Lecture 1 (“Orientation”):
Introductory Comments
►Course overview
►Syllabus
►Navigating Canvas
►Miscellaneous/Questions
►Content (time permitting)
Overview of Physics 0175
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This course is part of your “Integrated Curriculum”
(calculus, chemistry, engineering, physics)
Introductory Physics compressed into two terms (four
credits each)
Chapter 21: Coulomb’s Law
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Electric charge
Electric charge and the structure of matter
Conductors and insulators
Induced charge
Coulomb’s Law
Induced Charges and Shell Theorem
Experiments showing
that there are two kinds
of charges: positive and
negative.
Like charges repel.
Unlike charges attract.
Masses & charges of the particles
from which atoms are constituted:
Electron:
me = 9.109 x 10-31 kg
qe = - 1.602 x 10-19 C
Proton:
mp = 1.673 x 10-27 kg
qp = + 1.602 x 10-19 C
Neutron:
mn = 1.675 x 10-27 kg
qn = 0.000 C
The Millikan oil-drop experiment:
A classic series of
experiments (1910-1913)
that were the first
precision determination
of the charge of the
electron and established
the fact that charge was
quantized. what does
“quantized” mean?
Two important principles to
remember:
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Conservation of charge – the algebraic sum
of all the electric charges in any closed
system is constant
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Quantization of charge – every observable
amount of electric charge is always an
integer multiple of the electron charge or the
proton charge
Conductors vs. insulators:
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Conductors permit electric charge to move
easily. (For example: copper, silver, gold;
in fact, most metals)
Insulators do not permit electric charge to
move easily. (For example: plastic, wood,
glass; in fact, most non-metals.)
Superconductors and
semiconductors:
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Superconductors offer no resistance at all to the
movement of electric charge. (Special alloys at very
low temperatures.)
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Semiconductors allow electric charge to flow under
certain conditions, sometimes in one direction but
not in the opposite direction. (Insulators that are
“doped” with different atoms; very important in
modern electronics.)
Coulomb’s Law:
(1785—Charles Augustin de Coulomb)
Electric force between two charges:
Direction of F:
Magnitude of force:
q1  q2
F k
2
r
•Attractive if the charges are
opposite
•Repulsive if the charges are
alike
k = 8.988 x 109 N•m2/C2 (in S.I. units)
The constant k that appears in Coulomb’s
Law is often expressed in terms of
another, constant ε0 which is called the
permittivity of free space (i.e. vacuum):
k

1
4 0
 8.988  109  9.0  109 N  m 2 / C 2
 0  8.854  1012 C 2 / N  m 2
Electric Force vs. Gravitational Force:
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Both forces decrease as (1/r2)
Both forces are along the line joining the point
masses or point charges
The electric force is much stronger than the
gravitational force for charged particles
(recall that G=6.67x10-11 N•m2/kg2 )
The gravitational force is always attractive, while
the electric force can be attractive or repulsive
Remember Newton’s Third
Law:
Remember: forces are vectors!
Therefore, if a point test charge q0 is surrounded
by more than one other point charges q1, q2, q3, …
one must calculate the individual force vectors
that each of the surrounding charges exerts on the
test charge and add these vectors to find the total
force:
Illustrative Example #2.1: The figure shows four
situations in which two charged particles are
fixed in place along the x-axis. In which of
these four situations is there a point to the left of
the particles where an electron will be in
equilibrium?
Illustrative Example #2.2: The figure shows four
configurations of five charged particles evenly spaced
along the x-axis. Rank the configurations according to
the magnitude of the net electrostatic force on the central
particle.
Illustrative Example #2.3: What is the
net force that q1 and q2 exert on q3?
(1-D problem)
q1 = +1.0 nC
q2 = -3.0 nC
q3 = +5.0 nC
Remember: forces are vectors!
Therefore, if a point test charge q0 is surrounded
by more than one other point charges q1, q2, q3, …
one must calculate the individual force vectors
that each of the surrounding charges exerts on the
test charge and add these vectors to find the total
force:
Vector Addition of Forces Suggested Method:
Charging a metal sphere by
induction:
This would not work if the sphere were not a conductor!
Charging a metal sphere by
induction:
Demo 23d
Shell Theorem in
Electrostatics:
•A shell of uniform charge attracts or repels a
charged particle that is outside the shell as if
all the shell’s charge were concentrated at its
center.
•If a charged particle is located inside a shell
of uniform charge, there is no net electrostatic
force on the particle from the shell.
Illustrative Example #3.2: The three configurations
shown below all involve a charged particle and a
uniformly charged spherical shell. The charges are
given and the radii of the shells are indicated. Rank
the configurations according to the magnitude of the
force on the particle due to the presence of the shell.
Where does charge reside on a
conductor?
If an isolated spherical shell conductor carries a
net charge (either positive or negative), the excess
charge spreads uniformly over the outer surface of
the conductor. Since that surface has the greatest
surface area, this allows the like charges to
separate as much as possible.
If two identical conductors carrying
different net charges come into contact,
how do their net charges change?
Their combined net charge will be divided
equally between them.
+1e
+1e