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Phy 2049: Physics II
Syllabus, grades and exams
Electricity, Magnetism, optics
Charge
Force
Constants
F k
q1q2
r12
2
k  9  109
1

4o
2049, summerC, ‘07
Phy 2049: Homework
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Because the summer semester is quite
short, the homework assignments are
rather large. I would suggest that you
do as many as you can at the earliest
chance possible. I would certainly urge
you not to try them all the night before
they are due.
Phy 2049: Homework
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Physics is a little like riding bicycle. No
one has learned how to ride a bicycle
by reading an owner’s manual.
It is very hard in the beginning, but
then it becomes instinctive. That is
when you start to enjoy the scenery
Try doing homework in a group.
Phy 2049: Homework
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Coulomb’s law
Addition of forces as vectors
Induction and contact
Electric fields and field lines
Charges and Polarization
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Two kinds of charges, positive and negative,
unit is 1.6x10-19 Coulombs.
Like Charges repel. It takes force to hold
them in place.
Induction:
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Polarization is attractive. Like charges move
back and unlike charges move up.
Contact leads to repulsion. Only like charges are
left to be redistributed.
Natural particles
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Electrons are very light (m = 9.11 x10-31 kg).
Their charge is 1.6x10-19 Coulomb.
Protons and neutrons are same weight
(nearly so, m = 1.67x10-27 kg = 1/N ).
Protons are positively charge but the
neutrons are neutral.
Phy 2049: Physics II
F k
q1q2
r12
2
k  9  10
1

4o
9
rˆ
• Coulomb’s Law
• F in Newtons, r in meter, q in
Coulomb and k is the fix.
• F is a vector, directed along a line
joining the charges.
• The force can be added like a
vector
Phy 2049: Physics II
Two equally charged particles are held 2.5X
10-3 m apart and then released from rest.
The initial acceleration of the first particle is
observed to be 5.0 m/s2 and that of the
second to be 11.0 m/s2. The mass of the
first particle is 6.3X10-7 kg.
What is the mass of the second particle?
[2.86e-07] kg
What is the magnitude of the charge of each
particle? [4.68e-11] C
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Phy 2049: Physics II
Two equally charged
particles are held 2.5X 10-3
m apart and then released
from rest. The initial
acceleration of the first
particle is observed to be 5.0
m/s2 and that of the second to
be 11.0 m/s2. The mass of the
first particle is 6.3X10-7 kg.
What is the mass of the second
particle?
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What is the magnitude of the
charge of each particle?
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M1a1= M2a2 = F = kq2/r2
M2= M1a1/a2 = 2.86x10-7 kg
q2 = r2 M1a1/k
q = 4.68x10-11 C
Phy 2049: Physics II
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In Fig. at right, the
particles have charges
q1 = -q2 = 298 nC and
q3 = -q4 = 280 nC, and
distance a = 4.0 cm.
What are the a) x
[0.607] N and b) y [0.303] N components of
the net electrostatic
force on particle 3?
Induction and Contact
What is the force between the balls in each case?
• How much charge is transferred when they touch?
• Calculate the force between the balls after they
have touched?
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Force and Electric Fields
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F = qE
Electric field is the force felt by
a unit positive charge
Electric field lines come out of
a positive charge and go into a
negative charge.
They do not cross
Summary
1.
2.
3.
Coulombs law
Properties of charges
Electric fields