Read Cavity Problems as Superposed Charge Distributions
A cavity inside a uniformly charged object is handled by superposition: start with the full object and add an equal, oppositely charged object where the material was removed.
Electricity questions depend on identifying charge, a conducting path and the condition that controls current or magnetic effect.
Separate charge, current and the path between them
Charge is a property · current needs a conducting path
Charge is a property; current needs a conducting path. Decide first whether the question concerns charging, resistance, current or earthing.
Build the replacement model
Full charged sphere plus a negative charged sphere occupying the cavity gives the real charge distribution. Keep each sphere's own centre and radius clear.
Use potential formulas at the required point
Potential is scalar, so add the contributions directly. Choose the inside-sphere or outside-sphere potential expression according to the point's position for each sphere.
Keep geometry separate
A point may be inside the large sphere but outside the small negative sphere. Evaluate each contribution independently.
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- ✓Replace the cavity with negative charge density.
- ✓Locate the point relative to both spheres.
- ✓Add scalar potentials with signs.
Do not subtract the cavity volume from the final formula directly; superposition requires the potential contribution from its displaced centre.
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