10
9
8
7
6
5
4
3
2
S31
1
S21
33
29
25
17
21
S11
13
9
Electrostatic Forces
5
1
0
S1
10
8
6
4
2
0
-2
-4
-6
S31
-8
S21
6
21
S11
16
11
6
1
-10
S1
Investigation #1 (p792-794)
What do you see in
the cartoon?
Turn and tell your
partner 2 things
Share out
Record in your
notebook
What do you think
caused the shock?
Record answer in your
notebook
p147
Investigation #1 (p792-794)
The study of lightning,
shocks, and static cling
can reveal important
physics
•Complete steps 1-9
•Get your notebook
stamped when you
finish today
P147
“Static” Electricity
When you comb your hair and…
… bring your comb over a pile of paper bits
http://nsdl.org
“Static” Electricity
What will happen?
A.
B.
Why did this happen?
http://nsdl.org
“Static” Electricity
The force felt by the paper bits is due to a
difference in charge on the comb
compared to the paper.
This “force of attraction” was first observed
by the Greeks who found that piece of
amber (“elektron”) attracted other objects
when rubbed.
http://nsdl.org
Match the diagram below with its correct
description.
1.
2.
3.
Balanced atom
Net Charge
Zero
Deficiency of
Electrons
Net Charge
Positive
A.
B.
http://nsdl.org
Excess of
Electrons
Net Charge
Negative
C.
“Static” Electricity
Usually charges balance each other out, and
nothing happens. But when two objects with like
charges (all positive or all negative) come together,
the charges repel and the objects move away from
each other.
Objects with opposite charges attract each other
because the different charges want to balance
each other.
Objects can get a negative charge by picking up
electrons from other objects.
http://nsdl.org
Common Misconceptions about
“Static” Electricity
Actually, the thing we call static electricity is an
imbalance in the amounts of positive and
negative charges found on the surface of an
object.
http://nsdl.org
Common Misconceptions about
“Static” Electricity
Lightning is like static electricity, except on a much
bigger scale. Both lightning and static electricity
happen because of the attraction between the
opposite charges.
http://nsdl.org
Electric charge in cloud
Possible lightning paths
Charge and image
The “Leader”
Main Stroke
Lightning
striking
tree
Lightning
striking
Empire
State
Building
Shoes of man struck by lightning
Man not hurt!!
Self Check?
True
False
True
False
Charges flow from less
negative to more
negative areas.
Like charges repel.
http://nsdl.org
Concept Check - Electrostatics
Two charged balls are repelling each other as they hang from the
ceiling. What can you say about their charges?
1. one is positive, the other is negative
2. both are positive
3. both are negative
4. both are positive or both are negative
Concept Check - Electrostatics
Two charged balls are repelling each other as they hang from the
ceiling. What can you say about their charges?
1. one is positive, the other is negative
2. both are positive
3. both are negative
4. both are positive or both are negative
The fact that the balls repel each other only can tell you that they
have the same charge, but you do not know the sign. So they can
be either both positive or both negative.
Concept Check - Electrostatics
From the picture, what can you conclude about the charges?
1.
have opposite charges
2.
have the same charge
3.
all have the same charge
4. one ball must be neutral (no charge)
Concept Check - Electrostatics
From the picture, what can you conclude about the charges?
1.
have opposite charges
2.
have the same charge
3.
all have the same charge
4. one ball must be neutral (no charge)
The PERIWINKLE and BLACK balls must have the same charge,
since they repel each other. The RED ball also repels the
PERIWINKLE , so it must also have the same charge as the
PERIWINKLE (and the BLACK).
Concept Checks – Conductors
A metal ball hangs from the ceiling by an insulating thread. The ball is
attracted to a positive-charged rod held near the ball. The charge of
the ball must be:
1.
2.
3.
4.
5.
positive
negative
neutral
positive or neutral
negative or neutral
Concept Checks – Conductors
A metal ball hangs from the ceiling by an insulating thread. The ball is
attracted to a positive-charged rod held near the ball. The charge of
the ball must be:
1.
2.
3.
4.
5.
positive
negative
neutral
positive or neutral
negative or neutral
remember
the ball is a
conductor!
Clearly, the ball will be attracted if its charge is negative.
However, even if the ball is neutral, the charges in the ball can be
separated by induction (polarization), leading to a net attraction.
Electromagnetic Charge
Positive and Negative Charge
Positive
(+)
Air
Rubber balloon
Human Hands
Sulphur
Asbestos
Hard rubber
Rabbit's Fur
Nickel, Copper
Glass
Brass, Silver
Human Hair
Gold, Platinum
Mica
Sulfur
Nylon
Acetate, Rayon
Wool
Polyester
Lead
Celluloid
Cat's Fur
Polyurethane
Silk
Polyethylene
Aluminum
Polypropylene
Paper
Vinyl
Cotton
Silicon
Steel
Teflon
Wood
Saran Wrap
Lucite
Sealing wax
Amber
Polystyrene
Polyethylene
Negative
(-)
Insulators and Conductors
Conductors are materials that electric charges
move easily and spread out evenly
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+
+ + ++
++
+
+
++
+
+ ++
+
+
Conductor
Nonconductor
Insulators are materials that electric charges
can not move through
Conductors (Q2 charged by direct contact)
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+
+
+
Q
+
+
+ +
+
+
+
+
+
+
+ +
+
+
+
+
+
+
+
+
Q/2
+ +
+
+
+
+
+
+
+ +
+
+
+
Q/2
+
+
+
Insulators (not charged by induction
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+
+
+
Q
+
+ +
+
+
+
+
+
+
+
Q/2
+ +
+
+
+
+
+
+
Q/2
Conductors (Q2 charged by induction)
+
+
+
+
+
+
+
+
+
+
+
+
+
+ +
+
+
+
+
+
+
+ +
+
Q
+
+ +
+
+
+
+
+
+
+
+ ++
+
+ ++
+
+
+
Q/2
+ +
+
+
+
+
+
+
+ +
+
+
+
Q/2
+
+
+
Methods of Charging
Direct contact
Results in a transfer
of charges
http://nsdl.org
Induction
Usually results in a
temporary
rearrangement of
charges
Concept Check – Coulomb’s Law
What is the magnitude of the force F2?
1.
2.
3.
4.
5.
1.0 N
1.5 N
2.0 N
3.0 N
6.0 N
F1 = 3N
Q
Q
F2 = ?
Concept Check – Coulomb’s Law
What is the magnitude of the force F2?
1.
2.
3.
4.
5.
1.0 N
1.5 N
2.0 N
3.0 N
6.0 N
F1 = 3N
Q
Q
F2 = ?
The force F2 must have the same magnitude as F1. This is due to
the fact that the form of Coulomb’s Law is totally symmetric with
respect to the two charges involved. The force of one on the other
of a pair is the same as the reverse. Note that this sounds
suspiciously like Newton’s 3rd Law!!
Concept Check – Electric Force
Two uniformly charged spheres are firmly fastened to and electrically
insulated from frictionless pucks on an air table. The charge on sphere
2 is three times the charge on sphere 1. Which force diagram correctly
shows the magnitude and direction of the electrostatic forces:
Concept Check – Electric Force
Two uniformly charged spheres are firmly fastened to and electrically
insulated from frictionless pucks on an air table. The charge on sphere
2 is three times the charge on sphere 1. Which force diagram correctly
shows the magnitude and direction of the electrostatic forces:
Concept Check – Coulomb’s Law (2)
If we increase one charge to 4Q, what is the magnitude of F1?
1.
2.
3.
4.
5.
3/4 N
3.0 N
12 N
16 N
48 N
F1 = 3N
F1 = ?
Q
4Q
Q
Q
F2 = ?
F2 = ?
Concept Check – Coulomb’s Law (2)
If we increase one charge to 4Q, what is the magnitude of F1?
1.
2.
3.
4.
5.
3/4 N
3.0 N
12 N
16 N
48 N
F1 = 3N
F1 = ?
Q
4Q
q1q2
F1  K 2  3N
Coulomb’s Law:
r
4q1  q2

q1q2
 4K 2 =4F1=12N
Now we have: F1  K
2
r
r
which is 4 times bigger than before.
Q
Q
F2 = ?
F2 = ?
Concept Check – Coulomb’s Law (3)
The force between two charges separated by a distance r is F. If the
charges are pulled apart to a distance 3r, what is the force on each
charge?
F
F
Q
Q
1. 9 F
2. 3 F
3. F
r
4. 1/3 F
?
?
5. 1/9 F
Q
Q
3r
Concept Check – Coulomb’s Law (3)
The force between two charges separated by a distance r is F. If the
charges are pulled apart to a distance 3r, what is the force on each
charge?
F
F
Q
Q
1. 9 F
2. 3 F
3. F
r
4. 1/3 F
F/9
F/9
5. 1/9 F
Q
Q
3r
q1q2
r2
q1q2
q1q2 1

K 2 = F
Now we have: F  K
2
9r
9
 3r 
which is 1/9 as big as before.
Coulomb’s Law:
F K
TWO kinds of electric energy
Static Electricity
Current Electricity
A few thousand volts
Van de Graaff
Born 1901
Invented static
electricity generator in
1929
This is the generator we
will use today
Here is a
bigger
Van de
Graaff
generator
An even bigger one!
A giant Van de Graaff generator
The biggest--25 Million Volts
Oak Ridge National Lab in Tennessee
Electrons jump off
the belt at the top
p150
Electrons jump
onto the belt at the
bottom
One of the properties of matter
Where the electrons go when two
things are rubbed together
Why does
your hair
do this??
Because all the
(-) charges
(electrons) repel!
Finish:
Lab handout
Give me a charge(van de graf)
Electric Charge Model
p151
Summary of things we know:
1.) There is a property of matter called electric
charge. (In the metric system its units are
Coulombs.)
2.) Charges can be negative (like electrons) or
positive (like protons).
3.) In matter, the positive charges are stuck in
place in the nucleus. Matter is negatively charged
w when extra electrons are added, and positively
charged when electrons are removed.
4.) Like charges repel, unlike charges attract.
5.) Charges travel in conductors, not in insulators
6.) Force of attraction or repulsion F=Kqq / r2
Coulomb’s Law