Q4. Assertion: no work is done in moving a charge along an equipotential surface. Reason: the field at every point of the surface is normal to it. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: the force qE has no component along the surface, and there is no potential difference between its points (NCERT page 54).
Q5. Can two equipotential surfaces with different potentials cross? Explain.
No. At a crossing point the potential would have two different values, which is impossible (reasoned from the definition, NCERT page 54).
Q6. Two parallel equipotential planes at 100 V and 60 V are 2.0 cm apart in a uniform field. Find the size and direction of the field.
E = (100 − 60)/0.020 = 2.0 × 103 V/m, normal to the planes, from the 100 V plane towards the 60 V plane (NCERT Eq. (2.21), page 55).