CBSE Class 12 · 2011 · All India · Electromagnetic InductionShort Answer

Two bar magnets are quickly moved towards a metallic loop connected across a capacitor

C

as shown in the figure. Predict the polarity of the capacitor.

Answer: Plate a becomes positive and plate b becomes negative.

Solution

As the two magnets approach, the flux through the loop changes rapidly, inducing a current in the loop that, by Lenz's law, opposes the approaching poles.
Tracing this induced current through the connecting leads shows it charges the capacitor so that the plate connected to the near side of the loop acquires positive charge.
Hence

a is positive and b is negative

. The capacitor holds this polarity as long as the flux keeps changing in the same sense.

CBSE Class 12 Board · 2011 · All India · Subjective

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