CLASS EXERCISE #3  15 June 2005
Consider the electrical force between pairs of electric charges as shown.
These are three independent problems, with different charges but the same
separation
in each case.
represents the same number of coulombs in each
<U
case.
Let
denote the force
/
Show the magnitude (in terms of
;
J5
U
<
Þ
J
22
e.g.,
2 or 3
) and direction of the force on each charge in each pair.
JÎ
J
1.
U Š oqqqqqqqqqqqp ‹ U
<
Solution:
UŠpJ
J o‹ U
2.
2
U Š oqqqqqqqqqqqp Š U
<
Solution:
2
2
2
JoqŠ U
UŠqp J
3.
2
2
U Š oqqqqqqqqqqqp ‹ U
<
Solution:
2
4
4
2
U Š qqqp J
J oqqq ‹ U
The electrical forces are attractive (towards the other charge) in cases (1) and
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 Summer '08
 Fretwell
 Charge, Force, Mass, Electric charge, 2J, oq S 2U, 2U S qqqp

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