PHY 2049: Physics II
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There is a proposal to move the final to
Monday (8/4) from Wednesday (8/6). Any
objections?
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PHY 2049: Physics II
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About bicycle riding: If some one shows
you a trick, it means that,
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
It is possible to do that trick
The person who showed you, can do the trick
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It does not mean that you can do the trick.
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Try it a lot of times and you can too.
PHY 2049: Physics II
Coulomb’s law:
 Two kinds of charges,
positive and negative,
unlike attract, like repel.
 Electrons and Protons:
q = ± 1.602 x 10-19 C
 Current I = dq/dt
1 Amp. = 1 C/s
 Add forces as vectors

F k
q1 q 2
r12
2
rˆ
PHY 2049: Physics II
A rule
Try to calculate the force magnitude by ignoring
the signs of charges. Then assign the
direction by looking at the signs.
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Never do both (using the charge signs in the
calculation of the force and then assign the
direction).
PHY 2049: Physics II
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Charges are
introduced
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by rubbing (depends)
Touching (same)
By induction
(opposite)
PHY 2049: Physics II
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Balls are touched and then separated. Rank
them by
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Charge left
Magnitude of Force before
After
Charge transferred 3,1,2
PHY 2049: Physics II
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d
Force on the
center charge
= 6kq2/d2
PHY 2049: Physics II
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
q1 = q4 = Q; q2 = q3 = q
what is Q/q so that the
force on 1 is zero
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
All charges same sign.
There is no solution.
Q and q different sign.
There is a possible
solution.
Is the answer the same if
the force on 3 is to be
zero
PHY 2049: Physics II
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
What is the force on
the central chlorine
ion?
Suppose that we put
in an electron that
sits on a cesium site
and neutralizes it,
which way does the
Chlorine move?
PHY 2049: Physics II
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Electric Field
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Force F = qE

E = k ∑ rˆ qi/ri2
•Electric field lines denote direction
•Their density represents magnitude
•They do not cross
PHY 2049: Physics II
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q1 = 2Q
q2 = - 2Q
q3 = -4Q
E at origin
PHY 2049: Physics II

Ek
z
qz
2
R

2 3/ 2
zˆ