We are here to do everything we can to help
you learn some important and interesting
physics !
If you work hard during the term, we will
work hard with you.
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A few basics about the course...
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Required Texts:
"Physics for Scientists and Engineers" Vol. 4, Knight
"Tutorials in Introductory Physics" McDermott, Shaffer
Please come see us if you have questions.
If our office hours do not work for you, email us to set
up another time to meet.
Do not wait until the end of the term to talk to us.
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Clicker Question
Clickers are required for this course.
• Group discussion of clicker questions is required!
Do you have a clicker with you?
• Make sure to register your clicker this week.
A) Yes, I have a clicker with me.
capa.colorado.edu/cgi- bin/RegisterAFS
B) No, I don’t have a clicker so I can’t vote .
• Make sure to select this class for registration.
)
A
• Clicker scores are extra credit. They can replace
half of your lowest mid-term exam score.
3 points for correct answer
2 points for incorrect answer
0 points for no answer
• Scores start counting Feb. 6.
• Lowest five classes are dropped.
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We will use the MasteringPhysics online homework system.
Many of you will be familiar with this from PHYS1110.
With your purchase of the textbook, you also get an access
code to the online homework.
Follow the directions below to register for the class.
MasteringPhysics (MP) homework is due every
week on Friday at 11 pm.
The first (very short) assignment is due
this Friday, January 20 !
This is mostly to verify that people can use the
system. If you have any problems, please let us
know ASAP.
Late homework is accepted by the online system,
and can be completed anytime later with a 50%
penalty.
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Tutorials:
We are going to have weekly tutorials in the recitation
sections using the U. Washington text.
There is a required weekly online pre-test
due every Monday at 11 pm. First one due
January 23 !
The web link will be posted from the main page.
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Each week you will do a different tutorial.
Attendance at tutorials is required.
Participation is also required.
Tutorials are a great way to introduce some hands
on learning.
Each week there is a tutorial homework which is
due the following week at the start of the next
tutorial.
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Please check which tutorial section you are signed
up for. You must attend only that specific section.12
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Students learn a lot more with tutorials !
Tutorials are like this....
Yes, these are
spinach plants!
You may not
always like it, but it
is always good for
you.
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R. Hake, ”…A six-thousand-student survey…” AJP 66, 64-74 (‘98).
Students get better grades when they
put the effort into tutorials !
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Exams:
There are three midterm exams in the evenings!
February 14, March 14, April 18.
The final exam is May 8.
It is your responsibility to make sure you have no
conflicts with these dates and times.
If you have special needs, let us know ASAP.
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Grading:
Physics Attitude Survey:
Please take the 10-12 minutes to fill it out.
Extra credit worth one homework problem.
15% for MasteringPhysics Homework
35% for the Midterm Exams (11.7% each)
35% for the Final Exam
5% for Tutorial Attendance and Participation
10% for Tutorial Homework
__________________________
100%
* up to 5% Replacement for Clicker Scores
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Now for some physics...
Font Check !
This is 32 point font.
Since ancient times people have been
aware of electrical phenomena.
This is 28 point font.
This is 24 point font.
This is 20 point font.
Lightening, sparks, ....
This is 18 point font.
This is 16 point font.
This is 14 point font.
This is 12 point font.
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Clicker Question
They even started to quantify its behavior
without knowing the underlying particles
having the property of electric charge.
This year we celebrate the _____ birthday of
Benjamin Franklin.
a) 50th
b) 100th
c) 200th
d) 300th
e) 400th
Benjamin Franklin was one of the scientific
leaders in this early, more quantitative,
advance in understanding.
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Birthday: January 17, 1706
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Some Experimental Facts:
Calling protons (+) and electrons (-) is a
convention.
Electric charge comes in two types, which we call
positive (+) and negative (- ).
Atoms consist of a heavy (+)
charged nucleus surrounded by
light (- )electrons.
We could just as easily have called electrons
(+) and protons (-), but Benjamin Franklin
chose the other way and so we are stuck
with it.
The nucleus consists of (+) protons
and neutral neutrons.
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Size Scales:
atomic size ~ 10-10 m
nuclei size ~ 10-15 m
In fact, Franklin thought that all discharges went
from positive to negative.
Thus, he believed that most
lightening strikes "tis the Earth
that strikes into the Clouds."
Mass Scales:
m(electron) << m(proton) ~ 1 : 1800
Electric Charge:
q(proton) =- q(electron)
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The charge of the proton is opposite in sign to
the electron, but exactly the same magnitude
to the best precision we have ever measured.
Units of Charge:
Charge of one electron:
q =- e =- 1.602 x 10-19 [C]
Kind of a nice symmetry!
SI unit of charge is the Coulomb [C].
Charge of one proton:
q = + e = + 1.602 x 10-19 [C]
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You might wonder about why the proton is almost 2000
times heavier than the electron, but has the same
charge magnitude?
In fact, the proton is made up of
quarks with charge +2/3 e and- 1/3 e.
How large is one Coulomb of charge?
number of electrons x charge per electron =
total charge
N x (1.602 x 10-19 [C]) = 1.0 [C]
N = 1.0 [C] / (1.602 x 10-19 [C])
However, we never see these quarks
outside the proton. They are mostly
important in very high energy reactions.
N = 6.3 x 1018 electrons in one [Coulomb] !
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Forces are quantified via Coulomb's Law:
Unlike charges attract each other.
v
QQ
F = k 12 2
r
Like charges repel each other.
Q1
Q2
r
How did early scientists demonstrate this?
where F is the force between two charges Q1
and Q2 and r is the distance between them.
* Note that this is different from gravity where
Newton found only attractive forces.
k=constant = 9.0 x 109 [Nm2/C2]
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Clicker Question
Clicker Question
Consider the following three situations, labeled 1, 2 and 3
+Q
+Q
+Q
r
1
+q
2r
2r
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2
+3q
3
+5q
Two protons are near each other. Each feels an electrostatic
repulsion of magnitude Felec and a gravitational attraction of
magnitude Fgrav, due to the other proton.
Which charge (+Q) feels
the largest force?
A) +q
B) +3q
C) +5q
D) two of the charges tie
for the largest size force.
As the charges are moved apart, the ratio
A) Increases
B) Decreases
C) Remains constant
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+
Felec
Fgrav
+
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Electric Charge is Conserved.
It is impossible to create or destroy net charge.
In fact, only in high energy reactions can we create
or destroy electrons or protons, but we always do so
in pairs to still conserve net charge !
Electron
q = -e
Positron
q = +e
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1)
2)
3)
4)
Electric charge comes as (+) and (- )
Objects in nature have charge +e or- e
Unlike charges attract, like charges repel
Electric charge is conserved
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Recall that forces are vectors.
Thus, they have magnitude and direction.
v
QQ
F = k 12 2
r
Coulomb's force acts along the
line between the two objects.
These are experimental facts.
Why are they true? We do not know.
People studying string theory for example are trying
to understand these facts at a deeper level.
If we have multiple charges, we must add the forces
together as vectors.
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Example: Net force on electron from two protons.
- e
r
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Clicker Question
Consider the charge configuration shown below.
What is the direction of the net force on the +q charge?
+e
+q
r
B
h
+e
s/2
Work out on the board.
+Q
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A
s/2
- Q
C
E
D
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Metals (Cu, Al, Au, Ag, Fe, ...) conduct electricity.
In metals some electrons (conduction electrons) can
move freely thru the metal. If there is a net Coulomb
force, the conduction electrons flow as a current.
Material Properties:
All materials are made of atoms, but have very
different electrical properties.
Inner core electrons are strongly bound.
Outer electrons are often unbound in
metals and move freely. Metals often
have 1 or 2 conduction electrons per
atom. Chemists say "valence electrons".
We contrast metals versus insulators.
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Insulators (plastic, wood, ceramics, ...) do not conduct
electricity.
All the electrons are tightly bound and do not move
much- even in the presence of strong Coulomb forces.
Insulators can have an induced charge due to induced
dipole moments !
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Clicker Question
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Neutral Atom
Metals (conductors) are:
-
A) Grouped in a vertical
banded region.
B) Grouped in a horizontal
banded region
C) Randomly in the chart
++++
-
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-
Polarized Atom with
External Coulomb Force
+
+
+
+
-
++++
-
-
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Demonstration with Wood 2x4 and charged plastic rod.
Some molecules have a permanent dipole moment,
water (H20) for example.
With an external Coulomb force, the dipole
moments align.
Try the bending water demonstration at home with a
comb or balloon. Impress your friends.
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* Induced charge always produces an attractive force!
The opposite charge is always closest, thus resulting
in attraction.
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Clicker Question
Two socks are observed to attract each other. Which, if any, of
the first 3 statements MUST be true? (emphasis on MUST)
A)
B)
C)
D)
The socks both have a non-zero net charge of the same sign.
The socks both have a non-zero net charge of opposite sign.
Only one sock is charged; the other is neutral.
None of the preceding statements MUST be true.
Answer: (D). None of the preceding statements must be true.
Either B) or C) could be true.
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New Concept: Electric Field
Note that the -E field exists even if there is no test
charge there to "measure it."
Surrounding every charge is an electric field (E).
Similarly there is a gravitational field around the earth,
even if there is no "test mass" nearby to measure the
pull of earth's gravity.
Definition: The electric field at a point in empty space
is a vector quantity where if we placed a test charge
+q at that point, we would measure a force F.
v
E=
v
Fon.q
q
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v
v
v
Fon.m mg v v F GM
g
=
= 2
gravitational field =
=
=g
r
m
m
m
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What is the -E field around a point charge +Q?
Q = "source charge"
distance = r
q = "test charge"
v
1) Force on q (if it is there) F =
v
2) Electric Field E =
-E field always points in the direction that a "positive
test charge" would move (if it were there).
kqQ
rˆ
r2
v
kQ
F 1 kqQ
=
rˆ = 2 rˆ
r
q q r2
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Clicker Question
Which is the correct direction of the electric field
created by charge- Q?
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How to visualize these fields?
E-field around a positive point
charge always points away
from the charge !
-Q
+Q
A
B
E
C
D
Answer = B
E-field points in direction that
positive test charge would move.
E-field also decreases in
magnitude with distance.
v kQ
1
E = 2 rˆ ∝ 2
r
r
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Two views:
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Michael Faraday came up with the
novel idea of "fields".
"Action at a Distance"
Coulomb's Law suggests that two charges exert a
force on each other through empty space.
"Field
- View"
Charge 1 creates an E
- field around it. Charge 2 feels
a force due to that field. And vice versa.
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He is one of the best
experimentalists in the history of
physics, but knew very little formal
mathematics.
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Clicker Question
- field and superposition principle for vectors.
E
Just as was true with force vectors, we can find the
total net -E field by adding the E- fields due to the
individual charges.
v
v v
v
v
kQ
Enet = E1 + E2 + E3 + ... = ∑ Ei = ∑ 2 i rˆi
ri
i
i
Since
y
+Q
C
+Q
+2Q
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v
v Fon.q
E=
q
A point in empty space is near 3 charges as shown. The
distances from the point to each charge are identical.
A
B
x
The E-field direction is:
A) Some angle less than 45 deg.
below the +x-direction.
B) 45 deg. below the +x-direction.
C) Along the +y-direction.
D) Some other angle
E) E-field is zero at that point.
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, if we place a real charge "q" at
the spatial point on the test charge, we can calculate
the force on "q" as:
v
v
F = qE
We will find that it is often easier to calculate the
- field from a set of charges first, and then the force
E
on a charge moving through that field second.
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