Unformatted text preview: Slide 9
Physics 212 Lecture 2, Slide + 23 + + Physics 212 Lecture 2, Slide 10
Physics 212 Lecture 2, Slide Checkpoint 3 A positive test charge q is released from rest at a distance r away from a charge of +Q and a
distance 2r away from a charge of +2Q. How will the test charge move immediately after being
released?
A. To the left B. To the right C. Stay still D. Other INTERESTING: statement is correct, but given in support of “to the left” !! “although the force is double for the right charge
although
for being 2Q, the force is also quartered for
being 2r away. “
being
“The distance is squared in the denominator.
The
For the +Q, that leaves 1/1. for the +2Q, that
leaves 2/4=1/2. The +Q is therefore stronger
in this instance.”
in “the 2q is 2r away and the q is r away, so the
the
twos cancel out and the test charge will have
the same amount of force from each side”
the
12 Physics 212 Lecture 2, Slide 11
Physics 212 Lecture 2, Slide Example
+q P d
q “Just some more examples would be cool” What is the direction of the electric field at point P, the unoccupied corner of the square? +q
d (A) (B) Calculate E at point P. (E) Need to
know d & q Qi
ˆ
E = ∑ k 2 ri
ri
i q
Ex = k 2 −
d 20 (D) (C) E = 0 Need to
know d q
Ey = k 2 −
d ( π
cos 2
4
2d ( π
sin 4 q q
2d ) ) 2 Physics 212 Lecture 2, Slide 12
Physics 212 Lecture 2, Slide Continuous Charge Distributions “my head is spinning, what does the dQ mean?” Summation becomes an integral (be careful with vector nature) Qi
ˆ
E = ∑ k 2 ri
ri
i dq
ˆ
E = ∫k 2 r
r
WHAT DOES THIS MEAN ?? Integrate over all charges (dq)
r is vector from dq to the point at which E is defined Linear Example: λ = Q/L
dE pt for E
r
charges
25 dq = λdx
dq
Physics 212 Lecture 2, Slide 13
Physics 212 Lecture 2, Slide Charge Density “What exactly is charge density, how is...
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 Spring '08
 SELEN
 Charge, Electrostatics, Electric charge, Fundamental physics concepts, Ri

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