, normal to the paper. $A B$ is connected to a spring which is stretched to $A^{\prime} B^{\prime}$ and then released at time $t=0$. Explain qualitatively how induced e.m.f. in the coil would vary with time. (Neglect damping of oscillations of spring)
A [CBSE Comptt. I 2018]
The induced emf, therefore varies with time sinusoidally.
[CBSE Marking Scheme 2018]
Alternate Answer :
Arm $A B$ executes SHM under the influence of restoring force developed in the spring, consequently and induced emf is produced across the ends of moving $\operatorname{arm} A B$ which varies sinusoidally.) (Given full credit for the above part if the student explains qualitatively without using mathematical equations)
Q. 3. A wire in the form of a tightly wound solenoid is connected to a $d c$ source and carries a current. If the coil is stretched so that there are gaps between successive elements of the spiral coil, will the current increase or decrease? Explain.
U] [SQP I 2017] [NCERT Exemplar]
Show Answer
Solution:
Ans. Lenz’s law allows the emf to induce in spirals that opposes and lead to decrease in magnetic flux by increasing the current. This is because when the coil is stretched and air gaps are created between successive elements of the spiral coil, magnetic flux will leak through the gaps. As magnetic flux decreases, current increases to oppose the decrements of flux.
