Lenz’s Law

1. Definition

Lenz’s Law states that the direction of the induced current in a conductor is such that it opposes the change in magnetic flux that caused it.

1.1 Formula

E = -N × (ΔΦ / Δt) where:

• E = induced electromotive force (EMF) in volts
• N = number of turns in the coil
• ΔΦ = change in magnetic flux (in Weber)
• Δt = change in time (in seconds)
• The negative sign represents the opposition (as per Lenz’s Law).

1.2 Explanation

When the magnetic environment of a coil changes (e.g., moving a magnet toward or away), an EMF is induced.
The resulting current generates its own magnetic field that opposes the original change.

1.3 Example

• Magnet toward coil → Coil current produces a repelling magnetic field.
• Magnet away from coil → Coil current produces an attracting magnetic field

1.4 Video Example

Explanation of the experiment:
Two aluminum rings are mounted on a pivot:

• Closed ring: Moves away when a magnet approaches, and toward it when the magnet is removed.
• Open ring: Shows little to no movement because current cannot circulate.

2. Usage

Lenz’s Law helps determine the direction of induced currents when a conductor is exposed to a changing magnetic field.
Applications:

• Electric Generators : Predict induced voltage polarity.
• Transformers : Understand opposing currents affecting energy transfer.
• Electromagnetic Braking : Calculate braking force via eddy currents.
• Induction Heating : Design metal heating systems.
• Magnetic Levitation : Control repulsive forces for suspension.

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3. Limitations

• No magnitude prediction : Only gives direction (magnitude requires Faraday’s Law).
• Ideal assumptions : Assumes perfect conductors and no energy loss.
• Needs changing magnetic flux : Constant fields induce no current.
• Material dependent : Efficiency varies with conductor properties.
• Ignores relativistic effects : At extreme speeds or strong fields, more advanced theory is needed.

4. Video Explanation

video coming soon