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What is Lenz's law?

Class 12 PhysicsNCERT chapter: Electromagnetic Inductionइसे हिंदी में पढ़ें

Exam-ready answer

Lenz's law states that the polarity of the induced emf is such that it tends to produce a current which opposes the change in magnetic flux that produced it. This is why Faraday's law carries a negative sign, ε = −dΦB/dt. Lenz's law is a consequence of the law of conservation of energy.

Explanation

Consider a bar magnet with its north pole moving towards a closed coil. The flux through the coil increases. The induced current flows so that the face of the coil facing the magnet becomes a north pole; seen from the magnet's side, the current is anticlockwise. This north face repels the approaching north pole and opposes its motion.

When the north pole is pulled away, the flux through the coil decreases. Now the induced current reverses: the near face of the coil becomes a south pole, the current is clockwise as seen from the magnet, and the coil attracts the receding north pole. In both cases the induced current opposes the change in flux, not the flux itself.

Suppose the induced current helped the change instead. Then a tiny push would make the magnet speed up on its own, producing more and more current and energy from nothing. That would violate conservation of energy. In reality, work must be done against the opposing force to move the magnet, and this mechanical work appears as electrical energy, which is finally dissipated as heat in the coil.

Points to remember

  1. Lenz's law: the induced current opposes the change in magnetic flux that causes it.
  2. Lenz's law gives the direction of induced emf or current; Faraday's law gives its magnitude.
  3. The negative sign in ε = −dΦB/dt represents Lenz's law.
  4. Magnet approaching a coil: the near face becomes the same pole (repulsion); magnet receding: the near face becomes the opposite pole (attraction).
  5. Lenz's law follows from conservation of energy: the mechanical work done is converted into electrical energy.

Exam tip

Draw the magnet and coil, label the pole formed on the coil's near face, and show the current direction with arrows. End with the line 'Lenz's law is a consequence of conservation of energy.'

Questions students also ask

Does the induced current oppose the magnetic flux or its change?
It opposes the change in flux. If the flux is increasing, the induced current creates a field opposite to the original field. If the flux is decreasing, the induced current creates a field in the same direction as the original field, resisting the fall.
Is there an induced current if the coil is open?
No current flows in an open circuit, but an emf is still induced across its open ends. Its polarity is still given by Lenz's law: it is the polarity that would drive a current opposing the change if the circuit were closed.
Why is it harder to move a magnet into a closed coil than an open coil?
In a closed coil, the induced current produces a magnetic force opposing the motion of the magnet, so extra work is needed. In an open coil no current flows, so there is no opposing magnetic force and the magnet moves in easily.

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