Verified Solution • 2025–26 Session moderate

QPages 275-276: Q7: HC Verma: Concepts of Physics - Volume 2 Permanent Magnets Pages 275-276: Q7 Solution: Two bar magnets are placed close to each other with their…

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often incorrectly generalize that magnetic forces never do work, confusing the Lorentz force on a point charge (where $\vec{F} \perp \vec{v}$) with the interaction between macroscopic magnetic dipoles. You must recognize that the kinetic energy gain comes from the decrease in the magnetic field's potential energy, not from work done by the magnetic force on the moving charges themselves.

Two bar magnets are placed close to each other with their opposite poles facing each other. In absence of other forces, the magnets are pulled towards each other and their kinetic energy increases. Does it contradict our earlier knowledge that magnetic forces cannot do any work and hence cannot increase kinetic energy of a system?

Step-by-Step Verified Solution

**Answer:** No, it does not contradict the earlier knowledge. **Step-by-Step Explanation:** Magnetic forces on moving charges do no work because they are perpendicular to velocity. However, for two bar magnets attracting each other, the force is transmitted through the magnetic field, and the work done comes from the potential energy stored in the magnetic field configuration. The increase in kinetic energy is accompanied by a decrease in magnetic potential energy, so energy is conserved [citation:3].

All Questions in this Chapter

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