Transcript Electrostatics 12

Electrostatics
History
• The word electricity
comes from the
Greek elektron which
means “amber”.
• The “amber effect” is
what we call static
electricity.
History
• Ben Franklin made the
arbitrary choice of calling
one of the demo
situations positive and
one negative.
• He also argued that when
a certain amount of
charge is produced on
one body, an equal
amount of the opposite
charge is produced on
the other body…
Charge Concepts
• Opposite charges attract, like charges
repel.
• Law of Conservation of Charge:
– The net amount of electric charge produced in
any process is zero. thanks Ben!!!
• Symbol: q, Q
• Unit: C, Coulomb
Elementary Particles
Particle
Charge, (C)
Mass, (kg)
electron
-1.6x10-19
9.109x10-31
proton
neutron
+1.6x10-19
0
1.673x10-27
1.675x10-27
• If an object has a…
+ charge  it has less electrons than normal
- charge  it has more electrons than normal
Ions and Polarity
• If an atom loses or gains valence electrons
to become + or - , that atom is now called
an ion.
• If a molecule, such as H2O, has a net
positive charge on one side and negative
charge on the other it is said to be polar
Why does…
Chemistry work?
Physics!!!
The electrostatic forces between ions (within
molecules) form bonds called ionic bonds…all
bonds are ionic; others, like covalent, are to a
much lesser degree so that you can ignore the
ionic properties of that type of bond.
Why does…
Biology work?
Physics!!!
The intermolecular electrostatic forces
between polar molecules make such
things as the DNA double helix possible.
Types of materials
1.
2.
3.
4.
Conductor: a material that
transfers charge easily (ex.
Metals).
Insulator: a material that
does not transfer charge
easily (ex. Nonmetals)
Semiconductors:
somewhere between 1 & 2
(ex. Silicon, carbon,
germanium).
Superconductors: some
metals become perfect
conductors below certain
temperatures
Ways to Charge
• By Friction: two objects rubbed against each other may
cause a transfer of charge from one to another
(triboelectrification)
• Result: each object has a net charge which is equal and opposite to
the other
• By Conduction: contact occurs between charged object
and neutral object.
• Result: two objects with same charge
• By induction: no contact occurs between charged
object and neutral object.
• Result: two objects with opposite charge
• Credit Card: You may use Visa, Master Card, or
American Express
• Result: Debt from high interest rates
Charging by Friction
POSITIVE
Rabbit's fur
Glass
Mica
Nylon
Wool
Cat's fur
Silk
Paper
Cotton
Wood
Lucite
Wax
Amber
Polystyrene
Polyethylene
Rubber ballon
Sulfur
Celluloid
Hard Rubber
Vinylite
Saran Wrap
NEGATIVE
When insulators are rubbed together, one
gives up electrons and becomes positively
charged, while the other gains electrons
and becomes negatively charged.
Materials have different affinities for
electrons. A triboelectric series rates this
relative affinity.
A material will give up electrons to
another material below it on a
triboelectric series.
Common examples of charging by friction:
• small shocks from a doorknob after walking
on carpet with rubber-soled shoes
• plastic foodwrap that sticks to a container
• sweater pulled over your head that sparks
• laundry from the dryer that clings
• balloon rubbed with hair sticks that to a wall
click for applet
Charging by Conduction
When a charged conductor makes contact with a
neutral conductor there is a transfer of charge.
CHARGING NEGATIVELY
Electrons are transferred from
the rod to the ball, leaving them
both negatively charged.
CHARGING POSITIVELY
Electrons are transferred from
the ball to the rod, leaving
them both positively charged.
Remember, only electrons are free to move in solids.
Notice that the original charged object loses some charge.
Conduction
Charging by Induction
Induction uses the influence of one charged object to
“coerce” charge flow.
Step 1. A charged rod is brought
near an isolated conductor. The
influence of the charge object
polarizes the conductor but does
not yet charge it.
Step 2. The conductor is
grounded to the Earth,
allowing charge to flow out
between it and the Earth.
Charging by Induction (cont.)
Step 3. The ground is removed
while the charge rod is still
nearby the conductor.
Step 4. The rod is removed
and the conductor is now
charge (opposite of rod).
An object charged by induction has the opposite sign
of the influencing body.
Notice that the original charged object does not lose charge.
Induction
Polarization
Conduction or Induction
A
B
Lightning
Becomes very
“negative”
Becomes very
“positive”
Lightning (1)
Lightning (2)
Lightning (3)
Lightning (4)
Lightning (5)
Lightning (6)
Lightning Video compliments of
Tom A. Warner, ztresearch.com
640x480 pixels
7,200 images per second
0.15 seconds recording time
Visual aspects:
-stepped leaders
-dart leaders
-return strokes
-continuing current
Lightning
Rod
Simulator
Lightning striking the
Empire State Building
Van de
Graaff
electrostatic
generator:
simulates
lightning
from cloud to
ground
Electric Forces and Electric Fields
CHARLES COULOMB
(1736-1806)
MICHAEL FARADAY
(1791-1867)
Electric Force
AKA: Coulomb’s Law
Using a torsion balance,
Coulomb found that:
the electric force
between two charges
is proportional to the
product of the two
charges and inversely
proportional to the
square of the distance
between the charges.
Electric Force
Electric Force
q1q2
F E  kc 2
r
•
•
•
•
q  charge, C
r  distance between charges, m
FE  Electric Force, N  VECTOR
kc coulomb constant, 8.99x109Nm2/C2
The Electrostatic Force
EXAMPLE 1 - Find the force between these two charges
9.0  10 5  10


9
Fe
6

C 8  10 6 C
0.04 m 2

Fe  225 N
The negative signs means force of attraction,
but does not indicate left or right direction
EXAMPLE 2 - Find the net force on the left charge
9.0  10 5  10


9
Fe
Fe  360 N
6

C 5  10 6 C
0.025 m 2

(force of repulsion)
Fnet  Fleft  Fright
Fnet  360 N  225 N  135 N, to the left
Electric Field
The electric force is a field force, it applies force
without touching (like the gravitational force)
In the region around a charged object, an Electric
Field is said to exist
Electric Field
Rules for Drawing Electric Field Lines
1. The lines must originate on a positive
charge (or infinity) and end on a negative
charge (or infinity).
2. The number of lines drawn leaving a positive
charge or approaching a negative charge is
proportional to the magnitude of the charge.
3. No two field lines can cross each other.
4. The line must be perpendicular to the
surface of the charge
Electric Field
FE
E
q0
•
•
•
•
•
•
becomes
q
E  kc 2
r
E  electric field strength, N/C  VECTOR
q0  + test charge, C
q  charge producing field, C
r  distance between charges, m
FE  Electric Force, N  VECTOR
kc coulomb constant, 8.99x109Nm2/C2
E-Field vs g-field
E  Field

 Fe
E
q0
g  field

 Fg
g
m0
Electric Field Strength
Field Theory Visualizes Force At A Distance
DEFINITION OF
GRAVITATIONAL
FIELD
DEFINITION OF
ELECTRIC
FIELD
force
g field 
mass
g
E field 
force
charge
Fe
E
q0
Fg
m
q0 is a small, positive test charge
Electric field is a vector quantity
E field points toward negative charges
E field points away from positive charges
SI unit of electric field
click for
applet
newton
N

coulomb C
Van der Graff Generator
Conductors in Electrostatic Equilibrium
• When no net motion of charge occurs within a
conductor, the conductor is said to be in
electrostatic equilibrium
• There are four properties to consider when
looking at conductors
Property 1
• The electric field is zero everywhere inside the
conducting material
– Consider if this were not true
• if there were an electric field inside the
conductor, the free charge there would
move and there would be a flow of charge
• If there were a movement of charge, the
conductor would not be in equilibrium
Property 2
• Any excess charge on an isolated conductor
resides entirely on its surface
– If some excess of charge could be placed inside
the conductor, the repulsive forces would push
them as far apart as possible, causing them to
migrate to the surface
Property 3
• The electric field just outside a charged
conductor is perpendicular to the conductor’s
surface
– Consider what would happen if this was not
true
• The component along the surface would
cause the charge to move
• It would not be in equilibrium
Property 4
• 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)
Conductors in Electrostatic
Equilibrium
1. The electric field is zero everywhere inside a
conductor.
2. Any excess charge on an isolated conductor
resides entirely on the outside surface of the
conductor.
3. The electric field just outside the charged
conductor is perpendicular to the conductor’s
surface.
4. On an irregularly shaped conductor, charge
tends to accumulate where the radius of
curvature is the smallest, i.e. AT SHARP
POINTS.
Charge resides along the surface
=> Charges try to get as far away
as possible
E=0
Otherwise charges
would be moved
around
(Not equilibrium)
Perpendicular otherwise there
would be a force acting on the
charges along the surface
Charge accumulates at
smallest curvature
Electric Potential Difference (a.k.a. Voltage, Potential Difference)
The difference of potential between two points is defined as
the work done to move a charge from a point of lower
potential to a point of higher potential
Potential Difference
V
W
q
Where:
V is the difference in potential between two points
W is the work done in moving a charge in Joules
Units:
Joules
 volts
Coulombs
q is the charge being moved in coulombs
Electronvolts (eV)
The Joule is a large unit of energy…much too large
a unit to use when moving elementary charges
around.
The amount of work done to move a single
elementary charge across a potential difference of
one volt is called an electronvolt
V
W
q
1volt 
W
(1.6 x1019 C )
W  1.6 x10 19 J
1eV  1.6 x10 19 J