electrostatics

1. The question clearly says that the ruler is metallic and John holds it with his bare hand. As such, the ruler is earthed. Any charge that produced by rubbing on the ruler will escape away to earth through John's hand.

2. Yes.
Electrostatic force F is proportional to (Q-q)q. When q = 0, then F = 0 N

3. Judging from the path of the point charge that it curves inward, it is clear that the force is attractive. Hence, statement 2 is correct.

The distance OB > OA, hence the potential energy at B > potential energy at A. Hence statement 3 is correct.

Because of the above, kinetic energy at B < kinetic energy at A, i.e. speed of charge at B < speed at A. Hence, statement 1 is wrong.

4. Yes, you are right.
The electrostatic potential energy given by attractive force is -ve.



2014-10-05 13:41:04 補充：
Your suppl questions:
1. The strip is initially charged. It induces opposite charges on the ruler and then be attracted to the ruler.
2. I suppose you said there were charges induced on the neutral sphere B which produced attraction between A and B...

2014-10-05 13:46:30 補充：
(cont'd)...
The above is only true when the separation between the spheres is comparable to the sphere diameter. If the separation is large, the effects of the induced +ve and -ve charges on B cancel away, and produces no net force on A.

2014-10-05 13:47:10 補充：
(cont'd)...
The word "tiny" sphere is used in the question, and it indicates that the sphere dimensions can be taken as small compared with separation r.

2014-10-05 13:52:21 補充：
3. It is beacuse the charge possesses a non-zero initial speed at A in the direction AWAY from O.
The attractive force provides a centripetal force for the charge to turn around, but not strong enough to pull the charge to come closer to O.