Mastery of electric charges and fields is foundational for both CBSE board examinations and competitive tests (JEE/NEET). This chapter develops quantitative tools — Coulomb's law, superposition, field of continuous distributions and Gauss's law — that recur across electrostatics problems and appear in multi-concept questions requiring spatial reasoning and vector addition.
Beyond routine calculations, questions in examinations probe deeper: interpreting field graphs, recognizing symmetry, and applying Gauss's law in non-intuitive contexts. Practising such problems builds the analytic thinking and estimation skills essential to high-scoring board answers and for tackling tougher competitive problems.
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10
Questions
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Marking Scheme
Q1. Two point charges are placed on the –axis: at and at . At what point on the line between them (measured from ) is the net electric field zero?
Q2. The measured variation of the magnitude of electric field with distance from the centre of an object is: for and for . Which of the following charge distributions best explains this behaviour?
Q3. Two identical point charges are fixed at coordinates and with . What is the magnitude of the resultant electric field at the point ? (Use .)
Q4. Assertion (A): An isolated hollow conducting spherical shell is electrically neutral. If a point charge is now placed at the geometric centre of the cavity, an induced charge appears uniformly distributed over the inner surface and appears on the outer surface.
Reason (R): The total induced charge on the inner surface equals because the conductor was initially neutral and induced charges must sum to cancel the enclosed charge.
Q5. A thin circular ring of radius carries total charge uniformly. Find the magnitude of electric field on the axis of the ring at a point which is at a distance from the centre. (Use .)
Q6. For a uniformly charged thin ring of radius and total charge , the axial electric field magnitude is for a point on the axis at distance from the centre. At which (measured from the centre) is maximum?
Q7. A uniformly charged solid sphere of radius has unknown total charge . From an experimental graph inside the sphere one reads . Using , estimate (take ).
Q8. Two very large parallel insulating sheets carry uniform surface charge densities and and face each other. If , what is the magnitude of the electric field in the region between the sheets? (Take .)
Q9. Assertion (A): For a uniformly charged solid sphere of radius the electric field at an interior point a distance from the centre varies as (for ).
Reason (R): This linear variation arises because all the charge on a uniformly charged solid sphere is concentrated on its surface.
Q10. Two point charges and are fixed at and respectively. Consider a spherical Gaussian surface of radius centred at the origin with so that neither charge lies inside the surface. Which statement is correct?