Transcript Electrostatics Review

Electrostatics
Review
EQ: What causes you to get an
“electric shock” when you walk
across the carpet on a cold
winter’s day and reach for the
door knob?
Electrostatics
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Clothes tumble in the dryer and cling together.
You walk across the carpeting to exit a room and receive a door knob
shock.
You pull a wool sweater off at the end of the day and see sparks of
electricity.
During the dryness of winter, you step out of your car and receive a
car door shock as you try to close the door.
Sparks of electricity are seen as you pull a wool blanket off the
sheets of your bed.
You stroke your cat's fur and observe the fur standing up on its end.
Bolts of lightning dash across the evening sky during a spring
thunderstorm.
And most tragic of all, you have a bad hair day.
These are all static electricity events - events that can only be
explained by an understanding of the physics of electrostatics.
Electrostatics
Electrostatics:
The study of electric charges that can
be collected and held in one place.
The study of static electricity, where
static electricity is electricity that is
confined to one area.
Charge & Mass
Charged Objects
After Rubbing:
Positively Charged
Negatively Charged
Glass and wool
Hard rubber and plastics
Charges are not created!
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Like Charges
Opposite Charges
+ -
Repel
Attract
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+
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Conductors and Insulators
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Insulator
A material through which a charge will not move
easily.
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Conductor
A material that allows charges to move about
easily.
Air can be a conductor
Lightning Storms
Theorems of Electrostatics
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All unbalanced charge flows to the outside
surface of a conductor.
Charge density is higher near corners,
points.
There is no unbalanced charge inside of
conductors
Charge will flow from a point of higher
density to a point of lower density until the
charge densities at the two points are
equal.
Electrostatic Force
How strong is Electric Force?
Compare it to Gravity!
What do we Know?
 There are two kinds of electric charges:
positive and negative.
 Charges exert forces on other charges at a
distance.
 The force is stronger when the charges are
closer together.
 Like charges repel; opposite charges attract
Charging
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Friction – charging a body by rubbing.
Conduction – charging a neutral body
by touching it with a charged body.
Induction – charging an object without
touching it.
Grounding – the process of connecting
a body to Earth to eliminate excess
charge.
Coulomb’s Law
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The force that act between two or
more charged objects.
Force depends on distance
Force depends on charge
Charge denoted by “q” and the distance
between charges denoted by “r”.
Coulomb’s Law
The magnitude of the force between
charge qA and charge qB, separated by
a distance r is proportional to the
magnitude of the charges and
inversely proportional to the square of
the distance between them.
F is proportional to qA qB
r2
Coulomb’s Law
The unit of charge is a Coulomb
“q” symbol for charge
1 Coulomb = 6.24X1018 electrons or protons
Coulomb’s Constant K = 9 X109 Nm2
C2
F = qAqB
K
r2
The force between two charges is equal to Coulomb’s
constant, times the product of the two charges,
divided by the square of the distance between
them.
Electric Field
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A property associated with the space
around a charged object.
Its interaction with another charged
object in that field is manifested in the
electric or Coulomb Force.
We represent the E-Field graphically
with rays.
Electric Field
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The field flows outward, away from a
positively - charged object.
The field flows inward, towards a
negatively -charged object.
Strength is indicated by spacing of
lines
Electric Field
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The strength of an electric field is equal to the force on a positive test
charge divided by the strength of the test charge
E=F
q’
E is the electric field intensity
generated by a field charge q
q’ is the small test charge placed in the E-field to measure the
strength of the field at some point q’<<<q
F is the Coulomb or electric force generated by the field on the
test charge q’
Units N/C
Electric Field
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A charge (q), known as the field charge generates the electric
field (E). IN ORDER TO MEASURE THIS FIELD we introduce
a second significantly smaller charge (q’), called the test
charge, into the field and observe the resultant Coulomb
Force (F) exerted on this test charge.
Although we may be able to deduce things about it, we never
actually know anything about the field charge itself.
An alternative formula may be derived by substitution of the
Coulomb’s Force equation into the electric field equation,
resulting in
E=Kq
r2
Where the strength of the electric field at some distance (r)
from a field charge (q) may be determined.
Comparison of FG and FQ
F = Kq1q2
d2
F = G m1m2
d2