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Electric ChargesElectric Charges : the basis of electricity is: the basis of electricity is
charge.charge.
- The charge on an- The charge on an
atom is determinedatom is determined
by the subatomicby the subatomic
particles that makeparticles that make
it up.it up.
on- has a positive charge and is located in the nucleus.
Neutron- has no charge (is neutral) and is also
located in the nucleus as it fills in the spaces
between the protons.Electron- has a negative charge and is located
outside of the nucleus in an electron cloud around

Particle charges:Particle charges:
- Electrons and protons have
the same magnitude of charge
- Electron (-e): -1.60 x 10-19
C
- Proton (+e): +1.60 x 10-19
C
- This is why electrons are
forced to orbit around the
nucleus.
- Electrostatic Forces hold
atoms together.
- The Law of Charges- Like

Subatomic Particle SizesSubatomic Particle Sizes
A proton and neutron have about the sameA proton and neutron have about the same
mass -mass - 1.67 x 101.67 x 10-27-27
kgkg
An electron has a much smaller mass -An electron has a much smaller mass - 9.11 x9.11 x
1010-31-31
kgkg
To put this into perspective it’s like
comparing the sizes of a penny and
large bowling ball. The proton is
obviously the bowling ball and the
electron is represented by the penny.

How do atoms become “charged ?”How do atoms become “charged ?”
- Atoms become charged when they- Atoms become charged when they
become more positive or more negative.become more positive or more negative.
- How can this happen?- How can this happen?
- Remove or add a proton or an electr
- Protons and neutrons are
bound together
by the Strong Nuclear Force
and it is very
- Electrons, however, can be more

IonsIons
An atom
with a
deficiency of
electrons is
positively
charged.
An atom with
an excess of
electrons is
negatively
charged.
OMS DO NOT GAIN OR LOSE PROTONS

Charge is a fundamental quality likeCharge is a fundamental quality like
mass.mass.
- Charge is denoted as q.
- Charge has a fundamental unit of a Coulomb
(C).
- Charges are usually really really small numbers
(10-
).
- So what is 1 C?
An object would have to have 6.25 x 1018
extra
electrons to amount to –1 C of charge.
A lightning bolt is estimated to carry a charge
of 10 C.

Charges can ONLY be in multiples ofCharges can ONLY be in multiples of ee
Remember:
• -e = an electron = -1.60 x 10-19
C
• +e = a proton = +1.60 x 10-19
C
An object that has a net charge of 8.0 x 10-19
C has
a net charge of what multiple of e? Hint: How
many electrons would need to be removed to
create this charge?
et charge would be +5e, 5 electrons were rem

Multiples of Charges ChartMultiples of Charges Chart
1e 1.6 x 10-
19
2e 3.2 x 10-
19
3e 4.8 x 10-
19
4e 6.4 x 10-
19

Electrostatic ForceElectrostatic Force
- This is a non-contact force (like the
gravitational force except instead of two
masses exerting force on each other the
two objects charges exert a force of
repulsion or attraction).
- ANY charged object can exert the
electrostatic force upon other objects-
both charged and uncharged objects.

Coulomb’s Law -Coulomb’s Law - formula forformula for
electrostatic forceelectrostatic force
Again this is similar
o the gravitational force…
Fg = GmM
r2
charge (q) is now
responsible for the force
Fe = kq1q2
r2
Just like G was a constant so is k.Just like G was a constant so is k.
k is thek is the electrostatic constant and =and =
9 2 2

this…the relationship between the gravitational force and th
from the object…this is the inverse square law
2x 3x 4x
1/4 = 2.24 1/9 = 1.09 1/16 = 0.61
1x
9.81 m/s2
FFgg
rr22
Fg = GmM
r2

Try this…Try this…
The distance between a proton
and an electron in a hydrogen atom
is 5.3 x 10-11
m. Find both the
gravitational force and the
electrostatic force between the two
particles.

ANY charged object, whetherANY charged object, whether
positively charged or negativelypositively charged or negatively
charged, will have ancharged, will have an
ATTRACTIVE interaction with aATTRACTIVE interaction with a
neutral object.neutral object.
A balloon when rubbed on your
head becomes charged by picking
up extra electrons from your hair.
That same balloon, because it is
charged, will attract a neutral
object like pieces of paper.
-
+
-

So we are able to predict theSo we are able to predict the
charge on objects based on theircharge on objects based on their
interaction with other objects.interaction with other objects.
They can either bothThey can either both
be positive or bothbe positive or both
be negative.be negative.
They can haveThey can have
opposite charges oropposite charges or
one object is chargedone object is charged
and the other isand the other is

- On two occasions, the following charge
interactions between balloons A, B and C are
observed. In each case, it is known that balloon
B is charged negatively. Based on these
observations, what can you conclude about the
charge on balloon A and C for each situation.
positive or neutral
positive
positive (if it was neutral it wouldn’t rep
positive

Why does the balloon stick to theWhy does the balloon stick to the
wall?wall?
- When a balloon is
rubbed with a piece of
cloth electrons are
transferred between
the two objects.
Usually the balloon
attracts extra
electrons and then

When the balloon is placed against the wall
the excess electrons will repel the electrons
in the wall and be attracted to the positive

What happens to your hair when you rubWhat happens to your hair when you rub
a balloon on your head ?a balloon on your head ?
• The balloon, after
being rubbed and
then pulled away,
removes some of the
electrons in your hair
which give each
strand a positive
charge. Like charges
want to repel and
each strand is
repelling from the

Getting ShockedGetting Shocked
- As you walk across a carpet, electrons are
transferred from the rug to you.
- Now you have extra electrons.
- Touch a door knob (conductor) and ZAP!
- The electrons move from you to the knob.

LightningLightning
- Lightning is a- Lightning is a
REALLY big shock.REALLY big shock.
- Positive charges- Positive charges
tend to go up,tend to go up,
negative charges tendnegative charges tend
to go down.to go down.
- When the attraction- When the attraction
reaches a critical levelreaches a critical level
you get a lightningyou get a lightning
bolt.bolt.

Objects that tend toObjects that tend to give up electronsgive up electrons
and becomeand become positivepositive ::
- Glass- Glass
- Nylon- Nylon
- Fur- Fur
- Hair- Hair
- Wool- Wool

Objects that tend toObjects that tend to attract electronsattract electrons andand
becomebecome negativenegative ::
- Rubber- Rubber
- Polyester- Polyester
- Styrofoam- Styrofoam
- Saran- Saran
WrapWrap
- PVC- PVC

Insulators and ConductorsInsulators and Conductors
- Different materials hold electrons- Different materials hold electrons
differently.differently.
- Insulators- Insulators don’t let electrons movedon’t let electrons move
around within the material freely.around within the material freely.
Ex. Cloth, Plastic, Glass, Dry Air,Ex. Cloth, Plastic, Glass, Dry Air,
Wood, RubberWood, Rubber
- Conductors- Conductors do let electrons movedo let electrons move
around within the material freely.around within the material freely.

• A charged plastic rod is broughtA charged plastic rod is brought
close a neutral metal sphere. Howclose a neutral metal sphere. How
would the distribution of charges bewould the distribution of charges be
in the metal sphere?in the metal sphere?

• Which of the diagrams below best represents the chargeWhich of the diagrams below best represents the charge
distribution on a metal sphere when a positively chargeddistribution on a metal sphere when a positively charged
plastic tube is placed nearby?plastic tube is placed nearby?

Law of Conservation of ChargeLaw of Conservation of Charge
• Charges within a closed system may be transferred from oneCharges within a closed system may be transferred from one
object to another, but charge is neither created nor destroyed.object to another, but charge is neither created nor destroyed.

The diagram below shows the initial charges andThe diagram below shows the initial charges and
positions of three metal spheres, R, S, and T, on insulatingpositions of three metal spheres, R, S, and T, on insulating
stands.stands.
0e -8e +6e
R S T
Sphere R is brought into contact with sphere S and then removed. ThenSphere R is brought into contact with sphere S and then removed. Then
sphere S is brought into contact with sphere T and removed.sphere S is brought into contact with sphere T and removed.
What is the charge on sphere T after this procedure is completed ?What is the charge on sphere T after this procedure is completed ?
Note that the net
charge of the
system is -2e.

0e -8e
R S
When the spheres come in contact the charge will beWhen the spheres come in contact the charge will be
distributed evenly between both spheres.distributed evenly between both spheres.
-4e -4e +6e
T

0e -8e
S T
-4e +6e-4e
R
-4-4ee + 6+ 6ee ==
22
+2+2ee ==
22
++ee
+e +e

Note : that the charge of the system is conservedNote : that the charge of the system is conserved
- the initial charge is the same as the final charge.- the initial charge is the same as the final charge.
-4e +e +e
R S T
-4-4ee ++ ee ++ ee = -2= -2ee

Electroscopes - instruments used toElectroscopes - instruments used to
detect chargedetect charge
The yellow arms or leaves on both instruments willThe yellow arms or leaves on both instruments will
move to show the charge.move to show the charge.

Charged Electroscope
Excess of the same charge in both leavesExcess of the same charge in both leaves
causes them to diverge/repelcauses them to diverge/repel
+ +-
+-
+-
+
+
+-
+
Uncharged Electroscope
Leaves are neutral so they are notLeaves are neutral so they are not
diverging/repelling or converging/attracting.diverging/repelling or converging/attracting.
+-
+- +-
+-
+- +-

Steps in charging an electroscope bySteps in charging an electroscope by
INDUCTIONINDUCTION ::
1. Uncharged electroscope:1. Uncharged electroscope:
+-
+-
+-
+-
+-
+-
+-
+-
+-
Leaves are justLeaves are just
hanging straight down.hanging straight down.
Net charge is zero.Net charge is zero.

2. A negatively charged rod is brought near the electroscope:2. A negatively charged rod is brought near the electroscope:
+
+
+
+
+-
+-
+-
+-
+-
+-
-- -- --- ----- -- --
-- -- --- -- ----- - -
-
-
- Net charge is zeroNet charge is zero

-- -- --- -- ----- --
-- -- --- -- ----- -
3. Electrons move to the leaves:3. Electrons move to the leaves:
+
+
+
+
+-
+-
+-
-+
-+
-+
Net charge is still zeroNet charge is still zero
-
-
-
-

-- -- --- -- ----- --
-- -- --- -- ----- -
4. Leaves diverge:4. Leaves diverge:
+
+
+
+
+-
-+-
-+-
-+
-+-
-+-
Net charge of zeroNet charge of zero
+-
-+-
-+-
-+
-+-
-+-

-- -- --- -- ----- --
-- -- --- -- ----- -
5. The electroscope is grounded:5. The electroscope is grounded:
+
+
+
+
+-
-+-
-+-
-+
-+-
-+-
GroundingGrounding is theis the
process ofprocess of removingremoving
the excess charge onthe excess charge on
an object by meansan object by means
of the transfer ofof the transfer of
electrons between itelectrons between it
and another objectand another object
of substantial size.of substantial size.

-- -- --- -- ----- --
-- -- --- -- ----- -
6. Electrons go to the ground:6. Electrons go to the ground:
+
+
+
+
+-
+-
+-
-+
-+
-+
-- -- Net charge isNet charge is
now positivenow positive

-- -- --- -- ----- --
-- -- --- -- ----- -
7. Leaves converge and ground is removed:7. Leaves converge and ground is removed:
+
+
+
+
+-
+-
+-
-+
-+
-+
+-
+-
+-
-+
-+
-+

-- -- --- -- ----- --
-- -- --- -- ----- - +
+
+
+
+-
+-
+-
-+
-+
-+
8. Negatively charged rod is removed:8. Negatively charged rod is removed:

+
+
+
+
+
+-
+-
+
-+
-+
9. Electrons redistribute throughout the electroscope and9. Electrons redistribute throughout the electroscope and
move up toward the top:move up toward the top:
--

+
+
+
+
+
+-
+-
+
-+
-+
-
-
10. Leaves now have the same charge and diverge:10. Leaves now have the same charge and diverge:
+
+-
+-
+
-+
-+

Field Near a Negative ChargeField Near a Negative Charge
Note that the fieldNote that the field EE in the vicinity of ain the vicinity of a negativenegative
chargecharge –Q–Q isis towardtoward the charge—the direction that athe charge—the direction that a
+q+q test charge would move.test charge would move.
Force onForce on +q+q is withis with
field direction.field direction.
Force onForce on -q-q isis
against fieldagainst field
direction.direction.
E
Electric Field
.
r
++q
F
--
-- -
-
---Q
E
Electric Field
.
r
--q
F
--
-- -
-
---Q

The Magnitude of E-FieldThe Magnitude of E-Field
TheThe magnitudemagnitude of the electric field intensity at aof the electric field intensity at a
point in space is defined as thepoint in space is defined as the force per unitforce per unit
chargecharge (N/C)(N/C) that would be experienced by anythat would be experienced by any
test charge placed at that point.test charge placed at that point.
Electric Field
Intensity E
Electric Field
Intensity E
N
; Units
C
F
E
q
 
=  
 
TheThe directiondirection ofof EE at a point is the same as theat a point is the same as the
direction that adirection that a positivepositive charge would movecharge would move IFIF
placed at that point.placed at that point.

+
+
++
+
+
+
+Q
.
r
P
The E-Field at a distance r from aThe E-Field at a distance r from a
single charge Qsingle charge Q
++
++
+
+
++Q
.
r
P
Consider a test chargeConsider a test charge +q+q placedplaced
atat PP a distancea distance rr fromfrom QQ..
The outward force on +q is:The outward force on +q is:
The electric fieldThe electric field EE is therefore:is therefore:
2
F kQq r
E
q q
= = 2
kQ
E
r
=
++q
F
2
kQq
F
r
=
E
2
kQ
E
r
=

The Resultant Electric Field.The Resultant Electric Field.
The resultant fieldThe resultant field EE in the vicinity of a numberin the vicinity of a number
of point charges is equal to theof point charges is equal to the vector sumvector sum of theof the
fields due to each charge taken individually.fields due to each charge taken individually.
Consider E for each charge.Consider E for each charge.
+
- •q1
q2
q3
-
A
E1
E3
E2
ER
Vector Sum:
E = E1 + E2 + E3
Vector Sum:
E = E1 + E2 + E3
Directions are based
on positive test charge.
Magnitudes are from:
2
kQ
E
r
=

Electric Field LinesElectric Field Lines
++
++
+
+
++Q
--
-- -
-
---Q
Electric Field LinesElectric Field Lines are imaginary lines drawn inare imaginary lines drawn in
such a way that their direction at any point is thesuch a way that their direction at any point is the
same as the direction of the field at that point.same as the direction of the field at that point.
Field lines goField lines go awayaway fromfrom positivepositive charges andcharges and
towardtoward negativenegative charges.charges.

1. The direction of the field line at any point is1. The direction of the field line at any point is
the same as motion of +q at that point.the same as motion of +q at that point.
2. The spacing of the lines must be such that they2. The spacing of the lines must be such that they
are close together where the field is strong andare close together where the field is strong and
far apart where the field is weak.far apart where the field is weak.
+ -qq11 qq22
EE11
EE22
EERR
Rules for Drawing Field LinesRules for Drawing Field LinesRules for Drawing Field LinesRules for Drawing Field Lines

Examples of E-Field LinesExamples of E-Field Lines
Two equal butTwo equal but
oppositeopposite charges.charges.
TwoTwo identicalidentical
charges (both +).charges (both +).
Notice that linesNotice that lines leave +leave + charges andcharges and enterenter -- charges.charges.
Also,Also, EE isis strongeststrongest where field lines arewhere field lines are most densemost dense..

Summary of FormulasSummary of Formulas
The Electric Field
Intensity E:
The Electric Field
Intensity E: 2
N
Units are
C
F kQ
E
q r
= =
The Electric Field
Near several charges:
The Electric Field
Near several charges: 2
Vector Sum
kQ
E
r
= ∑

Work to Move a ChargeWork to Move a Charge
Work to move
+q from A to B.
++
++
+
+
++Q
∞
q
E
F +
••
ΑΒ
ra
rb
2a
a
kqQ
F
r
=
avg
a b
kqQ
F
r r
=
At A:
At B:
Avg. Force:
2b
b
kqQ
F
r
=
Distance: ra - rb
( )a b
a b
kQq
Work Fd r r
r r
= = −
1 1
b a
Work kQq
r r
 
= − 
 

Absolute Potential EnergyAbsolute Potential Energy
++
++
+
+
++Q
∞
q
E
F +
••
ΑΒ
ra
rb
1 1
b a
Work kQq
r r
 
= − 
 
Absolute P.E. isAbsolute P.E. is
relative torelative to ∞.
It is work to bringIt is work to bring
+q+q from infinity tofrom infinity to
point nearpoint near QQ—i.e.,—i.e.,
fromfrom ∞∞ to rto rbb
1 1
b b
kQq
Work kQq
r r
 
= − = 
∞ 
Absolute Potential
Energy:
kQq
U
r
=
0

Properties of SpaceProperties of Space
An electric field is a property of
space allowing prediction of the
force on a charge at that point.
;
F
E F qE
q
= =
The field E exist independently of
the charge q and is found from:
2
:
kQ
Electric Field E
r
=
E
Electric Field
++
++
+
+
++Q
r
E is a Vector

Electric PotentialElectric Potential
Potential
++
++
+
+
++Q
.
r
Electric potentialElectric potential is another property ofis another property of
space allowing us to predict the P.E. ofspace allowing us to predict the P.E. of
anyany charge q at a point.charge q at a point.
U
V
q
=
;
U
V U qV
q
= =
ElectricElectric
Potential:Potential:
The units are: joules per coulombjoules per coulomb (J/C)(J/C)
For example, if the potential isFor example, if the potential is 400 J/C400 J/C at pointat point PP,,
aa –2 nC–2 nC charge at that point would have P.E. :charge at that point would have P.E. :
U = qV = (-2 x 10-9
C)(400 J/C); U = -800 nJU = -800 nJ
P

The SI Unit of Potential (Volt)The SI Unit of Potential (Volt)
From the definition of electric potential asFrom the definition of electric potential as P.E. perP.E. per
unit chargeunit charge, we see that the unit must be, we see that the unit must be J/CJ/C. We. We
redefine this unit as theredefine this unit as the volt (V).volt (V).
1 joule
; 1volt =
1 coulomb
U
V
q
 
=  
 
A potential of one volt at a given point means that
a charge of one coulomb placed at that point will
experience a potential energy of one joule.
A potential of one volt at a given point means that
a charge of one coulomb placed at that point will
experience a potential energy of one joule.

Calculating Electric PotentialCalculating Electric Potential
Potential
++
++
+
+
++Q
.
r
kQ
V
r
=P
Electric Potential Energy andElectric Potential Energy and
Potential:Potential:
;
kQq U
U V
r q
= =
Substituting forSubstituting for
U, we find V:U, we find V:
( )kQq
r kQ
V
q r
= =
kQ
V
r
=
The potential due to a positive charge is
positive; The potential due to a negative
charge is positive. (Use sign of charge.)
The potential due to a positive charge is
positive; The potential due to a negative
charge is positive. (Use sign of charge.)

Potential DifferencePotential Difference
The potential difference between two points A and B
is the work per unit positive charge done by electric
forces in moving a small test charge from the point of
higher potential to the point of lower potential.
The potential difference between two points A and B
is the work per unit positive charge done by electric
forces in moving a small test charge from the point of
higher potential to the point of lower potential.
Potential Difference: VAB = VA - VB
Potential Difference: VAB = VA - VB
WorkAB = q(VA – VB) Work BY E-fieldWorkAB = q(VA – VB) Work BY E-field
The positive and negative signs of the charges may
be used mathematically to give appropriate signs.
The positive and negative signs of the charges may
be used mathematically to give appropriate signs.

Parallel PlatesParallel Plates
VA + + + +
- - - -VB
E+q
F = qE
Consider Two parallel plates of equalConsider Two parallel plates of equal
and opposite charge, a distanceand opposite charge, a distance dd apart.apart.
Constant E field: F = qEConstant E field: F = qE
Work =Work = FdFd = (qE)d= (qE)d
Also, Work =Also, Work = q(Vq(VAA – V– VBB))
So that:So that: qVqVABAB = qEd= qEd andand VAB = EdVAB = Ed
The potential difference between two oppositely
charged parallel plates is the product of E and d.
The potential difference between two oppositely
charged parallel plates is the product of E and d.

Summary of FormulasSummary of Formulas
;
kQq U
U V
r q
= =
kQ
V
r
= ∑
WorkAB = q(VA – VB) Work BY E-fieldWorkAB = q(VA – VB) Work BY E-field
;
V
V Ed E
d
= =
Electric Potential
Energy and Potential
Electric Potential
Energy and Potential
Electric Potential Near
Multiple charges:
Electric Potential Near
Multiple charges:
Oppositely Charged
Parallel Plates:
Oppositely Charged
Parallel Plates:

Made byMade by
• 1.Miss.Natnicha puemsuwan1.Miss.Natnicha puemsuwan No.24No.24
• 2.Miss.Thidarat Nuduang2.Miss.Thidarat Nuduang No.27No.27
• 3.Miss.Pornlada Chaijaroen3.Miss.Pornlada Chaijaroen No.31No.31
• 4.Miss.Laksanaporn Saleeon4.Miss.Laksanaporn Saleeon No.34No.34
• 5.Miss.Arthima Phaokong5.Miss.Arthima Phaokong No.38No.38

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