Q4. Assertion: the electric field of a dipole is zero because its total charge is zero. Reason: the fields of +q and −q nearly cancel far from the dipole. Is the assertion true? Is the reason true?
The assertion is false: the charges are separated, so their fields do not cancel exactly and the dipole has a field. The reason is true: far away they nearly cancel, which is why the field falls as 1/r (NCERT pages 23 and 25).
Q5. A small dipole of moment 2 × 10−8 C m is at the origin. Find the field 0.1 m away (a) along its axis, (b) along its equatorial line. Take 1/4πε0 = 9 × 109 N m2 C−2.
(a) E = 2kp/r3 = 2(9 × 109)(2 × 10−8)/(0.1)3 = 3.6 × 105 N C−1, along p. (b) E = kp/r3 = 1.8 × 105 N C−1, opposite to p (NCERT Eqs. (1.20) and (1.21), page 24).
Q6. What is a point dipole?
The limit in which the separation 2a shrinks to zero and q grows so that p = q × 2a stays finite; for it, the far-field formulas are exact at every nonzero distance; the ideal field is undefined at its origin (NCERT page 25).