JNTU B.Tech II ELECTRICAL ENGINEERING Previous Question Paper (March - 2022 - R13)

Lenz's law states that the direction of an induced electromotive force (EMF) and the resulting induced current in a closed circuit is always such that it opposes the change in magnetic flux that produced it. This law is a direct consequence of the Law of Conservation of Energy.

Mathematically, it is represented by the negative sign in Faraday's law of electromagnetic induction:

When the magnetic flux through a coil changes, an induced current flows in a direction that creates its own magnetic field to counteract the original change. For example, if the magnetic flux through a loop increases due to an approaching north pole of a magnet, the induced current creates a north pole on the face of the loop facing the magnet, creating a repulsive force that opposes the motion. Conversely, if the flux decreases, the induced current creates a south pole to attract the magnet and oppose its withdrawal.

Consider a straight conductor of length l carrying a current I placed in a uniform magnetic field of flux density B. Let the conductor be inclined at an angle θ with the direction of the magnetic field.

The force F experienced by a charge q moving with velocity v in a magnetic field is given by the Lorentz force equation:

In a conductor, the total charge Q moving through length l in time t is Q = It. The drift velocity v can be expressed as l / t. Substituting these into the force equation:

Simplifying the expression, we obtain the force on the conductor:

The direction of this force is determined by Fleming's Left-Hand Rule, which states that if the thumb, forefinger, and middle finger of the left hand are held mutually perpendicular, with the forefinger pointing in the direction of the magnetic field and the middle finger in the direction of the current, the thumb indicates the direction of the force exerted on the conductor.