A charge 26.41 mC is placed to the left of another charge 72.23 mC on a plane, a

Question

A charge 26.41 mC is placed to the left of another charge 72.23 mC on a plane, as shown in the figure. Another charge of -7.551 ?C and mass 36.31 g (depicted as a blue sphere) is placed at rest at a distance 33.89 cm above the right-most charge and released. What is the speed of the -7.551 ?C charge if it reaches the gray position labeled A?

http://img829.imageshack.us/img829/4831/qzyu.png
http://img829.imageshack.us/img829/4831/qzyu.png A charge 26.41 mC is placed to the left of another charge 72.23 mC on a plane, as shown in the figure. Another charge of -7.551 ?C and mass 36.31 g (depicted as a blue sphere) is placed at rest at a distance 33.89 cm above the right-most charge and released. What is the speed of the -7.551 ?C charge if it reaches the gray position labeled A?

Explanation / Answer

Solve with conservation of Energy

Initial PE = kqq/r + kqq/r

(9 X 10^9)(-7.551 X 10^-6)(72.23 X 10^-3)/(.3389) + (9 X 10^9)(-7.551 X 10^-6)(26.41 X 10^-3)/(.5066)

PE = -18027 J

Final PE = (9 X 10^9)(-7.551 X 10^-6)(72.23 X 10^-3)/(.2635) + (9 X 10^9)(-7.551 X 10^-6)(26.41 X 10^-3)/(.1130) = -34512 J

Now...

-18027 = -34512 + .5mv^2

16485 = (.5)(.03631)(v^2)

v = 953 m/s

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