Verified Solution • 2025–26 Session moderate

QPage 163: Q7: HC Verma: Concepts of Physics - Volume 2 Capacitors Page 163: Q7 Solution: When a dielectric slab is gradually inserted between the plates …

⚠️ Common Exam Traps & Conceptual Pitfalls

Students often incorrectly assume that inserting a dielectric always increases stored energy or that the force is repulsive; however, for an isolated capacitor (constant charge), the system minimizes energy by pulling the dielectric inward, so the force is attractive and directed into the capacitor.

When a dielectric slab is gradually inserted between the plates of an isolated parallel-plate capacitor, the energy of the system decreases. What can you conclude about the force on the slab exerted by the electric field?

Step-by-Step Verified Solution

**Answer:** The force on the slab is constant and directed inward (pulling it into the capacitor). **Step-by-Step Explanation:** The energy of the isolated capacitor decreases as the dielectric slab is inserted. The force is given by F=−dU/dxF = -dU/dx. Since the energy decreases linearly with insertion, the force is constant and positive (pulling the slab into the region of stronger field) .

All Questions in this Chapter

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