(a) A coil of insulated wire is connected to a galvanometer. What would be observed if a strong bar magnet with its south pole towards one face of the coil is

(i) moved quickly toward it?

(ii) moved quickly away from it?

(iii) held stationary near it?

(b) Name the phenomena involved.

(c) State the conclusion based on the observations in (i), (ii) and (iii).

Q: (a) A coil of insulated wire is connected to a galvanometer. What would be observed if a strong bar magnet with its south pole towards one face of the coil is (i) moved quickly toward it? (ii) moved quickly away from it? (iii) held stationary near it? (b) Name the phenomena involved. (c) State the conclusion based on the observations in (i), (ii) and (iii).

**(a) Observations:** * **(i) Moved quickly toward the coil:** A momentary deflection is observed in the galvanometer needle in one direction (e.g., to the left), indicating that a temporary current is induced in the coil. * **(ii) Moved quickly away from the coil:** A momentary deflection is observed in the galvanometer needle in the opposite direction (e.g., to the right), indicating that the direction of the induced current is reversed. * **(iii) Held stationary near the coil:** No deflection is observed in the galvanometer needle (it remains at zero), indicating that no current is induced when there is no relative motion. **(b) Phenomenon involved:** The phenomenon is **Electromagnetic Induction**. **(c) Conclusion:** An induced current is set up in a closed coil whenever there is relative motion between the coil and the magnet. This relative motion changes the magnetic field lines linking the coil, which induces a potential difference across its ends. The direction of the induced current depends on the direction of relative motion, and no current is induced when the relative motion stops.

Magnetic Effects of Electric Current

Electromagnetic Induction

Official Answer

(a) Observations:

(i) Moved quickly toward the coil: A momentary deflection is observed in the galvanometer needle in one direction (e.g., to the left), indicating that a temporary current is induced in the coil.
(ii) Moved quickly away from the coil: A momentary deflection is observed in the galvanometer needle in the opposite direction (e.g., to the right), indicating that the direction of the induced current is reversed.
(iii) Held stationary near the coil: No deflection is observed in the galvanometer needle (it remains at zero), indicating that no current is induced when there is no relative motion.

(b) Phenomenon involved:

The phenomenon is Electromagnetic Induction.

(c) Conclusion:

An induced current is set up in a closed coil whenever there is relative motion between the coil and the magnet. This relative motion changes the magnetic field lines linking the coil, which induces a potential difference across its ends. The direction of the induced current depends on the direction of relative motion, and no current is induced when the relative motion stops.

momentary deflectionopposite directionno deflectionelectromagnetic inductionrelative motioninduced currentmagnetic flux

Marking Scheme

1For correctly stating the observation in (i): momentary deflection in one direction. (0.5 marks)
2For correctly stating the observation in (ii): momentary deflection in the opposite direction. (0.5 marks)
3For correctly stating the observation in (iii): no deflection (remains at zero). (0.5 marks)
4For correctly naming the phenomenon as 'Electromagnetic Induction'. (0.5 marks)
5For stating the conclusion: relative motion between the magnet and coil causes a change in magnetic field lines/flux, which induces a current. (1 mark)

Hint

Recall that a galvanometer detects the presence of current. Current is only induced when there is a change in the magnetic field lines passing through the coil (i.e., when there is relative motion).

Quick Oral Answer

When a magnet is moved relative to a coil, the magnetic field lines passing through the coil change. This change in magnetic field lines induces an electric current in the coil, which is detected by the deflection of the galvanometer needle. This phenomenon is called electromagnetic induction.

Analysis & Explanation

The observations can be explained using Faraday's Law of Electromagnetic Induction and Lenz's Law:

Faraday's Law: The magnitude of the induced electromotive force (and hence the current) in a circuit is directly proportional to the rate of change of magnetic flux (magnetic field lines) linked with the circuit. When the magnet is moved quickly, the rate of change of flux is high, producing a noticeable deflection. When stationary, the rate of change of flux is zero, so no current is induced.
Lenz's Law: The direction of the induced current is always such that it opposes the change in magnetic flux that produces it.

When the South pole of the magnet approaches the coil, the face of the coil facing the magnet develops South polarity (clockwise current) to repel the approaching magnet.
When the South pole is pulled away, the face of the coil develops North polarity (anticlockwise current) to attract and oppose the receding magnet. This explains why the deflection direction reverses.

Common Mistakes

1Stating that the galvanometer shows a continuous deflection even when the magnet is kept stationary.
2Forgetting to mention that the deflection is 'momentary' or 'temporary'.
3Not linking the induction of current to the 'change' in magnetic field lines (relative motion).

Interesting Facts

This phenomenon was discovered independently by Michael Faraday in England (1831) and Joseph Henry in the United States (1832).

Electromagnetic induction is the fundamental principle behind the working of modern power generators, transformers, and induction cooktops.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Magnetic Effects of Electric Current" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Electromagnetic Induction" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.