Unformatted text preview: d W (by me)
U Wi 3 Q2 4 0 d Q d
. >2 Q 3 Q2 4 0 d W2 1 Q2 4 0 d . ( "
27 " W3 1 Q2 4 0 d 1 Q2 4 0 d 2 Q2 4 0 d
, F
' +: $ $ C
Q2 1 1 4 0d Q2 1 1 4 0d Q2 1 2 4 0d Q2 1 2 4 0d "
3 Q $ ; U U
U d d W by me
U Wi 1 Q2 4 0 d U
. ( . ( . (
29 1 Q . >2
W2 d Q 1 Q2 4 0 d 2 " " " 1 Q2 4 0 d W3 1 Q2 4 0 d 1 Q2 4 0 d 2 Q2 4 0 d ( A B C D 31  ( % LET'S DO THE CALCULATION !!
34 9 7
0 ' 7> + 0 ' ;
q X=0 2q X=d C $ " $ 7 >2 ' 7 7 I C x A) B) C) D) 0 1 2 3 37 H 7
0 $ $ ' ; " $ q X=0 2q X=d x A) B) C) D) E) A&B A&C B&C B&D A&D J 0 3 40 < $
r q X=0 d ( $
2q X=d 0
x U 1 Qq 4 0 r 1 4
0 2Qq r d Set U = 0
1 r 2 r d
Makes Sense! Q is twice as far from 2q as it is from +q
2 r
43 d $
r q X=0 (
dr $
2q X=d 3
x Setting U = 0 1 r 2 d r 2r d r r d 3 46 Makes Sense! Q is twice as far from 2q as it is from +q For a pair of charges:
Just evaluate U q1q2 k r r
q1 q2 (We usually choose U = 0 to be where the charges are far apart) For a collection of charges:
Sum up U q1q2 k r for all pairs...
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 Fall '12
 Energy, Force, Gravity, Potential Energy, Work, Electric charge, 2q

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