Name ______________________________________ Student ID _______________
last
first
II.
[25 points total] A Teflon (insulating) rod suspended by insulating strings is rubbed,
resulting in the Teflon rod gaining a uniform net negative charge. An initially
neutral aluminum (conducting) rod is then suspended near the Teflon rod with
similar insulating strings, as shown at right.
The rods never touch and no sparks jump between them.
A. [5 pts] Indicate the approximate charge distributions on the two rods by drawing
+ and – symbols on the figure below.
-- -- --- - -- - -- -- -- -- --
Insulating
strings
Teflon Aluminum
rod
rod
- --
+ ++
Teflon rod
Score________
Aluminum rod
Explain why you drew the charge distributions the way you did.
The Teflon rod is an insulator, so the negative charge on it remains uniformly distributed. The
negative charge in the Teflon rod attracts the positive and repels the negative charge in the aluminum
rod. Since the aluminum rod is a conductor, the charges are free to move because of the
attraction/repulsion from the Teflon rod.
B. [5 pts] Would the rods attract, repel, or not interact? Explain.
As described above, the end of the aluminum rod closest to the Teflon rod has a net positive charge
while the end furthest from the Teflon rod has a net negative charge. Since the Teflon rod is closer to
the positive than the negative charge in the aluminum rod, the attractive interaction is stronger than
the repulsive interaction. Thus there is a net attractive interaction between the rods.
C. [5 pts] Is the final net charge on the aluminum rod positive, negative, or zero? Explain.
Since no charged objects touched the aluminum rod, and no sparks jumped, the net charge on the
aluminum rod remains zero.
D. While the rods are hanging they are briefly touched by a person then released.
The rods never come into contact with each other.
i.
Teflon Aluminum
rod
rod
[5 pts] Indicate the approximate charge distributions on the two rods by
drawing + and – symbols on the figure below.
-- -- --- - -- - -- -- -- -- --
++++ +
Teflon rod
Aluminum rod
Explain why you drew the charge distributions the way you did.
The Teflon rod is an insulator, so the negative charge on it remains unchanged. The negative
charge in the Teflon rod attracts the positive and repels the negative charge in the aluminum rod.
Since the hand is also a conductor, some of the negative charge repelled by the Teflon rod would
go through the hand, and eventually into the ground. We can also think of positive charge from
the ground as being attracted to the Teflon rod, and thus moving into the aluminum rod.
ii. [5 pts] Would the rods attract, repel, or not interact? Explain.
Because negative charge was repelled into ground and positive charge was attracted out of
ground, the aluminum rod has a net positive charge. Positive and negative charges attract, thus
the two rods attract.
Physics 122B, Autumn 2009
Exam 1
EM-UWA122B094B-E1(ELS).doc