Download Charge - Ms. Gamm

It’s what we see…
Static Electricity
Charges
•Rubbing plastic or glass makes them
attractive to bits of paper or dust
•Benjamin Franklin decided to call the
charge on rubbed glass positive
Top & Bottom
Charges
•Objects charged the
same way will repel each
other
•Oppositely charged
objects attract each other
Charges
•All charged objects either
attract or repel each other
•This implies 2 types of
charges
Check yourself
Two small spheres hung on
threads attract one another as
shown. It is known that a
positively-charged rod will attract
ball A.
I. Ball A has a positive
charge.
II. Ball B has a negative
charge.
III. Balls A & B have opposite
Electricity
•One of the first objects to hold this charge
was amber
•The Greek word for amber is elektron
o Electricity
o Electron
Charge
•To understand Charge, let’s look at the atom
•Protons (nucleus) are
positively charged
•(repels charged glass)
•The electrons are
negatively charged
•(attracts charged glass)
Charge
•Even though protons are much bigger (2,000 x’s
bigger than electron), the – and + charges are
the same size.
•If an atom has equal number of electrons and
protons the charge is zero.
____?
•3 electrons (blue)
•3 protons (red)
•Net charge is zero
(neutral charge)
Charge Rules
•In order to be charged, an object must have extra
protons or electrons
•The following facts help understand charges:
1. In solids, positive charges do not move
protons are fixed in place
• But… Negative charges can move
Conductors
• Conductors: electrons move throughout the
material
• Because like charges repel the electrons and
protons will be as evenly spaced
• Conductors can be charged easily
Charge Rules
• Insulators: electrons mostly move around their
atom
• It is very difficult to charge an insulator because
insulators do not allow charges to transfer easily.
Examples of Conductors &
Insulators
Conductors
• Metals
Insulators
• Rubber
(copper, silver,
• Plastic
iron, gold)
• Glass
• Human bodies • Ceramics
Charge Rules
2. In fluids, both
protons and
electrons can
move all around
is wrong
• What
Protons
repel
• Charges
need
to
be
with
this
picture?
protons!
distributed evenly
the
• throughout
Electrons repel
liquid
electrons!
Charge Rules
3. Objects want to be neutral.
• If given a path to the earth, electrons will flow
out until the object is neutral.
• This is called grounding.
v
Brain Break!
Simpsons & Electrostatics
Charge by Rubbing
•Every atom attracts electrons (e-) a certain amount
•When you rub two objects together the object that
attracts electrons more will steal e-
+
−
Charge by Rubbing
•This stealer becomes negative (more e-) and the
other object becomes positive (lost e-)
What will happen to the charge when the bars
separate?
Transferring Charge
•This stealer becomes negative (more e-) and the
other object becomes positive (lost e-)
•Only works if there is contact
Conduction
① A charged conductor touches a neutral conductor
Total Charge = protons - electrons
Q1 = 3 protons - 7 electrons = -4 charge
Q1 = -4
Q2 = 4 protons - 4 electrons = 0 charge
Q2 = 0
Conduction
① A charged conductor touches a neutral conductor
Q1 = ?
Q2 = ?
Conduction
① A charged conductor touches a neutral conductor
② The electrons spread out so that the charges on
both are equal
Q1 = Q2 =
Q1 = -2
Q1 + Q2
2
Q2 = -2
Conduction
① A charged conductor touches a neutral conductor
② The electrons spread out so that the charges on
both are equal
Q1 = -2
Q2 = -2
What was wrong with this movie?
Induced Polarity
•A neutral balloon has no
effect on wall’s e•How can charged objects
attract neutral insulators?
•They induce a charge on
the neutral object.
Induced Polarity
•Let’s charge the balloon!
•Now the e- in the wall are
being repelled
•This leaves the exposed
wall acting positive
•Negative and positive
charges attract each other
Induction = Induced Polarity + grounding
• You charge an object without touching it with
another charged object (conduction)?
1. Induce polarity with a charged object
2. Ground the object
3. Remove the ground
Induced Induction!
polarization
Grounding
Visualizing Electric Charge pg 183
Neutral
Draw 3 units of charge
(protons & electrons)
In each bar
Induced Polarization pg 183
6 units of charge
(protons or electrons)
Q1 = 6
9 protons &
9 electrons total
Q2 = 0
Conduction pg 184
Q1 = 6+3
Initially Neutral
Q22 = 0
+3
What can happen
when charged
objects touch?
Electrons can transfer!
Charge has to equally distribute
Charging by Induction pg 185
Protons attract Electrons
9 protons &
9 electrons
total should
How many
electrons
enter from the ground?
The image below is an example of:
A.
B.
C.
Conduction
A.
Induction
B.
Induced Polarization
Charging by rubbing
Grounding
The image below is an example of:
A.
B.
C.
Conduction
A.
Induction
B.
Induced Polarization
Charging by rubbing
Grounding
The image below is an example of:
A.
B.
C.
Conduction
A.
Induction
B.
Induced Polarization
Charging by rubbing
Grounding
The image below is an example of:
A.
B.
C.
Conduction
A.
Induction
B.
Induced Polarization
Charging by rubbing
Grounding
The image below is an example of:
A.
B.
C.
Conduction
A.
Induction
B.
Induced Polarization
Charging by rubbing
Grounding
Induced Induction!
polarization
Grounding
Electrostatic Force
•A Force is a push or a pull
•Electric Force is 1039 times larger than gravity
•Coulomb studied charge with charged metal balls
• Let’s look at how the
amount of charge on
each object and the
distance effect the
electrostatic force
Coulomb’s Law
•Electrical force is proportional (
FE µ Q1Q2
• Particles with more
charge (±q)
experience a greater
force.
1
FE µ 2
d
• Charges separated by a
smaller distance experience
a larger force.
µ) to both
Coulomb’s Law
•These lead to one equation called Coulomb’s Law:
kQ1Q2
FE =
2
d
FE ºElectric Force in Newtons (N)
Q1,Q2 ºcharge in Coulombs (C)
d º distance between centers
in meters (m)
k º 9.0 ´10
9 N× m 2
C2
Coulomb’s Law
•A Newton is about equal to ¼ pound
•A Coulomb is the amount of charge in 6.25 x 1018
electrons or protons
kQ1Q2
FE =
2
d
Example 1
Two charged spheres, one with 1x1012
extra electrons and one with 2x1012 extra
protons are separated by 50 cm. What
force do they exert on each other?
Given:
Q1 = -1´10 electrons (
12
Q2 = 2 ´10 protons
12
d = 50cm
Want:
(
1m
100 cm
FE
)=
(
1 Coulomb
6.25´1018 electrons
1 Coulomb
6.25´1018 protons
0.50 m
) = -1.6 ´10
) = 3.2 ´10
-7
-7
C
C
Example 1
solve for FE
kQ1Q2
FE =
2
d
FE =
(9 ´10
9 N ×m 2
C2
-7
-7
)(-1.67 ´10 C)(3.2 ´10 C)
(0.50 m)
2
FE = - 0.0019 N
- force is attractive
+ force would be repulsive!
Example 2
By what factor does the Electric Force change by
if the distance between 2 charged spheres is
doubled?
1
Given:
+q
d = is doubled
+q
d
+q
2d
+q
Unknown:
how does FE change?
FE µ
d
2
1
1 1
FE final µ
=
2
2
(2d) 4 d
1
FE final = FE
4
1
FE final will be of the original
4
Time to Check Yourself!
pg. 188 #1-6
Skip #9
(unless you want extra credit)
Answers
1.
2.
3.
4.
5.
6.
A
C
A
C
E
A
7. B
8. A
9. D
10.B
5. When two small charged spheres are separated by
2.0 m, the electric force of attraction between them is
6.0 N. If the charge on each sphere is doubled and
the separation is reduced to 1.0m, the force of
attraction will now be:
qq
Given:
d = is halved
q1 & q2 = doubled
FE = 6N
Unknown:
how does FE change?
FE = 6N µ
1 2
2
d
2q1 2q2 4q1q2
=
FE final µ
2
2
1
1
d
( d)
4
2
4
FE final =
FE =16FE
1
4
FE final will be16 times FE
Start Homework!
#1-4 pg. 190
Hint: #4)μC = 10-6 C
Discussion Question 1
Two identical charges Q, separated by a
distance R, feel an electric force F. By
what factor does the force change if
both charges are doubled (2Q) and the
distance is doubled (2d)?
(A) 2F
(D) F/2
(B) 4F
(E) F/4
(C) F
The Electric Field
•All charged objects produce electric fields
•We know that a positive charge will repel another
positive charge and attract a negative one
•If we draw lines to represent this we would be
drawing an electric field
The Electric Field
Let’s look at the rules for drawing field lines:
① Field lines always point in the direction that a
positive test charge would move
The Electric Field
② Field lines always intersect charged objects at
right angles
The Electric Field
② Field lines always intersect charged objects at
right angles
The Electric Field
③ Field lines never intersect each other
The Electric Field
④ The closer that field lines are to each other, the
greater the electric field
The Electric Field
Example:
•Draw the electric Field Lines
Time to practice
Go to pg. 253
Electric Field in Conductors
•Imagine a charged circle
•Where can charges move to get farthest away?
Electric Field in Conductors
•Imagine a charged circle
•Where can charges move to get farthest away?
•To the edges!
•The excess charge on a conductor will always be
on the surface
Electric Field in Conductors
•How would the field effect a lone charge in the
middle?
Discussion Question
Why do you think these warnings are on gas
pumps?
60
60
Electric Potential
•Which is worse, 120 volts
or 25,000 volts?
•It depends….
•Energy causes pain… not voltage
•Voltage is potential energy per charge
E
so
V=
q
E = Vq
1 volt = 1 Joule of energy per 1 Coulomb of charge
1 Joule
1 volt =
Coulomb
Electric Potential = Voltage
•Basic outlet is 120 V
•The flow of an outlet is about 20
Coulombs/sec
•The energy released in 1 sec
would be:
E = Vq
E = (120V)(20C) = 2,400 J
Amount of energy to lift 75 gal of water 1 m high!
Electric Potential
•Van de Graff generator carries up to 400,000 V!
•But, it only flows 10 μCoulombs/sec
•The energy released in 1 sec would be:
E = Vq
5 J
-6
E = (4 ´10 C )(10 ´10 C) = 4 J
Amount of energy to lift 2 cups of water 1 m high
Time to practice
Go to pg. 200