2) Initially both a small pith ball, with a conductive coating, that hangs from
ID: 3893321 • Letter: 2
Question
2) Initially both a small pith ball, with a conductive coating, that hangs from a string and
a plastic PVC rod are neutral. The pith ball is rubbed against the rod with enough friction
to charge both objects.
a) How should the charge state of the pith ball compare to the charge state of the rod after
the two objects are rubbed together. Explain your reasoning.
b) The charged pith ball ball is then brought into contact with a second identical and
initially uncharged pith ball. After touching the two pith balls repel and when placed so
that their centers are 5 cm apart the repulsive force is measured to be 0.006 N. Use this
information to determine the amount charge on the rod.
c) Consider a system consisting of the charged rod and either (but not both) of the
charged pith balls (after they
Explanation / Answer
a) After being rubbed together the charges on both the bodies will be same and opposite. This is because since the number of electron in both the bodies is conserved, thus if one of the bodies gain N electrons , the other body has obviously lost N electrons. Thus the chatge on both the bodies is same in magnitude but opposite in nature, If one is charged to +n C , the other is charged to -n C.
b) F = k* q1 * q2 /r^2
=> 0.006 =8.98755*10^9* q^2/(5/100)^2
=> q = sqrt((0.006/(8.98755*10^9))*(5/100)^2) = 4.08530955*10^-8 C
this is the charge on one ball
thus charge on both balls = 2* 4.08530955*10^-8 = 8.1706191*10^-8 C
thus charge on the rod = -8.1706191*10^-8 C
c) each of the balls have half the total charge. Hence one ball will have half the charge of the rod. On coming in contace, the charge will try to neurtalize and only half of the charge in the rod will get neutralized, Hence charge in the system willl be unlike the rod (half to be specific)
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