Read Cavity Fields: Use Superposition, Not Symmetry
A cavity in a uniformly charged sphere is solved by superposition: add a full positive sphere and an equal negative sphere at the cavity centre.
Electric Field in an Off-Centre Cavity is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Start with the condition
Inside a uniform sphere, E is rho r/(3 ε₀). Subtract the two linear fields using their different centre coordinates.
Build the governing relation
The position-dependent terms cancel inside the cavity, leaving a uniform field set by the displacement of the two centres.
Finish with a physical check
Check the limiting case before selecting an option: remove the special feature in the diagram or make the relevant parameter very large or very small. The result should reduce to a familiar relation.
OlympIQ exam lens
- ✓State the physical condition before choosing a formula.
- ✓Keep direction, sign and constraint information explicit.
- ✓Test one limiting case before committing to an option.
The cavity is not centred, so applying spherical symmetry around the cavity itself gives the wrong field.
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