Question

Figure shows two bulbs \[{{B}_{1}}\] and \[{{B}_{2}}\], resistor R and inductor L. When the switch S is turned off

(A). Both \[{{B}_{1}}\] and \[{{B}_{2}}\] die out promptly
(B). Both \[{{B}_{1}}\] and \[{{B}_{2}}\] die out with some delay
(C). \[{{B}_{2}}\] die out promptly but \[{{B}_{1}}\] with some delay
(D). \[{{B}_{1}}\] die out promptly but \[{{B}_{2}}\] with some delay

Answer 1

Hint: In this given circuit, we have been using an inductor and resistor. The inductor can produce some induced emf according to Lenz's law and that will oppose the cause for the change of magnetic flux. This will help to maintain current some more time in the circuit.

Complete step by step answer:
In this question, we can see our circuit was in ON state. Now we have to find out the effect when the switch turned off. This will act as an LR circuit. When the switch is turned off, the current decreases but not suddenly becomes zero. The inductor in the circuit will produce some magnetic flux when the circuit is closed. This magnetic flux creates induced emf in the circuit. So, when the switch is turned off, according to Lenz's law the induced emf will oppose the change in current. So, it won’t allow a sudden decrease in current. Therefore, the bulb connected with the inductor i.e. \[{{B}_{2}}\] will die out with some delay.
But in the case of the bulb \[{{B}_{1}}\], the bulb will die out promptly. Since when the voltage becomes zero current also becomes zero. So \[{{B}_{1}}\] will die out promptly. So, the correct answer is D.

Additional information:
Lenz’s law can be defined as the direction of the induced current that will oppose the change that causes it. It is an extension of Faraday’s law. It represents the application of Newton’s third law in the electromagnetic circuits. “For every action, there is an equal and opposite reaction”.
Mathematical formula of Lenz’s law can be written as,
\[\varepsilon =-N\dfrac{d\phi }{dt}\], where N is the number of turns and \[\phi \] is the magnetic flux.

Note: If you are not observing the circuit, you will get the wrong answer. Since, in this case, the presence of an inductor brings the idea of Lenz’s law. So, it can make an impact on the circuit even if it is in the off state. If it is a resistor instead of an inductor, then the bulb will die out suddenly. You can think of the iron box case as the inductor. It will take some time to release the heat from the surface even after the switch off.