2.17 POINTS TO PONDER

1. Electrostatics deals with forces between charges at rest. But if there is a force on a charge, how can it be at rest? Thus, when we are talking of electrostatic force between charges, it should be understood that each charge is being kept at rest by some unspecified force that opposes the net Coulomb force on the charge.

2. A capacitor is so configured that it confines the electric field lines within a small region of space. Thus, even though field may have considerable strength, the potential difference between the two conductors of a capacitor is small.

3. Electric field is discontinuous across the surface of a spherical charged shell. It is zero inside and (σ/ε0) n cap outside. However, electric potential is continuous across the surface, equal to q/4πε0R at the surface.

4. The torque p vector ⨯ E vector on a dipole causes it to oscillate about E vector. However, only if there is a dissipative mechanism, the oscillations are damped and the dipole eventually aligns itself with E vector.

5. Potential due to a point charge q at its own location is not defined - it is infinite.

6. In the expression qV (r vector) for potential energy of a charge q, V (r vector) is the potential due to external charges and not the potential due to q. As seen in point 5, this expression will be ill-defined if V (r vector) includes potential due to a charge q itself.

7. A cavity inside a conductor is shielded from outside electrical influences. It is worth noting that electrostatic shielding does not work the other way round; that is, if you put charges inside the cavity, the exterior of the conductor is not shielded from the fields by the inside charges.