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Let
x
be the coordinate of
P
, the point where the field vanishes. Then, the total electric
field at
P
is given by
()
21
2
2
02
1

1
4()
qq
E
xx
πε
⎛⎞
=−
⎜⎟
−
−
⎝⎠
.
If the field is to vanish, then
2
2
2
22
2
11
(
)
.


q
x
x
q
−
=⇒
=
−
−−
Taking the square root of both sides, noting that 
q
2
/
q
1
 = 4, we obtain
70 cm
2.0
20 cm
x
x
−
= ±
−
.
Choosing –2.0 for consistency, the value of
x
is found to be
x
=
−
30 cm.
7. At points between the charges, the individual electric fields are in the same direction
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This note was uploaded on 06/07/2010 for the course PHYS 344 taught by Professor Bb during the Spring '10 term at The Petroleum Institute.
 Spring '10
 BB
 Physics, Charge, Electric Fields

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