Download chapter-19-1 - High Point University

Review
You have two neutral pieces of tape stuck together.
You rip them apart and notice that each one is
charged (because each one is attracted to your
neutral hand). Without doing an experiment, we
know that they MUST be oppositely charged. Why?
Electrostatic Force
The observation that “like” charged particles repel and
“unlike” charged particles attract comes from
experiment. It’s not a memorized fact nor a definition.
Experiment shows that for two charged particles
if the particles are in a vacuum
Coulomb’s Law
Fon 2
q2
Fon 1
q1
The electrostatic force is attractive
(toward each other) for like
charges.
Coulomb’s Law
Fon 2
q2
Fon 1
q1
The electrostatic force is repulsive
(away from each other) for unlike
charges.
Comparison with Gravitational Force
OR
m1
m2
q1
q2
At the atomic scale, the electrostatic force
dominates interactions of charged particles
The mass of a proton is 1.7 x 10-27 kg. The mass of an
electron is 9.1 x 10-31 kg. (Note: a proton is roughly
2000 times more massive than an electron.) Compare
the gravitational force and electrostatic force of a
proton on an electron if they are 0.5 x 10-10 m apart.
Poll
Two positively charged spheres exert a force of 0.1 N
on each other as shown below. What is the direction
of the electrostatic force on Sphere A (due to Sphere
B)?
qA
qB
Poll
Sphere A has a charge 1 C. Sphere B has a charge
10 C. On which sphere is the electrostatic force the
largest (in magnitude)?
qA
1. Sphere A
2. Sphere B
3. Neither, because the
force on each sphere is
the same (in magnitude)
qB
Poll
Given a small positive charge and a large negative
charge, which figure best represents the forces
that the charges place on each other?
1)
+
−
4)
+
−
2)
+
−
5)
+
−
3)
+
−
6)
+
−
Poll
−
+
−
An assembly is composed of a positive and a negative
charge of equal magnitude connected by a rigid rod.
The assembly is placed near another negative charge.
What is the direction of the net force on the assembly?
1) The net force is directed toward the right.
2) The net force is directed toward the left.
3) The net force is zero.
Superposition of Forces in 1-D
• If multiple charges are present, the net force on any one
charge is equal to the vector sum of the forces due to
each of the other charges.
• If the charges all lie in a straight line (x-axis), all of the
forces will act along the + or – x-axis.
– Use + sign for forces acting in positive x-direction.
– Use – sign for forces acting in negative x-direction.
– Net force will simply be the sum of all forces and will either be in
the + or – x-direction.
Example
Q1 = +7.5C
+
Q2 = +4.0C
−
+
0.20 m
Q3 = -4.0C
0.20 m
What is the net force on Q1 due to Q2 and Q3?
x
Superposition of Forces in 2-D
•If the charges do not lie in a straight line then all of the
forces must be resolved into x and y components before
addition.
–Choose orientation of x and y axes.
–Resolve each force into x and y-components.
–Use + and – signs to indicate whether each component is in the
positive or negative x (or y) direction.
–The x-component of the net force will be the sum of the xcomponents of all the forces.
–The y-component of the net force will be the sum of the ycomponents of all the forces.
–Find magnitude and direction of net force from Pythagorean
Theorem and tanq=(Fy/Fx). (Note q is the angle between Fnet and
the x-axis.)
Example
y
0.50 m
−
What is the net force on Q1 due to Q2 and Q3?
−
+
Q2
Q1 = -4.5C
Q1
0.50 m
Q3
x
Q2 = +5.0C
Q3 = -8.2C
Electric Field
How does q2 know that q1 is there?
q1
q2
q1 creates an electric field everywhere in space
around it. This electric field exerts a force on a charged
particle.
Electric field due to a positively charged
particle.
The electric field due to a
positively charged particle
points away from the
particle and decreases with
distance squared.
Electric field due to a negatively charged
particle.
The electric field due to a
negatively charged particle
points toward the particle
and decreases with
distance squared.
Force on a charged particle by an electric
field
The force by an electric field on a charged particle is
If q is positive, then the
force on the particle is
parallel to the electric field.
If q is negative, then the
force on the particle is
opposite (i.e. antiparallel)
to the electric field.
q
Poll
The electric field at some location and instant of time is in
the +y direction. In what direction is the force on a
proton at this same location?
Poll
The electric field at some location and instant of time is in
the +y direction. In what direction is the force on an
electron at this same location?
Poll
The electric field at some location and instant of time is in
the +y direction. Which is larger, the force on a proton or
the force on an alpha particle (i.e. helium nucleus with
two protons) at this same location?
Poll
The electric field at some location and instant of time is in
the +y direction.
1
If this electric field is
CREATED by a positive
charge, +q, where might
that charge be located?
4
2
3
Poll
The electric field at some location and instant of time is in
the +y direction.
1
If this electric field is
CREATED by a negative
charge, -q, where might that
charge be located?
4
2
3
Poll
A proton placed at location A experiences an electric force in
the direction shown below. What is the direction of the electric
field at location A?
Poll
At location C there is an electric field in the direction
indicated, due to charges not shown. A chloride ion
(Cl−) is placed at location C. What is the direction of
the electric force on the chloride ion?
Poll
At location D there is an electric field in the direction
indicated, due to charges not shown. An electron is
placed at location D. What is the direction of the
electric force on the electron?
Poll
An electron is placed at location A. What is the direction of
the electric field at location B, due to the electron?
Poll
A proton is placed at location B. You have calculated the
electric field at location A, due to the proton. To draw an
arrow representing the electric field you calculated, what
should you do?
1.
2.
3.
4.
Put the tail at point A.
Put the tail at point B.
Put the tip at point A.
Put the tip at point B.
Superposition
If there are many charged particles, then the electric field
at a point in space is the sum of the electric fields
created by all of the charged particles independently.
+
P
+
M
Poll
What is the direction of
the net electric field at
point P due to the
dipole?
P
Poll
What is the direction of
the net electric field at
point P due to the
dipole?
P