Electromagnetic Induction | Class 12 Physics Notes

6.5 LENZ'S LAW AND CONSERVATION OF ENERGY

Lenz's law, formulated by Heinrich Friedrich Lenz in 1834, provides the direction of the induced emf and current resulting from electromagnetic induction. It states that the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change in magnetic flux that produced it. This is represented mathematically by the negative sign in Faraday's law.

For example, when the North pole of a bar magnet is pushed towards a coil, the magnetic flux through the coil increases. The induced current flows in a direction that creates a magnetic field opposing this increase, effectively repelling the approaching magnet. Conversely, when the magnet is pulled away, the induced current direction reverses to oppose the decrease in flux, attracting the magnet back. This opposition ensures that energy conservation is not violated.

If the induced current were to aid the change in flux, it would lead to a perpetual increase in motion and energy without external work, violating the conservation of energy principle. Instead, the opposing induced current requires work to be done to move the magnet, and this work is dissipated as heat due to the resistance of the coil.

Figures 6.6(a) and (b) illustrate the directions of induced currents for different motions of magnets relative to coils. Lenz's law is fundamental in understanding the behavior of induced currents and the energy transformations involved in electromagnetic induction.

Example 6.4 applies Lenz's law to determine the direction of induced currents in loops of different shapes moving into or out of magnetic fields. Example 6.5 discusses scenarios involving stationary loops, loops moving in electric fields, and loops moving out of magnetic fields, emphasizing the conditions under which emf is induced.

📊 Diagram: See figure_8: (a); figure_9: (b); figure_10: FIGURE 6.7

🧪 Activity: Using Lenz's law to predict directions of induced currents in various moving loops and understanding energy conservation in electromagnetic induction.

🔗 Connection: This section prepares for understanding motional emf and the forces involved in moving conductors in magnetic fields, discussed next.