Q.State and explain Lenz's law in the light of principle of conservation of energy.

Showing the 12 most recent of 26 on this concept.

1. CBSE 2026Set V11 mark
   Q.Lenz's law gives the __________ of induced emf. Fill in the blank choosing the appropriate answer from the bracket: (photons, diffraction, polarity, monopoles, greater than unity, less than unity)

   ›Reveal solutionSolution

   direction (polarity)

   ✓Final answer

   direction (polarity)

   Lenz's law states that the induced emf (and hence the induced current) is directed so as to oppose the change in magnetic flux that produces it. Thus Lenz's law gives the direction (polarity/sense) of the induced emf; it is a consequence of the conservation of energy.

2. CBSE 2026Set ANNUAL1 mark
   Q.Lenz's law obeys the conservation law of ______.

   ›Reveal solutionSolution

   Lenz's law says the induced effects always oppose the cause producing them, which is required so that work must be done against the induced opposition - i.e. energy is conserved, never created for free.

   If the induced current instead aided the change in flux (rather than opposing it), the change would accelerate itself, and the system could generate energy from nothing, violating the law of conservation of energy. Because the induced current opposes the very change that produces it (Lenz's law), external work must always be done to drive the change, and that work is what gets converted into electrical energy. This is why Lenz's law is a direct consequence of conservation of energy.

   ✓Final answer

   conservation of energy.

3. CBSE 2026Set ANNUAL1 mark
   Q.Which law determines the direction of an induced current?

   ›Reveal solutionSolution

   Lenz's law gives the direction of induced current: it always opposes the very change that causes it (a statement of energy conservation).

   Faraday's law gives the MAGNITUDE of the induced emf (emf = -dphi/dt), but it is Lenz's law that fixes its DIRECTION. Lenz's law states that the direction of the induced current is always such as to OPPOSE the change in magnetic flux that induces it - for example, if flux through a loop is increasing, the induced current flows so as to create its own magnetic field opposing that increase. This is ultimately a consequence of the law of conservation of energy: if the induced current instead aided the change, flux (and hence energy) would build up without any external work being done, which is impossible.

   ✓Final answer

   Lenz's law.

4. CBSE 2025Set 55/6/11 markMCQ
   Q.A vertically held bar magnet is dropped along the axis of a copper ring having a cut as shown in the diagram. The acceleration of the falling magnet is: (A) zero (B) less than $g$ (C) $g$ (D) greater than $g$

   ›Reveal solutionSolution

   

   Because the copper ring has a cut (it is not a closed loop), no induced current can flow. Without induced current, there is no magnetic braking force, so the magnet falls with acceleration $g$.

   The key idea here is Lenz’s law and what it requires. Lenz’s law says that an induced current flows in a direction that opposes the change causing it. That opposition creates a retarding force on a moving magnet — but only if the current can actually flow. A cut in the ring breaks the conducting path, so no current circulates. No current means no opposing magnetic field, and therefore no upward force on the falling magnet.

   Many students memorise that a magnet falling through a copper ring slows down, but they forget the ring must be a closed loop. An open ring is just a curved piece of metal — it cannot sustain a current.

   Let’s walk through it step by step.

   1. What happens when a magnet moves near a conductor?

      A changing magnetic flux through a conducting loop induces an emf (Faraday’s law). If the loop is closed, this emf drives a current. That current produces its own magnetic field, which opposes the motion of the magnet (Lenz’s law). The result is a retarding force, so the magnet falls with acceleration less than $g$.

   2. What is different here?

      The copper ring has a cut — it is not a closed loop. An emf is still induced across the gap (a potential difference appears), but without a complete conducting path, no current can flow.

      Watch out

      A common mistake is to think that an induced emf automatically means a retarding force. It does not. Without current, there is no magnetic field from the ring, and therefore no force on the magnet.

   3. What forces act on the magnet?

      Only two forces: gravity downward ($mg$) and possibly a magnetic force from the ring. Since no current flows, the magnetic force is zero. The net force is just $mg$, so by Newton’s second law, the acceleration is $g$.

   4. What if the ring were closed?

      Then the induced current would create an upward magnetic force, reducing the net downward force. The acceleration would be less than $g$ — that’s option (B). But here the ring is open, so that doesn’t happen.

   Tip

   Think of it this way: a cut ring behaves like a broken circuit. An emf appears across the break, but no electrons complete the loop. No loop current = no opposing field = free fall.

   ✓Final answer

   The acceleration of the falling magnet is $g$, so the correct option is (C).

5. CBSE 2025Set ANNUAL1 markMCQ
   Q."The polarity of induced emf is such that it tends to produce a current in a direction which opposes the change in magnetic flux that produces it" - This law is based on conservation of which physical quantity?

   (a) Mass

   (b) Charge

   (c) Energy

   (d) Momentum

   ›Reveal solutionSolution

   Lenz's law is a statement of the law of conservation of energy applied to electromagnetic induction.

   Lenz's law says the induced current opposes the change in flux that produces it. If the induced current instead aided the change, the flux (and hence the current and the force on the source) would grow without any external work being done — creating energy from nothing. Because the induced current opposes the change, external work must be done against this opposition to sustain the change, and this work is exactly the electrical energy delivered. This is why Lenz's law is a direct consequence of conservation of energy.

   ✓Final answer

   (c) Energy

6. CBSE 2025Set IMPROVEMENT1 mark
   Q. What will be the direction of the induced current in the situation shown in the following figure?

   ›Reveal solutionSolution

   Using Lenz's law, the induced current opposes the approaching magnet by making the near face of the coil repel it.

   In the figure, the bar magnet's S-pole faces the coil and the magnet is moving towards the coil, so the magnetic flux linked with the coil (directed towards the approaching S-pole) is increasing. By Faraday's law, an emf is induced in the coil, and by Lenz's law the induced current must flow in a direction that opposes this increase in flux — i.e. it must oppose the relative approach of the magnet and the coil.

   This means the induced current must make the face of the coil nearer the magnet behave like an S-pole as well, since like poles (S facing S) repel each other, opposing the magnet's approach (equivalently, this can be seen as the induced current creating a magnetic moment that opposes the increasing flux through the coil, consistent with the negative sign in $\varepsilon = -\dfrac{d\phi}{dt}$).

   Applying the right-hand (curled-fingers) rule for the coil so that its near face acts as an S-pole (field lines entering that face, i.e. pointing from p-end towards q-end inside the winding as drawn), the induced conventional current flows from q to p within the coil (i.e., from p to q through the external resistor r).

   ✓Final answer

   The induced current opposes the approaching S-pole (coil's near face becomes an S-pole too), flowing from q to p through the coil, by Lenz's law.

7. CBSE 2025Set D1 markMCQ
   Q.Which of the following laws is based on the principle of energy-conservation? (A) Ampere's law (B) Faraday's law of electrolysis (C) Lenz's law (D) None of these

   ›Reveal solutionSolution

   Lenz's law fixes the direction of the induced current so it opposes the flux change; this opposition is exactly what energy conservation demands.

   Lenz's law states that the induced e.m.f./current always opposes the change in magnetic flux that produces it. If the induced current instead aided the change, the flux would grow without any external work, creating energy from nothing — a violation of the conservation of energy.

   Because the induced current opposes the motion, external work must be done to maintain the change, and this work reappears as the electrical energy of the induced current. Hence Lenz's law is a statement of energy conservation applied to electromagnetic induction.

   ✓Final answer

   (C) Lenz's law.

8. CBSE 2025Set ANNUAL1 markMCQ
   Q.Lenz's law is based on conservation of physical quantity

   (a) momentum

   (b) angular momentum

   (c) energy

   (d) charge

   ›Reveal solutionSolution

   Lenz's law fixes the direction of induced current so that it opposes its own cause; if it did not, the induced current would help the change along and energy could be created from nothing, violating energy conservation.

   Lenz's law states that the direction of an induced EMF (and induced current) is always such as to oppose the change in magnetic flux that produces it. If the induced current instead aided the change, the flux (and hence the current) would keep increasing without any external work being done — creating energy from nothing. Since this is forbidden, the opposing direction given by Lenz's law is exactly what conservation of energy requires: work must be done against the induced effects to change the flux, and that work is converted into electrical energy.

   ✓Final answer

   (c) energy.

9. CBSE 2025Set ANNUAL1 mark
   Q.A magnet is brought towards a coil as shown in the adjoining figure. In which direction will the induced current be?

   ›Reveal solutionSolution

   The approaching south pole increases the flux linked with the coil; by Lenz's law the induced current opposes this by making the near face of the coil itself a south pole, repelling the magnet.

   As the bar magnet's S pole moves toward the coil, the magnetic flux (directed into the S pole's field lines, i.e. towards the magnet) through the coil increases. By Lenz's law, the induced current must flow in a direction that opposes this increase — i.e. it must create a magnetic field that repels the approaching magnet. Since the approaching pole is South, the coil's near face must become a South pole too (like poles repel), so as to push back against the magnet's motion and oppose the flux increase.

   Using the right-hand (or clock) rule for a solenoid: to make the face nearer the magnet a South pole (field lines entering that face), the induced current, viewed from the magnet's side, flows clockwise.

   ✓Final answer

   The induced current flows so as to make the coil's near end a South pole (repelling the approaching magnet) — i.e. clockwise as viewed from the magnet's side — in accordance with Lenz's law.

10. CBSE 2025Set ANNUAL1 markMCQ
    Q.Which of the following gives the polarity of the induced emf?

    (i) Biot-Savart Law

    (ii) Lenz's Law

    (iii) Ampere's circuital Law

    (iv) Fleming's right-hand Rule

    ›Reveal solutionSolution

    Lenz's law gives the direction (polarity) of an induced emf: it always opposes the change in flux that produces it.

    Faraday's law gives the magnitude of the induced emf, $|\varepsilon| = N\dfrac{d\Phi_B}{dt}$, but by itself does not fix its sign/direction. Lenz's law supplies that: the induced current flows in the direction such that the magnetic field it creates opposes the change in flux causing it — this is expressed by the minus sign in $\varepsilon = -N\dfrac{d\Phi_B}{dt}$, and is a consequence of the conservation of energy.

    Biot–Savart law and Ampere's circuital law relate currents to the magnetic fields they produce (not induced emf), and Fleming's right-hand rule is only a mnemonic for motional emf direction, not the general law. Lenz's law is the general statement of polarity for any induced emf.

    ✓Final answer

    (ii) Lenz's Law — the induced emf always opposes the change in flux producing it.

11. CBSE 2025Set ANNUAL1 mark
    Q.Which principle is obeyed by Lenz's law?

    ›Reveal solutionSolution

    The induced current always opposes the change that produces it, which is exactly what conservation of energy demands.

    Lenz's law states that the direction of an induced emf/current is such that it opposes the change in magnetic flux that produces it. If it instead aided the change, the induced current would reinforce the flux change, drawing more current, more force, indefinitely — creating energy from nothing. Because the induced effect always opposes the cause, external work must be done against this opposition to sustain the change, and that work is what converts to electrical energy. This is precisely the requirement of the principle of conservation of energy.

    ✓Final answer

    Lenz's law is a consequence of (obeys) the principle of conservation of energy.

12. CBSE 2025Set ANNUAL1 markMCQ
    Q.A current starts flowing from A to B as shown in the figure : The direction of the induced current in the loop is :

    (a) Clockwise

    (b) Anticlockwise

    (c) No current is induced

    (d) Dependent on the radius of the loop

    ›Reveal solutionSolution

    As the current in the straight wire builds up, the into-the-page flux it creates through the nearby loop increases; by Lenz's law the induced current opposes this increase, so it flows anticlockwise.

    The current in the straight wire flows from A (left) to B (right). Using the right-hand thumb rule, at the location of the loop (just below the wire) this current produces a magnetic field directed into the page.

    As the current in the wire starts flowing (i.e. grows from zero), the flux through the loop, directed into the page, is increasing with time.

    By Lenz's law, the induced current in the loop must oppose this increase, i.e. it must create its own magnetic field out of the page inside the loop.

    Applying the right-hand rule to the loop itself: to produce a field out of the page, the induced current must flow anticlockwise (as viewed from the side the field emerges towards).

    ✓Final answer

    (b) The induced current in the loop flows anticlockwise, opposing the increasing into-the-page flux from the growing current in wire AB (Lenz's law).