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UNIT 9 -
ELECTROSTATICS
Chapter 18
TYPES OF ELECTRIC CHARGE
Static
(Unit 9)
Charge
at rest
Collection
Dynamic
Charge
Such
of charge static electricity
(Unit 10)
in motion : current
as in electrical circuits
ATOMS
 Structure
 Proton

Positive charge object found in the nucleus
 Neutron

Neutral object found in the nucleus

Nucleus will also have a positive charge
 Electron

Negatively charged object found orbiting the nucleus
of an atom

Can move from object to object or through a material.
THE CHARGE OF AN ATOM
If
an atom has an equal number of protons
and electrons it is considered electrically
neutral.
Ions
(charged atoms) contain a different
number of protons and electrons

Fewer electrons – positively charged

More electrons – negatively charged
QUANTITY OF CHARGE
Unit
for charge – Coulomb (C)
The
charge on a single electron is −1.6 × 10−19 𝐶
and the charge on a single proton is +1.6 × 10−19 𝐶
To
determine the total charge of an atom…
 Subtract
the number of electrons from the number
of protons = excess protons
 Multiply
by +1.6 × 10−19 𝐶
EXAMPLE #1
A. Determine the charge on an object that has
4.0 × 1020 more electrons than protons.
 4.0 × 1020
B.
−1.6 × 10−19 = −64 𝐶
Determine the charge on an object that has 5.2 ×
1017 protons and 2.8 × 1016 electrons.

5.2 × 1017 − 2.8 × 1016 = 4.92 × 1017 excess protons
 4.92 × 1017
1.6 × 10−19 = .079 𝐶
EXAMPLE #2
An
object carries an estimated negative charge
of 2 Coulombs. Determine the number of excess
electrons.
# 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠
# 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠
1.6 × 10−19 = #𝐶𝑜𝑢𝑙𝑜𝑚𝑏𝑠 so…
=
2
1.6×10−19
= 1.25 × 1019 electrons.
WAYS TO CHARGE OBJECTS
Friction
The
frictional charging process results in a transfer
of electrons between the two objects that are
rubbed together.
 Rubbing
 Charged
objects.
a balloon on your hair, feet across the carpet
objects exert an electric force upon other
 Non-contact
force (like gravity)
WAYS TO CHARGE OBJECTS
Friction
 Opposites
attract and likes repel
 Since
one object becomes negatively charged and
the other positively charged, the two objects will cling
together
 Charged
objects and neutral objects are attracted to
each other as well.
WAYS TO CHARGE OBJECTS - FRICTION
EXAMPLE #3 – WWW.PHYSICSCLASSROOM.COM
CLASSIFICATION OF MATERIALS
 Conductors
 Materials
with an ability to transfer charge from one object to
another (or particle to particle) easily
 Charge
 Ex:
is distributed evenly across the surface of the object
copper, aluminum, water
 Insulators
 Materials
 Charge
 Ex:
that resist the flow of charge
is not evenly distributed across the surface
wood, glass, rubber
CLASSIFICATION OF MATERIALS
Semiconductors
 Materials
 Often
times based on temperature
 Exactly
 Ex:
intermediate in the ability to carry charge
4 valance electrons
silicon, germanium
CHARGING BY INDUCTION
Induction
Redistribution
of charge due to the presence of
nearby charged objects.
Charging
without contact between objects
Polarization
– the positive and negative charges
separate from each other.
Law
of Conservation of Charge – the overall
charge in the system is the same before and after
the charging process.
CHARGING BY INDUCTION
CHARGING BY CONDUCTION
Conduction
Charging
Van
by contact
de Graaff generator
Polarization
– the positive and negative charges
separate from each other.
Law
of Conservation of Charge – the overall
charge in the system is the same before and after
the charging process.
CHARGING BY CONDUCTION
ELECTRIC FORCE AND COLOUMB’S LAW
The
interaction between charged objects is a
non-contact force that acts over some
distance of separation.
 Depends
on the two charges and the distance
between them.
Electric
force is a vector quantity with both
magnitude and direction.
ELECTRIC FORCE AND COLOUMB’S LAW
Direction is determined by opposites attract
and likes repel.
ELECTRIC FORCE AND COULOMB’S LAW
Coulomb’s
Law let’s us calculate the magnitude of
the electric force between two charges:
𝑘𝑄1 𝑄2
𝐹=
𝑑2
𝑄1
d
and 𝑄2 are the charges in Coulombs
is the distance between the two charges
k
is Coulomb’s constant, which is 9.0 × 109 𝑁𝑚2 /𝐶 2 for
air.
+
is repulsive force, and - is attractive force.
EXAMPLE PROBLEM #1
Two
balloons are each charged to -6.25 nC
(−6.25 × 10−9 𝐶). They are held apart at 61.7 cm.
Determine the magnitude of the electric force of
repulsion between them.
𝐹
=
(9.0×109 )(−6.25×10−9 )(−6.25×10−9 )
.6172
−7
9.23 × 10
𝑁 repulsive force.
= 9.23 × 10−7 𝑁
EXAMPLE PROBLEM #2
Two
balloons with charges of 3.37 µC (3.37 × 10−6 𝐶)
and -8.21 µC (−8.21 × 10−6 𝐶) attract each other with
a force of 0.0626 N. Determine the distance
between the two charges.
−0.0626
𝑑 2
𝑑
=
=
(9.0×109 )(3.37×10−6 )(−8.21×10−6 )
𝑑2
(9.0×109 )(3.37×10−6 )(−8.21×10−6 )
−0.0626
= 1.99 𝑚
= 3.98
ELECTRIC FIELD
 Charged
objects create electric fields – alterations in the
space around them.
 Objects
that enter the space will feel the charge, but the
electric field exists regardless.
 If
Q is the charge of the object and q is a test charge, the
magnitude of electric field is
𝐸=
𝐹𝐸
𝑞
=
𝑘𝑄𝑞
𝑟2
𝑞
=
 The
𝑘𝑄
𝑟2
greater the test charge, the more force it will feel as a
result of the same electric field.
 Direction
depends on whether charges are +/-
REVIEW PROBLEM #1
REVIEW PROBLEM #2
REVIEW PROBLEM #3
REVIEW PROBLEM #4
REVIEW PROBLEM #5
REVIEW PROBLEM #6
REVIEW PROBLEM #7
Calculate
the charge of an object that has 1.4 × 1015
protons and 3.2 × 1015 electrons
REVIEW PROBLEM #8
An
object that has a charge of 0.5 C has how many
more protons than electrons?
REVIEW PROBLEM #9
Calculate
the electric attractive force between 2
objects that have charges of 3.2 × 10−5 𝐶 and −5.2 ×
10−6 𝐶 and are a distance of 0.25 meters apart.
REVIEW PROBLEM #10
A
test charge of 4.2 × 10−6 C experiences a force of
0.6 N when it is a distance of 0.1 m away from an
object.
A.
What is the electric field intensity created by the
object?
B.
What must be the charge of the object?
C.
What force would the test charge experience if it
was twice as far away from the object?
D.
If it had twice the charge?
ANSWERS
1.
A,B,C (Y might be + or N, so Z could be anything)
2.
A is rubber (insulator) and B is copper (conductor)
3.
D (Polarization)
4.
D (Protons don’t move!!!)
5.
B,D,G
6.
0.294 C
7.
−2.88 × 10−4 C
ANSWERS CONTINUED
8.
9.
3.125 × 1018 excess protons
24 N attractive force
10.
A.
1.43 × 105 N/C
B.
1.59 × 10−7 C
C.
0.15 N
D.
1.2 N