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Lecture 1 agenda:
Electric Charge.
Just a reminder of some things you learned back in grade school.
Coulomb’s Law (electrical force between charged particles).
You must be able to calculate the electrical forces between one or more charged particles.
The electric field.
You must be able to calculate the force on a charged particle in an electric field.
Electric field due to point charges.
You must be able to calculate electric field of one or more point charges.
Motion of a charged particle in a uniform electric field.
You must be able to solve for the trajectory of a charged particle in a uniform electric field.
Electric Charge
Read about electric charge in sections 21.1 and 21.2 in your
text. You should have learned this material in your prior
academic career. If you haven’t, there is important information
you need to learn now!
There are two kinds of charge.
+
-
 like charges repel
 unlike charges attract
 charges can move but charge is conserved
Law of conservation of charge: the net amount of electric
charge produced in any process is zero. (Not on your starting equation
sheet, but a fact that you can use any time.)
Although there are two kinds of charged particles in an atom,
electrons are the charges that usually move around.
+
-
Protons are roughly 2000 times more massive than
electrons and are typically bound inside nuclei.
Charges are quantized (come in units of e= 1.6x10-19 C).
The charge of an electron is –e = –1.6x10-19 coulombs.
The charge of a proton is +e = +1.6x10-19 coulombs.
That’s all the lecture time I’ll devote to sections 21.1 and 21.2.
Lecture 1 agenda:
Electric Charge.
Just a reminder of some things you learned back in grade school.
Coulomb’s Law (electrical force between charged
particles).
You must be able to calculate the electrical forces between one or more charged particles.
The electric field.
You must be able to calculate the force on a charged particle in an electric field.
Electric field due to point charges.
You must be able to calculate electric field of one or more point charges.
Motion of a charged particle in a uniform electric field.
You must be able to solve for the trajectory of a charged particle in a uniform electric field.
Coulomb’s Law
Coulomb’s law quantifies the magnitude of the electrostatic*
force.
Coulomb’s law gives the force (in newtons) between charges q1
and q2 (in units of coulombs), where r12 is the distance in meters
between the charges, and k=9x109 N·m2/C2.
q1q 2
F k 2
12
r12
*Moving charged particles also exert the Coulomb force on each other.
a note on starting equations
q1q 2
F k 2
12
r12
is on your starting equation sheet.
In general, you need to begin* solutions with starting equations.
You may begin with any correct variant of a starting equation.
QA QB
For example, F  k
E
D2
is “legal” and may be used.
Don’t get hung up about starting a problem with an equation
which is an exact copy of one from the OSE sheet.
*“Begin” does not mean that a starting equation has to be the first thing that appears
on your paper. It might be several lines before you use a starting equation.
Force is a vector quantity. Your starting
equation gives the magnitude of the force.
Use your diagram for the problem to figure
out the direction. If the charges are opposite
in sign, the force is attractive; if the charges
are the same in sign, the force is repulsive.
q1q 2
F k 2
12
r12
This equation just gives the
magnitude of the force.
If a problem asks you to calculate a force, assume that means
both magnitude and direction (or else all components).
1
Also, k 
40
where
0  8.85 1012
C2
.
2
Nm
Remember, a vector has a magnitude and a direction.
Coulomb’s Law is valid for point charges. If the charged objects
are spherical and the charge is uniformly distributed, r12 is the
distance between the centers of the spheres.
r12
-
+
I just told you it’s OK to
use Coulomb’s Law for
spherically-symmetric
charge distributions.
If more than two charges are involved, the net force is the vector
sum of all forces (superposition). For objects with complex
shapes, you must add up all the forces acting on each separate
charge (calculus!!).
+
+
+
-
-
-
Example: a positive charge Q1 = +Q is located a distance d
along the y-axis from the origin. A second positive charge
Q2 = +Q is located at the origin and a negative charge Q3 = -2Q
is located on the x-axis a distance 2d away from Q1. Calculate
the net electrostatic force on Q1 due to the other two charges.
To be worked at the blackboard. You should apply the expert
techniques you learned in Physics 1135 when you work
Physics 2135 problems.