Download Chapter 15 Electric Charge, Forces, and Fields

Chapter 15
Electric Charge, Forces, and
Fields
Sections for Chapter 15
Electric Charge
Electrostatic Charging
Electric Force
Electric Field
Conductors and Electric Fields
Gauss’s Law for Electric Fields
What is an “electric charge”?
Yeah all that is good, but what is it?
15.1 Electric Charge
Electric charge is a fundamental
property of matter; electric
charges may be positive or
negative.
The atom consists of a small
positive nucleus surrounded by a
negative electron cloud.
15.1 Electric Charge
Why do protons, electrons, and
neutrons have differing charges?
Particle Physics Time.
15.1 Electric Charge
Proton:
Cool!
15.1 Electric Charge
Proton:
15.1 Electric Charge
Neutron:
15.1 Electric Charge
Neutron:
15.1 Electric Charge
Electron:
15.1 Electric Charge
What’s with the unit for mass?
MeV/c2?
eV = unit for energy
c = speed of light
What does this have to do with mass?
15.1 Electric Charge
1 eV = 1.6e-19 J
c = 3e8 m/s
Mass = E/c2
Mass of electron = 9.1e-31
15.1 Electric Charge
SI unit of charge: the coulomb, C. All charges are
integer multiples of the charge on the electron:
15.1 Electric Charge
Like charges repel; unlike charges attract.
15.1 Electric Charge
Charge is conserved:
The net charge of an isolated system
remains constant.
If one object has lost charge, you can be sure
another object has gained it.
Kind of like: force, mass, energy, momentum, everything.
Practice Questions
1. You shuffle across the carpet and develop a
net positive charge.
a. Do you have a deficiency or an excess of electrons?
b. If the charge the carpet acquired has a magnitude
of 2.15 nC, how many electrons were transferred?
2. Why don’t we typically exchange protons?
3. What is the net charge on a typical Carbon
atom? Nitrogen? Fermium?
15.2 Electrostatic Charging
Conductors transmit charges
readily.
Semiconductors are
intermediate; their
conductivity can depend on
impurities and can be
manipulated by external
voltages.
Insulators do not transmit
charge at all.
Van de Graaff
15.2 Electrostatic Charging
An electroscope may be used to determine if an
object is electrically charged.
http://en.wikipedia.org/wiki/Triboelectric_effect
15.2 Electrostatic Charging
Methods of Charging:
• Friction
• Conduction
• Induction
• Polarization
15.2 Electrostatic Charging
Charging by friction: This is
the process by which you get
“charged up” walking across
the carpet in the winter. It is
also the process that creates
“static cling” in your laundry,
and makes it possible for you
to rub a balloon on your hair
and then stick the balloon to
the wall.
15.2 Electrostatic Charging
An electroscope can be
given a net charge by
conduction—when it is
touched with a charged
object, the excess charges
flow freely onto the
electroscope.
15.2 Electrostatic Charging
An electroscope may also be charged by induction,
if there is a way of grounding it while charge is being
induced.
15.2 Electrostatic Charging
Charge may also be moved within an object—without
changing its net charge—through a process called
polarization.
15.3 Electric Force
The force exerted by one charged particle on
another is given by:
15.3 Electric Force
K is known as the “Coulomb Constant”
and is a proportionality constant.
Charles-Augustin de Coulomb
1736 – 1806
French physicist for
whom the SI unit of
charge, coulomb, is
named.
Noted for his work in the
fields of electricity and
magnetism
k = 9.00x109 N·m2/C2
1. A point charge of +2.5 nC is 0.25 m from a
charge of +1.4 nC, what is the electric force
on each particle?
2. A point charge of +3.4 nC is 0.50 m from a
charge of -2.2 nC, what is the electric force
on each particle?
15.3 Electric Force
If there are multiple point charges, the force
vectors must be added to get the net force.
k = 9.00x109 N·m2/C2
Find the net electric force acting upon each
of the charges for the following problems:
Charge 1: +4.5 nC (0m, 0.3m)
Charge 2: +3.0 nC (0m, 0.3m)
Charge 3: +2.5 nC (0.4m, 0m)
k = 9.00x109 N·m2/C2
Find the net electric force acting upon each
of the charges below
Charge 1: +2.5 nC (3.0 cm, 0 cm)
Charge 2: +1.0 nC (0 cm, 0 cm)
Charge 3: +5.5 nC (0 cm, -5.0 cm)
15.4 Electric Field
Definition of the electric field:
The direction of the field is the direction the
force would be on a positive charge.
15.4 Electric Field
Charges create electric fields, and these fields in turn
exert electric forces on other charges.
Electric field of a point charge:
15.4 Electric Field
For multiple charges, the total electric field is found
using the superposition principle:
For a configuration of charges, the total, or net, electric
field at any point is the vector sum of the electric fields due
to the individual charges.
15.4 Electric Field
It is convenient to represent
the electric field using electric
field lines, or lines of force.
These lines are drawn so the
field is tangent to the line at
every point.
15.4 Electric Field
Rules for drawing electric field lines:
1. Closer lines mean a stronger field.
2. The field is tangent to the lines at every point.
3. Field lines start on positive charges and end on
negative charges.
4. The number of lines entering or leaving a charge
is proportional to the magnitude of the charge.
5. Field lines never cross.
15.4 Electric Field
Electric field lines of a dipole:
15.4 Electric Field
r = 0.015m
Practice!
+
A +4.5 nC charge is fixed in place.
1) What is the electric field created by this
charge 1.5 cm to the right of it?
(don’t forget direction!)
2) If a -2.0 nC charge is placed in the above
location, what force will it experience?
15.4 Electric Field
r = 0.015m
Practice!
+
1) What is the electric field caused by the
-2.0 nC charge 2.0 cm below it?
2) What is the electric field caused by the
4.5 nC charge 2.0 cm below the -2.0
nC charge?
3) What is the overall electric field 2.0 cm
below the -2.0 nC charge?
r = 0.020m
The -2.0 nC charge is now also fixed in place
and a +1.5 nC charge is added to the
previous system
-
15.4 Electric Field
Electric field lines due to very large parallel
plates:
Electric Fields!
Exclamation
because we’re
excited about it
1. You place a charged particle in an electric field
and it experiences no net force, which of the
following could be true?
A. It is far enough from another charge that it is
unaffected q ---/ /---?
B. It is in-between two charges q---?---q
C. It is next to two charges q---q---?
D. There is no other charged object ?
E. The object placed has no charge q 0 q
Bold and
underlined
because Physics
is AWESOME
Electric Fields!
2. Thirteen electrons, a proton, and Bilbo Baggins
(no charge) set out on a grand adventure. Find
the electric field 4.5 cm away from this merry
group.
3. A +5.0 nC particle is placed to the left of another
charged object and is experiencing a 6.2E9 N
force. Find the field acting upon the +5.0 nC
particle and determine the charge of the other
object.
15.4 Electric Field
Electric field lines due to like
charges: (a) equal charges; (b)
unequal charges.
Electric Field Applet
15.5 Conductors and Electric Fields
Electric charges are free to move within a conductor;
therefore, there cannot be a static field within the
conductor:
The electric field is zero inside a charged conductor.
Excess charges on a conductor will repel each other,
and will wind up being as far apart as possible.
Any excess charge on an isolated conductor resides
entirely on the surface of the conductor.
15.5 Conductors and Electric Fields
There cannot be any component of the electric field
parallel to the surface of a conductor; otherwise
charges would move.
The electric field at the surface of a charged conductor is
perpendicular to the surface.
15.5 Conductors and Electric Fields
The force from neighboring charges is less when the
curvature of the surface is large:
Excess charge tends to accumulate
at sharp points, or locations of
highest curvature, on charged
conductors. As a
result, the electric
field is greatest at
such locations.
15.6 Gauss’s Law for Electric Fields: A
Qualitative Approach
Gaussian surface
Completely surrounds a point
charge and intercepts the
same number of field lines
regardless of its shape.
For a positive charge, the lines
exit the surface; for a negative
one they enter it.
15.6 Gauss’s Law for Electric Fields: A
Qualitative Approach
If a greater amount of charge is enclosed, more
field lines cross the surface.
15.6 Gauss’s Law for Electric Fields: A
Qualitative Approach
Here, surface 1 surrounds the positive
charge and has lines exiting it.
Surface 2 surrounds the negative charge
and has lines entering it.
Surface 3 does not enclose any charge,
and the same number of lines exit as enter.
Surface 4 encloses both charges; as they are equal in
magnitude, the same number of lines exit the surface as
enter it.
15.6 Gauss’s Law for Electric Fields: A
Qualitative Approach
The underlying physical principle of Gauss’s law:
The net number of electric field lines passing through an
imaginary closed surface is proportional to the amount of
net charge enclosed within that surface.
This can be used to show
that excess charge on a
conductor must reside on
the surface.
Summary of Chapter 15
Like charges repel; unlike charges attract.
Charge is conserved.
Electrons move freely inside conductors, but not
inside insulators.
Objects may be charged electrostatically by
friction, conduction, or induction.
Polarization is the separation of positive and
negative charge within an object.
Summary of Chapter 15
Coulomb’s law:
The electric field, a vector, is the force per
unit charge. Electric fields from multiple
charges add by superposition.
Electric field lines are used to represent the
electric field.
A conductor has zero electric field inside
and has all excess charge on its surface.
Summary of Chapter 15
The electric field is always perpendicular to
the surface of a conductor.
The charge density and electric field are
greatest on a conductor where the curvature
is largest.
Chapter 15: Overview
I.
Electric Charge
•
•
II.
How to calculate based upon electrons/protons
Static charging: friction, conduction, induction, polarization
Electric Force
•
Force between two or more charges
Reminder:
III. Electric Fields
•
•
Fields define force per unit charge
Field lines rules
IV. Charged Objects
•
•
Rules of charged conductors
Gaussian surfaces
Vector Math
Equations
q = ne
F = kq1q2/r2
E = F/q
E = kq/r2