Quiz on reading.
1. Coulomb’s law describes
a. the force between electrical charges
b. how dry your socks get in the dryer
c. the friction force between items of clothing
d. The operation of a conveyor belt
HW 9 Saturday by noon
Next Tuesday: special guest lecturer
2. Electric charges
a. can never move from one object to another
b. can never move inside an object
c. Can move inside an object but not from one object to another
d. Can move both inside an object and between objects,
depending on what the objects are.
Today: electricity
Next week: flashlights
3. True or false: a conveyor belt is safer because in contains
strips of metal wires.
Answers: 1 a 2 d 3 a
a) True b) False
Static electricity and lightning
(moving around electric charges)
Source of electricity: charges
Electrostatic demos
Why is this happening?
most of the volume is in a
“cloud” of electrons, with negative charges.
The atom
A new kind of force (not discussed in class yet, anyway).
Depends on the distance between objects.
99.9% of mass is in the nucleus,
neutrons (neutral/no charge) and
protons (positive charge)
Electrostatic forces.
Depends on the pieces that make object up, not the objects as a whole.
Most atoms have equal number of protons
and electrons,
so total charge is zero. The same is true of
most ordinary objects made of atoms.
You can pull electrons off atoms. This gives unbound (free) electrons
(negative charge) & atoms that are positively charged.
like charges repel
-
Force:
Repelled from other charge
opposite charges attract
What force does charge “A” feel?
Depends on charge of A (qA),
charge of B (qB), and distance between them (r).
A
B
Observed Behavior:
• more charge, more force
• closer together, more force
In equations:
Forceof B on A = kqAqB
r2
k is Coulomb constant = 8.99 x 10 9 N m2/C2
qA and qB are amount of charge in coulombs (C )
1 Coulomb = 6 x 10 18 electron charges!
+
Electric Hockey Simulation!
Forceof B on A = kqAqB
r2
Place charge (B), see charged puck (A) fly away when 2 cm away.
Now place charged puck (A) now 1 cm away from placed charge (B),
compared to previous situation force on A will be:
a. half as large, b. same size, c. twice as large d. four times larger
e. something else.
d. four times larger since force depends on
1/(distance between charges) 2 …
distance smaller, force larger
Add another charge to B, right on top of first. Force on
“puck” (A) is
a. ½ b. same c. x 2 d. x 4 e. something else.
c. x 2 because force on A goes like (charge of A x charge of B),
… changed charge of B from 1 to 2.
1
B
proton
A
electron
-
0.001 m
apart
+
what is force between them?
calculate and write down
answer.
Forceof B on A = kqAqB
r2
k is Coulomb constant = 8.99 x 109 N m2/C2
qA and qB are amount of charge in coulombs (C )
1 Coulomb = 6 x 1018 electron charges!
B
proton
A
electron
-
0.001 m
apart
+
what is force between them?
calculate and write down
answer.
Forceof B on A = kqAqB
r2
k is Coulomb constant = 8.99 x 109 N m2/C2
qA and qB are amount of charge in coulombs (C )
1 Coulomb = 6 x 1018 electron charges!
electron qA = -1 charge x (1C/6 x 1018 charges) = -1.6 x 10-19 C,
proton qB = 1 charge x (1C/6 x 1018 charges) = +1.6 x 10-19 C
so Force = (8.99 x 109 N m2/C2) (-1.6 x 10-19 C) (1.6 x 10-19 C)
(1 x 10-3 m)2
= -2.3 x 10-28 N m2 = -2.3 x 10-22 N
1 x 10-6 m2
B
proton
A
electron
-
0.001 m
apart
+
what is force between them?
calculate and write down
answer.
Forceof B on A = kqAqB
r2
Electrical materials:
In a conductor (think: metal),
free electrons can move around
so Force = -2.3 x 10-22 N
What does the minus sign mean?
-
-
a) Force points to left
b) Force points to right
Electrical materials:
Electrical materials:
In a conductor (think: metal),
free electrons can move around
In an insulator, electrons remain bound to atoms.
-
+
+-
+
2
Demo of pie plates stacked on Van de Graff machine.
Turn in on, thereby put lots of extra electrons on it.
Here is the van de Graaf machine. Its purpose is to put a lot
of negative charge on this conducting sphere. Where does all
the cahrge go?
a)
b)
c)
d)
e)
What will happen to the pie plates?
a. nothing, plates will stay where they are.
b. all the plates will fly off at same time
and stick to each other.
c. top plate will fly off, then next to top
will fly off, etc.
d. all will fly off at same time and separate.
To the middle of the sphere
All around the surface of the sphere
To the top of the sphere
To the bottom of the sphere
Somewhere else
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
Van de Graff
Answer b: negative charges repel, and are free
to move around. They will get as far apart as
possible.
Belt moves free electrons
-up to metal ball…
-
demo of pie plates stacked on Van de Graff machine.
-
- -- ---
-
just like pom-poms …top one flies
off first because charges pileup on
edges, so lots of negative charges
on top plate repels negative charges
on Van de Graph.
-
like charges repel
Cut away view ..
Electrons pile
Up on outer surface
Forceof charges on VG on top plate = k qTopPlate qVandegraphBall
r2
-
c. top plate will fly
off, then next to top
then next etc.
- - - -- -- -- --- - - -- --
Balloon demo-- rub a balloon and stick it to the wall.
What attracts the balloon to the wall?
Sweater and Balloon applet
Predict what charges are on balloon and sweater.
a. both have extra plus charges.
b. both have extra minus charges
c. balloon has extra plus or minus charges, sweater neutral,
d. sweater has extra plus or minus charges, balloon neutral
e. either sweater has extra minuses and balloon extra pluses
or balloon extra minuses and sweater extra pluses.
do experiment and find out. Answer is e
Rub a second balloon on the sweater.
The two balloons will ..
a. attract b. repel, c. not exert a force on each other
Answer b. opposite charges attract
Move charged balloon close to wall. What will happen?
a. wall is neutral (no extra pluses or minuses) so will not be affected.
b. minus charges in wall will move away, pluses towards balloon
c. plus charges in wall will move away, minuses towards balloon.
d. minus charges in wall will move away, pluses don’t move.
lots of extra
electrons on
balloon.
qA
-
-
-
+ qpluses
+
+
+
+
+
+
- qminuses
-
Forceon Balloon = kqAqpluses + kqAqminuses
r2
r2
Pull from plus charges is greater than the Push from minus
charges
because the negative charges are farther away (r is larger, so
F is smaller).
3
Electrostatic dust rag (think SwifferTM).
Rub it on surface, it’s very good at attracting electrons (negative
Charges).
What kind of dust will this negatively
charged rag pick up best?
a. Only dust with positive charges.
b. Only dust with negative charges.
+ - c. The rag will pick up all dust equally.
d. The rag will pick up dust with positive
charges, and also neutral dust particles,
+
just not as well.
e. The rag will pick up dust with negative
charges, and also neutral dust particles,
just not as well.
-
Voltage Difference is measure of
how much energy you need to add (work)
or how much energy is released
when charged object to go from one place to another.
If you need to add energy, then
increase in Electrostatic Potential Energy (EPE)
If energy is released, then
decrease in Electrostatic Potential Energy (EPE)
Change in EPE = charge x voltage difference = qV
(like G. P. E. with charge instead of mass, V instead of h)
q
--
--
no voltage difference
between plates. No change
in EPE to move from one to
other.
V=
0
Electrostatic dust rag.
Rub it on surface, it’s very good at attracting electrons (negative
Charges).
What kind of dust will this negatively
charged rag pick up best?
-
++
-
ans. d. The rag will clearly pick up
positively-charged dust.
It can also pick up neutral dust, but has to
move the charges first, so this is not as good.
Voltage Difference
Change in Electrostatic PE = charge x voltage difference = qV
++
++
++
++
++
-- q
-- --- q
--V
Change in EPE = qV . Suppose V = +100Volts difference
If Charge = q = 2 electrons = -2 charges x (1.6 x 10-19 C/charge)
then …
Change in EPE= -3.2x10-19Coulombs x (100Volts) = -3.2 x 10-17 J.
After charged object hits plate, where did this energy go?
a. Kinetic Energy b. Thermal Energy c. Gravitational PE
d. Pressure PE
b. Thermal energy. But just before hit, kinetic so qV=1/2 mv 2
Bring uncharged metalized mylar balloon up to vandergraff.
Predict what will happen:
1. before it touches
2. after it touches
------------------------------------------------------------------------------------------------
before touching
a. not affected by Vandegraaf b. attracted to it c. repelled
before touching.
positive charge pulled closer,
feels largest force.
Wins over repulsion of
negative because closer.
so attracted
-
+
----------------------------------------------------------------------------------------------------
Forceon Balloon = kqballplusesqVG + kqballminusesqVG
r2
r2
Pull from plus charges Greater than Push from minus charges
because negative charges farther away (r is larger, so F is
smaller).
after touching
a. not affected by Vandegraaf b. attracted to it c. repelled
4
A -------
B
-----------------------------------------------------------------General
-----------------Forceof B on A = kqAqB
-----------------r2
For this case,
After touching.
Extra minuses transferred
to ball. Why?
Getting pushed off Vandegraff
by other negatives.
Forceballoon = kqballqVG
r2
Once extra minuses on balloon, then balloon and VG repel.
5