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Electrostatics
Static Electricity
 Static Electricity – Stationary electricity,
study of charges at rest.
Origin of Charges
 Comes from the atom: The removal or
addition of electrons from an atom gives an
atom a net positive or negative charge (ion).



Proton (p)
Neutron (n)
Electron (e-)
Origin of Charges
 Elementary charge (e) – Smallest observable
charge. Found on an electron or proton.
(Charge is “quantized”.)



Electron = -1e
Proton = +1e
Neutron = 0 e
Origin of Charges
 As example, an atom that has lost two
electrons has a charge of +2 e.
 An atom that has gained three electrons
would have a charge of –3 e.
Charge
 SI unit of charge is the Coulomb (C).



q = symbol for charge
1 Coulomb (C) = 6.3 x 1018 elementary charges (e)
1e = 1 e / 6.3 x 1018 e/C
= 1.6 x 10-19 C
(see reference tables pp. 1)
Charge
 Opposite Charges Attract.
 Like Charges Repel.
 Law of Conservation of Charge – Charge can
neither be created or destroyed.
Separation of Charges:
#1 Friction
 Friction will rub electrons off one material on
to another. Object A gets a net positive
charge and object B gets a net negative
charge.
Separation of Charges:
#2 Conduction (contact charging)
 Charged object in direct contact with another
object. Electrons flow from neg object to pos
object.
Separation of Charges:
#2 Conduction (contact charging)
 Ex – cereal & ruler.
Polarization – or –
separation of charges.
(cereal is still has a net
neutral charge)
Separation of Charges:
#2 Conduction (contact charging)
 Pith Balls
Separation of Charges:
#2 Conduction (contact charging)
 Ex - Electroscope.
Electroscope polarizes in step II. Note: only negative
charges (e-) move. + signs mean an ABSENSE of electrons,
or a net pos charge.
Grounding
 Grounding - Contact to Earth to add or
remove electrons, giving a charged object a
neutral charge.
 Earth acts as a source to either donate or
receive excess e-’s to/from a charged object.
Grounding
 Grounding - Contact to Earth to add or
remove electrons.
 Ex – Negatively
charged electroscope.
Grounding
 Ex – Positively
charged electroscope.
Separation of Charges:
#3 Induction
 Charging without direct contact.
 Ex -
Charging: Pith Balls
 Pith balls pick up electric charge well
 Charged object brought near a pith ball will
polarize it (as in the electroscope). As a
result, pith ball is attracted to object.
 Charged object that touches a pith ball will
charge it. As a result, pith ball has same type
of charge and is repelled by object.
Pith Balls
 1. Charged object brought near neutral pith ball.
 2. Charged object touches pith ball.
 3. Charged object near charged pith ball.
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Lightning
 Lighting – Clouds
build up charges
through friction with
other clouds. When
the charge is too
great, it grounds
with the Earth.
Field vs Contact Force
 Examples of contact forces:


Normal force
Friction
 Example of field (non-contact) forces:



Electrostatic
Gravitational
Magnetic
Forces Between Charges
 Coulomb’s Law –
q1q2
Fe k 2
r
2
m
9
k
8
.99
x
10
N
 2
C
F = electrostatic force
k = electrostatic constant
r = distance between charges
(see reference tables pp1 & pp3 “Electricity & Magnetism”)
Example #1
 What is the force between one proton and
one electron where r = 0.001 m?
If F = pos (+) number, the force is repulsive.
If F = neg (-) number, the force is attractive.
Example # 2
 The nucleus of a gold atom consists of 79 protons. If all but
one of the orbiting electrons were removed from the neutral
atom, what would be the magnitude of the electrostatic force
exerted on the remaining electron when it is at a distance of
6.0 x 10-10 m from the nucleus?
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Electric Field
 The region where an electric force acts on a
charge. The direction of the field lines point
to the direction of which the force is directed
on a small positive test charge.
(qo = test charge)
Electric Field
 An electric field is said to exist anywhere a force is felt on a
positive test charge. Placing a positive test charge in a field and
observing its path creates a segment of a "field map." The movie
below demonstrates the tracing of the path of a positive test
charge.
Electric Field
 When you have traced the paths of that test charge many times in
many different places, you start to get an idea of what the field
map looks like. The completed field map is shown below.
Electric Field
 It is very important that you notice that the map lines have a
direction to them. The direction represents the motion of the
positive test charge when placed at different points around the
field. You should also notice that field lines never cross each
other. Even as we create other field maps, we find the field lines
never cross. The closer the field lines are to each other, the
greater the field intensity or field strength.
Electric Field
 Other Electric Fields: Single Positive Charge.
 What would the diagram look like if the charge was negative?
Electric Field
 Other Electric Fields: Charged parallel plates.
Electric Field Intensity
 Electric Field Intensity (Field Strength) –
Shows the strength of the electric field, or the
force per unit charge at a distance away from
a charged object.
Electric Field Intensity
 Equation:
Fe
E
q
Units: Newtons / Coulomb
 For Parallel Plates:
V
E
d
d = distance between plates
Units: Volts / m
- or - Newtons / Coulomb
Electric Field Intensity & Coulomb’s Law
Coulomb’s Law:
q1q2
Fe k 2
r
Electric Field Intensity:
F
q
q
1 q
q
e
1
2
1
E


k

k

k
2
2
2
q
r
q
r
r
2
(assuming test charge ‘q’ is ‘q2’ in Coulomb’s Law)
Voltage
 Electric Potential (Voltage)(Potential
Difference) – Change in PE per unit of
charge.
W
V
q
Units: Joules / C
- or – Volts (V)
Voltage
 Think of a Positive charge stuck on the negative side of a
parallel plate. A certain amount of force(F) will need to be
applied over a distance(d), work done (W=Fd), to move to
the charge (q) to the positive side. The plates essentially
provide a certain amount of potential energy per unit of
charge (a.k.a Voltage) to charges inside.
W
V
q
The Electronvolt
 When an elementary charge, ie/ an electron, is moved
through an E field through a potential difference of 1 Volt,
then the work done is 1.6 x 10-19 Joules
W
V
q
W  Vq

19
W

(
1
V
)(
1
.6
x
10
C
)

19
W1.6x
10 J
This is called the electronvolt

19
1
eV

1
.6
x
10J
See reference tables pp1
This is a unit of electrical energy!
How DO You Measure Electric Charge?
 Millikan oil drop experiment:
 What forces are acting on the oil drops that
are in the uniform electric field?
Millikan Exp’t (cont)
 By carefully balancing the gravitational and electric
forces on tiny charged droplets of oil suspended
between two metal plates (uniform electric field).
Knowing the electric field, the charge on the oil
droplet could be determined. Repeating the
experiment for many droplets, they found that the
values measured were always multiples of the same
number. They interpreted this as the charge on a
single electron: 1.602 × 10−19 coulomb
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