Chapter 19 Flashcards
Types of Charges
They are called positive and negative
Named by Benjamin Franklin
Like vs Opposite charge
Like charges repel and unlike charges attract one another
Electric charge is always _____
conserved
Charge is not created, only ________
exchanged
Objects become charged because ______
negative charge is transferred from one
object to another
Charge is quantized
All charge is a multiple of a fundamental unit of charge, symbolized by e,
with the value of
e = 1.6 x 10-19 C
o Quarks are the exception
o Electrons have a charge of –e
o Protons have a charge of +e
o The SI unit of charge is the Coulomb (C)
Conductors
are materials in which the electric charges move freely in response to an electric force
o Copper, aluminum and silver are good conductors
o When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material
Insulators
are materials in which electric charges do not move freely
o Glass and rubber are examples of insulators
o When insulators are charged by rubbing, only the rubbed area becomes
charged
o There is no tendency for the charge to move into other regions of the
material
Semiconductors
The characteristics of semiconductors are between
those of insulators and conductors
o Silicon and germanium are examples of semiconductors
Charging by Conduction
A charged object (the rod) is placed in contact with another object (the sphere)
o Some electrons on the rod can move to the sphere
o When the rod is removed, the sphere is left with a charge
o The object being charged is always left with a charge having the same sign as the object doing the charging
Charging by Induction
When an object is connected to a conducting
wire or pipe buried in the earth, it is said to
be grounded
o A negatively charged rubber rod is brought
near an uncharged sphere
o The charges in the sphere are redistributed
o Some of the electrons in the sphere are repelled from the electrons in the rod
o A grounded conducting wire is connected to the sphere
oAllows some of the electrons to move from the sphere to the ground
o The wire to ground is removed, the sphere is left
with an excess of induced positive charge
o Charging by induction requires no contact with the
object inducing the charge
Charge Polarization
The charged object (on the
left) induces charge on the
surface of the insulator
o This realignment of charge on the surface of an insulator is known as polarization
Light: Animation
A charged comb attracts bits of paper due to polarization of the paper
Coulomb’s Law
F = (k |q1||q2|) / (r^2)
ke is called the Coulomb Constant
ke = 8.9875 x 109 N m2/C2
Remember that force is a ____ quantity
vector quantity
Electrostatic Force
o Both are inverse square laws
o The mathematical form of both laws is the same
o Masses replaced by charges
o Electrical forces can be either attractive or
repulsive
o Gravitational forces are always attractive
o Electrostatic force is stronger than the
gravitational force
Coulomb’s Law for Multiple
Charges
o The resultant force on any one charge
equals the vector sum of the forces exerted
by the other individual charges that are
present.
o Remember to add the forces as vectors
If q is positive
charge repels
If q is negative
charge attracts
Electrical Field
E (r on top) = ((F (I on top)) / (q (subscript o) = (K (subscript e) Q) / r^2
Electrical Field SI units are
N / C
The electric field is a _____ _____
vector quantity
The direction of the field is defined to be the
direction of the electric force that would be
exerted on a _______
small positive test charge, qo,
placed at that point
Direction of Electric Field
o The electric field produced by a
negative charge is directed toward
the charge
o A positive test charge would be
attracted to the negative source charge
o The electric field produced by a
positive charge is directed away from
the charge
o A positive test charge would be repelled
from the positive source charge
Electric Field Lines
A convenient aid for visualizing electric field patterns
is to draw lines pointing in the direction of the field vector at any point
introduced by Michael Faraday
The field lines are related to the field in the following manners
o The electric field vector, , is tangent to the
electric field lines at each point
o The number of lines per unit area through a
surface perpendicular to the lines is proportional to the strength of the electric field in a given region
Electric Field Line Patterns
o Point charge
o The lines radiate equally in all
directions
o For a positive source charge, the
lines will radiate outward
o For a negative source
charge, the lines will point
inward
o The high density of
lines between the
charges indicates the
strong electric field in
this region
No two field lines can cross each other
An electric dipole
consists of two equal
and opposite charges
When no net motion of charge occurs within a
conductor, the conductor is said to be in ________ _______
electrostatic equilibrium
An isolated conductor has the following properties
o The electric field is zero everywhere inside the conducting material
o Any excess charge on an isolated conductor resides entirely on its surface
o The electric field just outside a charged conductor is perpendicular to the conductor’s surface
o On an irregularly shaped conductor, the charge accumulates at locations where the radius of curvature of the surface is smallest (that is, at sharp points)
QUICK QUIZ
A ping-pong ball covered with a conducting
graphite coating has a mass of 5.0 × 10−3 kg and a charge of 4.0 µC. What electric field directed upward will exactly balance the weight of the ball?
(g = 9.8 m/s2)
Electric Flux
Field lines penetrating an area A perpendicular to the field
o The perpendicular to the area A is at an
angle θ to the field
Electric Flux Equation
ΦE = E A cos θ
The product of EA is
the flux, Φ
When the area is constructed such that a
closed surface is formed, use the convention that flux lines passing into the interior of the volume are _________ and those passing out of the interior of the volume are _________
Pass into the interior of the volume are negative
Pass through the interior of the volume are positive
Gauss’ Law
Gauss’ Law states that the electric flux through any closed surface is equal to the net charge Q inside the surface divided by εo
Gauss’ Law Equation
ΦE = (Q inside)/(εo)
εo is the
permittivity of free space and equals
8.85 x 10^-12 C^2/Nm2
a Gaussian surface
The area in Φ is an imaginary surface, a Gaussian surface, it does not have to coincide with the surface of a physical object
Total flex equation
Total Flex = (q1+q2)/(εo)
Electric Field of a Charged Thin Spherical Shell
The calculation of the field outside the shell is identical to that of a point charge
E = (q/4pi(r^2)(εo) = ke (q/r^2)
The electric field inside the shell is zero
Electric Field of a Nonconducting
Plane Sheet of Charge
Use a cylindrical
Gaussian surface
The flux through the
ends is EA, there is no
field through the
curved part of the
surface (A = 2A0)
The total charge inside
is Q = σA0
Electric Field of a Nonconducting
Plane Sheet of Charge
Equation:
E = (σ)/(2εo)
Parallel Plate Capacitor
o The device consists of plates of positive and negative charge
o The total electric field between the plates is given by
E = σ/εo
o The field outside the plates is zero
QUICK QUIZ
A closed surface contains the following point
charges: 6 C, 4 C, –2 C, –4 C. The electric flux
coming out of the surface is:
