Q4. Assertion: a charge Q at the centroid of an equilateral triangle, with equal charges q at its corners, feels no net force. Reason: the three forces on Q are equal in size and at 120° to one another. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: any two of the forces add to a force equal and opposite to the third (NCERT Example 1.5, pages 12–13).
Q5. Two charges each push a third charge with a force of 2 N, and the two forces are at 60° to each other. What is the total force?
2F cos(θ/2) = 2 × 2 × cos 30° = 2√3 ≈ 3.5 N, along the bisector of the angle between the two forces (parallelogram law, NCERT page 12).
Q6. In NCERT Example 1.6, take q = 1 µC and l = 0.3 m. Find the force on each charge.
kq2 = 9 × 109 × 10−12 = 9 × 10−3 N m2, so F = 9 × 10−3/0.09 = 0.1 N. Each +q feels 0.1 N; −q feels √3 × 0.1 ≈ 0.17 N (NCERT pages 13–14).
Q7. Four point charges qA = 2 µC, qB = −5 µC, qC = 2 µC and qD = −5 µC sit at the corners of a square ABCD of side 10 cm. What is the force on a charge of 1 µC at the centre? (NCERT Exercise 1.6, page 42)
Zero. A and C are opposite corners with equal charges, the same distance from the centre, so their forces on the 1 µC charge are equal and opposite. B and D, also opposite and equal, cancel the same way (superposition, NCERT page 12).