Unit 1: Coulomb’s Law Today’s Concepts - College of …s...Physics 2112 Unit 1: Coulomb’s Law...
-
Upload
truongdung -
Category
Documents
-
view
235 -
download
7
Transcript of Unit 1: Coulomb’s Law Today’s Concepts - College of …s...Physics 2112 Unit 1: Coulomb’s Law...
Physics 2112
Unit 1: Coulomb’s Law
Today’s Concepts:
A) Coulomb’s Law
B) Superposition
Unit 1, Slide 1
Why are electrons fixed and stationary in plastics?
Nothing was difficult to understand. Videos were clear and concise.
Would like to see a variety of worked-out examples to strengthen
understanding.
The most difficult is listening to his voice.
donuts?
I think I understood this pre lecture for the most part.
HA AP Physics
i am confused about the forces relating to magnetism.
Feel ok about the check point. However was a bit confused in one of the
calculations, they didn't include one of charges and im not sure why. Would
be nice to do a few calculations in class. The lecture seemed fine.
You're going to be so proud of me! Like OMG. For the first time ever I actually
watched the whole pre-lecture and I actually thought about the Checkpoint
questions. Assuming I actually got these questions right, I didn't have trouble
with anything. :)
What we thought….
Electricity & Magnetism Lecture 2, Slide 2
Electric Charges
Electric charges come in two sign: + and -
Like sign charges repel, Opposite sign charges
attract.
Electric charge is “quantized”.
qe=1.602 X 10-19C
Charge of electron is exactly same as charge of
proton
Unit 1, Slide 3
Why?
Why?
Why?
How?!
Electric Charges
Electric charges is conserved
Unit 1, Slide 4
At Fermilab
-1 +1
anti-proton proton
Electric Charges
Tribo-electric
Series
Unit 1, Slide 5
Tend to lose electrons
when rubbed
Tend to gain electrons
when rubbed
When we place near the positively charged
glass rod, the ruler was attracted to it.
What will happen when it is placed near the
charged PVC rod?
A. It will be attracted to it
B. It will be repelled by it
C. There will be no effect
QUESTION
In our simulation, we have now rubbed the
sweater and the balloon, transferring charge
from one to the other. Both are pretty good
insulators. What can we say about the force
between the sweater and the balloon?
A. They will attract each other.
B. They will repel each other.
C. There will be no force between
them.
QUESTION
In our simulation, we have now rubbed the sweater
and the balloon, transferring charge from one to the
other, but the wall remained neutral. All three are
pretty good insulators. What can we say about the
force between the wall and the balloon?
A. They will attract each other.
B. They will repel each other.
C. There will be no force between them.
QUESTION
Conductors / Insulators
Conductors – charges are free to move
anywhere on the conductor
Insulators - charges remain where they
are place except in cases of “extreme”
force
Unit 1, Slide 9
The force on a charge due to another charge is proportional to the product of the charges and inversely proportional to the separation squared.
r
The force is always parallel to a line connecting the charges, but the direction depends on the signs of the charges:
q2
q2 q1
q1
Like signs repel
Electro-static Force
q2 q1
Opposite signs attractq2 q1
Unit 1, Slide 10
2
21
r
qqF
Coulomb’s Law
Electricity & Magnetism Lecture 1, Slide 11
2,12
2,1
212,1 r̂
r
qqkF
k = 9 X 109 N m2/C2
Unit of Charge is Columb (C)
(big!) qe=1.602 X 10-19C
Charles-Augustin de Coulomb – French engineer who was a pioneer in torsion and
soil mechanics.
A hydrogen atom is composed of a nucleus containing a single
proton, about which a single electron orbits. When the electron
is in the lowest orbital position, the electric force between the
two particles is 2.3 X 1039 greater than the gravitational force!
How would we adjust the separation between the two particles
so the electric and gravitational forces are equal?
A) We must move the particles farther apart.
B) We must move the particles closer together.
C) It can’t be done at any distance.
QUESTION
Coulomb’s Law
Electricity & Magnetism Lecture 1, Slide 13
2
2,12
2,1
212,1 r̂
r
qkqF
2,1F
Examples: If the charges have the same sign, the force by charge 1 on charge 2
would be in the direction of r12 (to the right).
q2q1 2,1r
If the charges have opposite sign, the force by charge 1 on charge 2 would be opposite the direction of r12 (left).
q1 q22,1F
2,1r
Our notation:
is the force by 1 on 2 (think “by-on ”)
is the unit vector that points from 1 to 2. 2,1F
12r̂
Electricity & Magnetism Lecture 1, Slide 14
Coulomb Force
What will the direction of the force be?
A) Attractive B) Repulsive
k 9 x 109 N m2 / C2
electron charge 1.6 x 10-19 Coulombs
NA 6.02 x 1023
Two paperclips are separated by 10 meters. Then you remove 1 electron from each atom on the first paperclip and place it on the second one.
122
12
21 r̂r
qqkF
Question
Unit 1, Slide 15
Which weight is closest to the approximate force between those paperclips (recall that weight = mg, g = 9.8 m/s2)?
A) Paperclip (1 g x g)
B) Text book (1 kg x g)
C) Truck (104 kg x g)
D) Aircraft carrier (108 kg x g)
E) Mt. Everest (1013 kg x g)
k 9 x 109 N m2 / C2
electron charge 1.6 x 10-19 Coulombs
NA 6.02 x 1023
Two paperclips are separated by 10 meters. Then you remove 1 electron from each atom on the first paperclip and place it on the second one.
122
12
21 r̂r
qqkF
Example 1.1 (forces between paper clips)
Unit 1, Slide 16
Two 1 gram paperclips are separated by 10 meters. Then you remove 1 electron from each atom on the first paperclip and place it on the second one.
What is the force between the two clips?
Example 1.2 (forces between students)
Unit 1, Slide 17
What would the force be between two 80kg students sitting two meters apart if the charge of the proton were 10-10% greater than the charge of an electron?
CheckPoint: Forces on Two Charges
Electricity & Magnetism Lecture 1, Slide 18
Two charges q = + 1 μC and Q = +10 μC are placed near
each other as shown in the figure below.
Which of the following diagrams best depicts the forces acting
on the charges:
CheckPoint Results: Forces on Two Charges
Electricity & Magnetism Lecture 1, Slide 19
A) Since they are both positive they have to repel each other, and I would think the one with the stronger charge would repel more.
B) Just like a fly having the same amount of force impact on a bus that the bus has on the fly, two charges will have equal and opposite effects on each other in accordance with Newton's Third Law.
C) The force of 2 (+10 uC) on 1 (+1 uC) is 10 times greater than the force of 1 on 2 consequently causing charge 1 to become displaced at a much larger magnitude than charge 2.
Two charges, -Q and +2Q are placed a distance R apart.
The magnitude and direction of the force F on the -Q charge
is shown by the arrow. Which of the following arrows
represents the approximate magnitude and direction of the
force on the particle with the larger (+2Q) charge?
A.
C. D.
E.
B.
+2Q
-Q
Question
Two charges, -Q and +2Q are placed a distance R apart. The
magnitude and direction of the force F on the -Q charge is
shown by the arrow. Next, the –Q charge is changed to +2Q.
Which of the following arrows represents the approximate
magnitude and direction of the new force on the bottom +2Q?
Question
Two charges, -Q and +2Q are placed a distance R apart. The
magnitude and direction of the force F on the -Q charge is
shown by the arrow. The charges are then moved to a
separation distance of R/2. Which arrow’s direction and length
represents the force on the +2Q charge in this new position?e
new force on the +2Q charge?
Question
q1
q2
q3
q4
If there are more than two charges present, the total force on any given charge is just the vector sum of the forces due to each of the other charges:
F2,1
F3,1
F4,1
F1
Superposition
Electricity & Magnetism Lecture 1, Slide 23
F2,1
F3,1
F4,1
F1
1F
...1,41,31,2 FFF
What happens to the magnitude of Force on q1 if its sign is changed?
A) |F1| increases
B) |F1| remains the same
C) |F1| decreases
D) Need more information to determine
Electricity & Magnetism Lecture 1, Slide 24
Question: Superposition
q1
q2
q3
q4
The direction of all forces changes by 180o – the magnitudes stay the same:
F2,1
F3,1
F4,1
F1
F2,1
F3,1
F4,1
F1
F2,1
F3,1
F4,1
F1
q1
q2
q3
q4
F2,1
F3,1
F4,1
F1
q1
q2
q3
q4
Electricity & Magnetism Lecture 1, Slide 25
1F
...1,41,31,2 FFF
1F
- ...1,41,31,2 ---- FFF
Electricity & Magnetism Lecture 1, Slide 26
-Q Q
Case 1 Case 2
Compare the magnitude of the net force on q in the two cases.
A) |F1 | > |F2|
B) |F1 | |F2|
C) |F1 |< |F2|
D) Depends on sign of q
CheckPoint: Compare Forces
Case 1 Case 2
Electricity & Magnetism Lecture 1, Slide 27
-Q Q
Case 1 Case 2
Compare the magnitude of the net force on q in the two cases.
A) |F1 | > |F2|
B) |F1 | |F2|
C) |F1 |< |F2|
D) Depends on sign of q
F1
F2 0
CheckPoint: Compare Forces
A) No matter the sign of charge q, there will be an attractive and repulsive force in case
1 (same direction), and two forces (attractive or repulsive depending on the sign of the
charge on q) that cancel so that the net force on q is zero.
C) the distance between the two particles from charge q is the same and the quantity of
charge on +Q and -Q is the same. so the magnitude of the net force will be the same
in both cases.
D) I believe that if the sign of q is negative, the force on q will be less than if the sign of q
is positive, as there will be two repelling forces acting on it rather than one repelling
and one attracting force.
Electricity & Magnetism Lecture 1, Slide 28
-Q Q
Case 1 Case 2
Compare the magnitude of the net force on q in the two cases.
A) |F1 | > |F2|
B) |F1 | |F2|
C) |F1 |< |F2|
D) Depends on sign of q
F1
F2 0
CheckPoint: Compare Forces
D) because if the central charge is neutral there is no net force on either. If
the central charge isnt neutral, the net force would be greater in case 1.
Good point. From now on, if I refer to a q, you can assume |q| = 0
CheckPoint: Force from Four Charges
A positive charge Q is placed at the
origin. Four other positive charges are
placed on a circle in the x-y plane
centered on Q as shown. The top
charge is located on the y axis and
has a magnitude of 3q. The three
bottom charges are symmetric about
the y axis as shown and each has
magnitude q.
What is the direction of horizontal force on Q?
A) Fx > 0 B) Fx 0 C) Fx < 0
x
y
3q
q q
q
Q
Electricity & Magnetism Lecture 1, Slide 29
CheckPoint Results: Force from Four Charges
What is the direction of horizontal force on Q?
A) Fx > 0 B) Fx 0 C) Fx < 0
x
y
3q
q q
q
Q
Electricity & Magnetism Lecture 1, Slide 30
A positive charge Q is placed at the
origin. Four other positive charges are
placed on a circle in the x-y plane
centered on Q as shown. The top
charge is located on the y axis and
has a magnitude of 3q. The three
bottom charges are symmetric about
the y axis as shown and each has
magnitude q.
Electricity & Magnetism Lecture 1, Slide 31
What is the vertical force on Q?
A) Fy > 0 B) Fy 0 C) Fy < 0
x
y
3q
q q
q
Q
Question
A positive charge Q is placed at the
origin. Four other positive charges are
placed on a circle in the x-y plane
centered on Q as shown. The top
charge is located on the y axis and
has a magnitude of 3q. The three
bottom charges are symmetric about
the y axis as shown and each has
magnitude q.
Electricity & Magnetism Lecture 1, Slide 32
What is the vertical force on Q?
A) Fy > 0 B) Fy 0 C) Fy < 0
x
y
q q
q
Q
Question
A) The y distance between the bottom left and bottom right charges is less than the y
distance of the 3q on top, so the net force in the y direction would be upwards
B) because the magnitude of the 3q charge will equal the magnitude of the three q
charges.
C) all force of the 3q is directed along the negative y axis. some of the force exerted on Q
by two of the q charges is directed along the x axis. therefore the net force is along the
negative y axi
A positive charge Q is placed at the origin. Four
other positive charges are placed on a circle in
the x-y plane centered on Q as shown. The top
charge is located on the y axis and has a
magnitude of 3q. The three bottom charges are
symmetric about the y axis as shown and each
has magnitude q.
3q
Example 1.3 (Force on charge)
Electricity & Magnetism Lecture 1, Slide 33
2
2,12
2,1
212,1 r̂
r
qkqF
q2
-30uC
+10uC
+25uC
q3q1
2m
2m
What is the force on q2?
Charge on a Conductor
Unit 1, Slide 34
Charges are free to move
on conductor
++
+ ++
+
+
+
+
+
+
+
+
+
+
+
Charges always on very
edge of conductor