Ncert Exercises & Solutions

Question 2.31: Answer carefully:

(a) Two large conducting spheres carrying charges Q₁ and Q₂ are brought close to each other. Is the magnitude of the electrostatic force between them exactly given by Q₁Q₂/4

π

ε_{0}

r², where r is the distance between their centers?

(b) If Coulomb's law involved 1/r³ dependence (instead of 1/r²), would Gauss's law be still true?

(c) A small test charge is released at rest at a point in an electrostatic field configuration. Will it travel along the field line passing through that point?

(d) What is the work done by the field of a nucleus in a complete circular orbit of the electron? What if the orbit is elliptical?

(e) We know that the electric field is discontinuous across the surface of a charged conductor. Is electric potential also discontinuous there?

(f) What meaning would you give to the capacitance of a single conductor?

(g) Guess a possible reason why water has a much greater dielectric constant (80) than say, mica (6).

(a) The force between two conducting spheres is not exactly given by the expression, Q₁Q₂/4

π

ε_{0}

r², because there is a non-uniform charge distribution on the spheres.

(b) Gauss's law will not be true if Coulomb's law involved 1/r³ dependence, instead of 1/r², on r.

If a small test charge is released at rest at a point in an electrostatic field configuration, then it will travel along the field lines passing through the point, only if the field lines are straight. This is because the field lines give the direction of acceleration and not of velocity.

(d) Whenever the electron completes an orbit, either circular or elliptical, the work done by the field of a nucleus is zero.

(e) No Electric field is discontinuous across the surface of a charged conductor. However, electricity is continuous.

(f) The capacitance of a single conductor is considered as a parallel plate capacitor with one of its two plates at infinity.

(g) Water has an unsymmetrical space as compared to mica. Since it has a permanent dipole moment, it has a greater dielectric constant than mica.

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