Question

Whenever the magnetic flux linked with a coil changes, an induced e.m.f. is produced in the circuit. The e.m.f. lasts
A. for a short time
B. for a long time
C. for ever
D. so long as the change in flux takes place

Answer 1

Hint: A magnetic flux linked with a coil is defined as the number of magnetic lines of force passing through the surface area. Faraday discovered that whenever a magnetic flux linked with a circuit changes an emf is developed in the circuit. This emf is called induced emf.

Complete answer:
We know that a current carrying wire produces a magnetic field around it and the intensity of magnetic field at any point around a current carrying conductor is given by the Biot-Savart’s law. The reverse effect is also possible. i.e. magnetic fields can produce electric current in a circuit which is first discovered by Michael Faraday and is known as Faraday's law of electromagnetic induction.

He discovered this effect by performing an experiment. Let us take a conducting loop connected to a sensitive ammeter and no other sources of emf or battery is included. So there will be no current in the circuit. However, if the bar magnet was moved towards the loop, a current suddenly appears in the circuit. The current will disappear when the magnet stops moving. If the bar magnet is moved away from the loop, a current will again appear suddenly, but in the opposite direction. If the experiment is carried out for a while, the following discovery will be noted:

1. A current will appear in the loop only if there is relative motion between the loop and the magnet the current disappears when the relative motion between them ceases.
2. Faster motion produces a greater current.
3. If moving the magnet's North Pole toward the loop causes, say, clockwise current, then moving the north pole away causes counterclockwise current. If the South pole is moved towards or away from the loop, then there will be a current in the loop. But the direction of current is reversed.

The current produced in the loop is called an induced current and the work done per unit charge to produce that current (to move the conduction electrons that constitute the current) is called an induced emf. And this process of producing the current and emf is called induction.
Magnetic flux is defined as the number of magnetic field lines passing through a given surface area. So the induced emf stays as long as the magnetic flux linked with the circuit changes.

So, the correct answer is “Option D”.

Note:
Whenever the magnetic flux linked with a circuit changes an emf is induced in the circuit. The emf stays in the circuit as long as the flux keeps changing. The nature of the induced emf is such that it opposes the cause due to how it is produced, i.e. the flux changes.