# Electrostatics by chenshu

VIEWS: 1 PAGES: 55

• pg 1
```									Electrostatics
 Atomic  Particles and Properties
 Interaction of Charge
 Conductors and Insulators
 Methods of Charging
 Coulomb‟s Law
 Electric Fields
 Electric Potential Energy
 Voltage (Potential Difference)
 Capacitors...
Try  to picture an atom...
Now, draw a picture of a
model of an atom that
you‟ve seen in the past.
If you haven‟t seen one,
make one up!
The Bohr model of the atom
Atomic Particles
All have     mass and charge
SI units?

Fundamental        unit of
charge:
“e” = 1.60 x 10 -19 Coulombs
chargeis quantized; it comes in set
amounts!
Electrons
negatively  charged (-e)
mass = 9.11 x 10-31 kg
“orbit” the nucleus in a
probability sphere (cloud)
held in place by electrostatic forces
held more weakly as “orbits”
increase
Protons
charged (+e)
positively
mass = 1.67 x 10 -27 kg
~2,000  times the mass of an
electron
alwaysoccur in the same
number as electrons
unless   it‟s an ion
Neutrons
neutral
electrically
mass ≈ 1.67 x 10 -27 kg
the    same as a proton!
may  or may not occur in
the same number as
protons
Quarks and Leptons
Of course, at this stage of our
scientific understanding, we
know that even protons,
neutrons, and electrons are
not elementary.
They are made up of quarks
and leptons
Charge is Quantized!
That is...electric charge will appear in packets or
multiples of “e”. Hence,
q = N·e
N is the # of charges, e is the charge of an electron
q is measured in Coulombs
Ex: How many protons would there be in one Coulomb of
positive charge?
q     1.0C
N              6.25  1018

E 1.6  10 C
19

That number of basketballs would fill the Earth 11,000 times over!!!
Think and Explain...
„Think about what happens when a
balloon is rubbed against your hair and
then placed next to the wall...
„Can you explain why this happens?
 Electric   charge
 An  electrical property of matter;
forces exist between two
charged objects.
 Just like “mass” is an intrinsic
property of matter and forces
exist between two masses.
 Likecharges repel;
opposite charges attract.
Separation of Charge
 How are electrostatic forces and
gravitational forces alike? How are
they different?
 Demo:       comb and paper bits…
 Charges       can be “separated”
by rubbing two things together
 literally
 this shows how friction is an
electrostatic force
 when charges
move objects
become electrified
Conservation of Charge

Law of Conservation of
Charge: During any process, the
net electric charge of an isolated
system remains the same.
„Electrons flow from regions of negative charge to
regions of positive charge.
„Read the Physics of electronic ink on page 524
Conductors and Insulators
  Not only can charge exist on an object,
it can move through an object
 Materials that readily allow the
movement of charges are called
conductors
 metals,   water
 outer shell electrons (valence) that can move
„freely‟ through the substance
 flows from e- surplus (-) to e- deficiency (+)
 Materials  that inhibit the movement of
charges are called insulators
   glass, ceramic, rubber, plastic
Back to the Balloon Question…
are pulled off of your hair.
 This causes the balloon to be charged negatively.
 When the negative balloon is placed near the neutral
wall the electrons in the wall rearrange.
 The electrons in the wall are repelled giving the
wall a local positive charge.
 Thus the negative balloon is attracted to the positive
wall.
Detecting Charge
Charge   is detected with an
electroscope
The Electroscope
 Ifa charged object comes in contact
with the electroscope, the leaves of
the electroscope push away from
each other. Why???
 But you can‟t tell what
kind of charge it is with an
electroscope. Why?
Methods of Charging
Friction       (rubbing)
 electronsget „rubbed off‟ leaving extra/not
enough electrons (net charge)
 excess   --> negative charge -- deficiency --> positive
charge
 carpets, static cling, the Van de Graff generator
 hard rubber & fur = negative charge
 glass & silk = positive charge
Methods of Charging
Conduction
chargingby contact
Charges flow from a charged
conductor to a neutral conductor
Induction
no contact is necessary!
Just move close to a neutral object and
charges rearrange. Why???
A   neutral object coming close to a charged
object can have its charges moved without
taking on electrons.
 The object is still
neutral while it
has a + and – region.
(polar molecules
in chemistry)
Coulomb’s Law
 The electrostatic force between two charged
particles is directed along a line joining their
centers. If the charges are similar the force
is repulsive, otherwise it is attractive.

k = 8.99 x 109
q1q2
F k 2                     N·m2/C2  F is
in Newton's
d
How is this similar to the gravitational force?
Coulomb’s Law...

double the distance, force drops by 1/4.

double the charges, force increases by a factor
of four.
Coulomb’s Law Example
Ex: How do the forces of gravity and
electrostatic repulsion compare between two
100.0 kg metal spheres separated by 10.0
cm and given a charge of 1.00 mC?
What does this say about levitation???
Coulomb’s Law Example
Ex: A gold atom normally contains 79
protons. If all of the electrons are removed
except for one, what force would it undergo
at a distance of 6 x 10-10 m from the
nucleus? Is this a very large force?

20cm           30cm

q1=-6µC     q2=+2µC           q3=-4µC
Determine the net electrostatic
force on q2 in the figure above.
Could charges q1 and q3 be
positioned so that the net force is
zero?
Determine the net
electrostatic force on q2
q1=-6µC
in the figure above.
Could charges q1 and
q3 be positioned so that
20cm
the net force is zero?

30cm
q2=+2µC          q3=-4µC
Speed of a Orbiting electron
satellite

+
planet                             nucleus

In a hydrogen atom one electron (-e) orbits around a proton (+e) at a
radius of 0.0529 nm. Use UCM techniques to calculate the orbital speed
(can you calculate gamma for this speed and determine if there are
relativistic effects?)
Electric Fields
 All charged bodies will affect other charges
when placed near each other. Explain…
 We say that a charged body sets up an
ELECTRIC FIELD around it (think of how a
massive body sets up a gravitational field
around it.)
 When a charge is placed in an electric field, it
experiences a Coulomb attraction/repulsion
 It will accelerate in the direction of the net
force. What would you have to do in order to
prevent the acceleration?
The Electric Field
 Just  like gravitational fields exist wherever there is
mass, electric fields exist wherever there is charge.
 SI   units: N/C
   The way we determine field strength is to place a (positive) point
charge
in the field and measure the
applied force.
 Electricfields are vectors:
Ex 8-11 and fig. 18.17                        E F
q0
Electric Field Lines
Conventions:
   begin on positive and end
on negative charges
   directed away from positive
charges and toward negative
charges (tells which way a
positive test charge would
go)
   the density of lines indicates
the strength of the field
An Electric Dipole (two positive charges)

 Where    is the field
the strongest?
The weakest?
 Can you draw the
field for a dipole
opposing charges?
A more complex field…
Neat Fact:
When   electric charge is placed on a
conductor. All the charge resides on
the surface.
This   is behind the idea of electric
shielding.
Think and Explain...
What   would
happen to you if
you were struck
by lightning in
Think and Explain...
What   must you do to lift a stone off the
ground? What happens to the stone’s
potential energy?
What must you do to move a positive test
charge closer to another positive charge?
What happens to the charge’s energy?
Remember...
 Tolift a stone you must do work. The
amount of work you do is equal to the
stone‟s change in G.P.E.
W = ∆G.P.E. = mgh - mgh0
 When  raised in a gravitational field a
stone has potential energy. That
energy depends on how high you
raise the stone above some reference
level.
Apply...
 How would the G.P.E. of the stone compare to that
of a large boulder raised to the same height?
 This dependence on mass could be overcome by
dividing by mass. Let’s call this new value the
gravitational potential (G).
G = G.P.E./m (units?)
 Now we can determine the gravitational potential at
any point above the earth for any sized object.
Electric Potential and Work
   Suppose we move a charge, +q, from A to B in
the field below. The work done to move q is
equal to the charge‟s change in electric potential
energy (E.P.E)

W = ∆E.P.E.
= EPEB-EPEA

 When the positive charge is moved from A to B
does it gain or lose EPE?
 How would the work done change for a larger
charge?
Electric Potential
 The  Electric Potential (or potential difference) is
also called Voltage. It is found by dividing the
Work done to move a charge by the magnitude of
the charge moved.
V = W/q = ∆EPE/q
 Voltage is measured in Volts (J/C)
 Voltage is the “push” that causes electrons to
move through wires
Field Lines and Equipotentials
 Equipotential   surfaces are places in an electric field
where the potential difference is the same
They   are perpendicular to the electric field lines.
 In  the diagram at
right, the field
lines are blue and
the equipot. Lines
are red.
Go Figure...
How  much work must be done to
move a 300 µC charge through a
potential difference of 6.0 V?
1.8 x 10-3 J
If GPE is zero at ground level,
where is EPE zero???
An infinite distance away!
Point Charges...
The   E.P.E. due a point charge
is:               qq0
E.P.E.  k
r
The   potential of a point charge
is:
Vk q
r
Go Figure...
•Ex: A 2.00 µC point charge is
located at the center of an equilateral triangle
with sides of 50.0cm
•What is the potential at each vertex?
•62,000V
•What is the EPE of a 1.00 µC charge placed
at a vertex?
•6.2x10-2 J
•How much work must be done to move the
test charge from one vertex to the next?
•None! Why???
Charge Distribution and Shape

„All charge resides on the surface of a conductor
(shielding)
„Charge spreads out evenly on a perfect conducting sphere
„Charge is more confined to original area on a non
conductor
„There is no potential diff. between any points on a
conductor
„The surface of a conductor is an equipotential surface
Charge Distribution and Shape

„Charge concentrates at points or sharp edges on a
conducting surface
„There can be no potential difference between any
two points on a conductor
„If the field intensity at a sharp edge is great
enough, gas around surface will ionize and electric
discharge will occur
„Spark plugs, lightning, lightning rods
St. Elmo‟s Fire
   St. Elmo's fire is a spark discharge from soaring
buildings. It is generated by a high voltage between
the ground and the air. St. Elmo‟s fire has been
observed among others from steeples, masts,
mountain tops and barbed wire fences. It is very rare.
When you see St. Elmo‟s fire near yourself, there is a
high danger of being struck by lightning. The high
voltage can also show up by your hair standing on
end. Although this may look "funny", you must
immediately leave your position as a flash of lightning
is imminent.
Go Figure...
Ex. How much work must be done to
place a 10.0 µC charge at each of the
corners of a square that is 50.0 cm on
each side? [Hint: place them one at a
time…]

~18J
Capacitors 1
Devices that store electric
energy are called capacitors.
The  most common type is made up
of two parallel plates separated by
some medium (called a dielectric).
The plates are usually rolled into a
cylindrical shape
Energy comes from the work it took
to separate the charge!
Capacitors 2
The charge “stored” on a
capacitor is given by the

q  CV
equations:

“C” is the capacitance of the capacitor,
a constant that depends on its makeup.
What are the units of C?
Capacitors 3
The capacitance, C, of a
capacitor can be found using:
 0A
C
d
κ is the dielectric constant, ε0 is the
permittivity of free space, A is the area
of the capacitor, and d is the
separation between the two sides
Capactors 4
 Ex:The electric field
strength between the
plates of a capacitor
can be found using:

E = V/d
where V is the voltage
across the plates and d
is the plate separation.
Capacitor Examples
Ex: What voltage is required to
store 7.2 x 10-5 C of charge on
the plates of a 6.0 mF
capacitor?
Ex: #40 on page 583
Capacitor Examples
Ex: How much charge can be stored by
a parallel plate capacitor made up of
3.0 cm x 4.0 cm sides, separated by a
distance of 0.025 mm when a voltage
of 12 V is placed across it and…
a.) no dielectric is used.
b.) ruby micra is used as a dielectric.
c.) What would the electric field strength be
between the plates of the capacitor?
Did you know???
Photo  Flashes use capacitors
Computer keyboards use
capacitors (p 573)
Capacitors are used to prevent
power surges in brown outs.
The greater the dielectric constant
the more charge can be stored
(Why?).
Oil and water can be used as
dielectrics, air too!
I’m shocked…
Can this be
the end,