Shielding and Conducting Cavities
Electrostatic equilibrium enforces E = 0 in conductor material and constant conductor potential. An empty closed cavity has no field from external sources; a charged cavity induces net -q on its inner surface.
Why this shows up in the exam
Faraday shielding · Field-line diagrams for charged cavities · Lightning protection by metal bodies
Learn the idea
Conductor material has zero electrostatic field, while a cavity field depends on charges inside that cavity. A metal enclosure blocks external static fields from its closed empty cavity. If a charge is placed inside, induced surface charge creates a generally nonzero cavity field while the metal remains field-free.
🧠 Memory hook: Metal is field-free; an empty cavity is shielded, a charged cavity is not empty.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- E_metal = 0 — field inside conductor material
- Q_inner induced = -q_inside — net induced charge on a cavity wall
How to approach it
- 1Separate cavity, metal, and exterior regions
- 2List charges inside each cavity
- 3Apply E_metal = 0 and Gauss surfaces in the metal
Common slip-ups that cost marks
- •Claiming every cavity has zero field
- •Allowing field lines to cross conductor material
- •Assuming induced surface density is uniform in an asymmetric cavity
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
Two point charges 1 microC and 2 microC are 1 m apart in vacuum. Take k = 9 x 10^9 SI. Find the force magnitude.
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