Two charges q1 and q2 are placed 30 cm apart, as shown in the figure. A third charge q3 is moved along the arc of a circle of radius 40 cm from C to D. The change in the potential energy of the system is q34 0k, where k is [figure]
- 8q1
- 6q1
- 8q2
- 6q2
Answer
(3) 8q2
Solution
On the arc, the distance of q3 from q1 stays 40 cm, so only the q2–q3 energy changes.
BC = 402 + 302 = 50 cm = 0.5 m, BD = 40 - 30 = 10 cm = 0.1 m
U = q2 q34 0 ( 10.1 - 10.5 )
= q34 0(10 - 2)q2
= q34 0 8q2
k = 8q2
Answer: (3)