5/9/2011
Phy 2049: Physics II
Phy 2049: Homework
Syllabus, grades and exams
Electricity, Magnetism, optics
F = k
Charge
Force
Constants
q1q 2
r12
k = 9 × 10
=
1
4 πε
2
9
o
Phy 2049: Homework
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 hard in the beginning, but then it
becomes instinctive. That is when you
start to enjoy the scenery
Try doing homework in a group.
Charges and Polarization
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:
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.
Polarization is attractive. Like charges move
back and unlike charges move up.
Contact leads to repulsion. Only like charges are
left to be redistributed.
Coulomb’s law
Superposition (Addition) of forces as
vectors
Induction and contact
Electric fields and field lines
Natural particles
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/NA ).
Protons are positively charged but the
neutrons are neutral.
Electrons move in a metal, hence induction.
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5/9/2011
Touching charges
Induction and Contact
Three identical
balls, 2q, q and –q
A touches B and
separated back.
B touches C and
separated back
Ratio of Force FAB
now to force FAB at
the start.
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?
•
Phy 2049: Physics II
F =k
q1q2
r12
2
k = 9 × 109
1
=
4πε o
• Coulomb’s Law
rˆ
• 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?
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?
What is the magnitude of the charge of each
particle?
Directions of Forces
M1a1= M2a2 = F = kq2/r2
M2= M1a1/a2 = 2.86x10-7 kg
q2 = r2 M1a1/k
q = 4.68x10-11 C
Between two
charged particles
forces are directed
along a line joining
them.
Can be added as
vectors
What is the magnitude of the
charge of each particle?
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5/9/2011
Phy 2049: Physics II
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) Fx and
b) Fy components of
the net electrostatic
force on particle 3?
Force and Electric Fields
•
•
•
•
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
3