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7/31/2019 45429_Electric Charge & Coulombs Law
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Welcome
to Physics 24.
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PHYSICS 24
Engineering Physics IIWinter/Spring 2009
Dr. Allan PringleCourse InstructorRoom 122 Physics, 341-4031http://www.mst.edu/~pringle
pringle@mst.edu
http://physics.mst.edu/classes/class_24.html
http://www.mst.edu/~pringlemailto:pringle@mst.edumailto:pringle@mst.eduhttp://www.mst.edu/~pringle7/31/2019 45429_Electric Charge & Coulombs Law
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Todays agenda:
Course overview.
Physics 23: a reminder.
Coulombs Law (electrical force between charged particles).After today, you must be able to calculate the electrical forces between two or morecharged particles.
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Important Note
The next few slides summarize importantinformation contained in the course handbook.
Please refer to the handbook for details.
In case of discrepancy between these notesand the handbook, the version of the
handbook on the Physics 24 web site is the
official word.
We will not use clickers this semester in Physics 24.Do not buy a clicker!
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Course Schedule
Physics 24 lectures are Monday and Wednesday.
Recitations are Tuesday and Thursday. Labs take placethroughout the week. Homework assignments are being
handed out today and are posted on the course web site.
There are three scheduled hour examinations and a final.See your syllabus for dates and times. Rooms will be
announced later.
Section Changes
For the first week of class, see the secretaries in the
department office (102 Physics) to make section changes.
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Hour Examinations and Final Exam
The four exams will be worth 200 points each.
The exams will cover concepts and definitions, assigned
problems with minor numerical changes, and problems similar
to those assigned but requiring a deeper understanding ofconcepts or more complex calculations.
Assigned reading material not covered in lecture is testable.
There will be a 50-point multiple-choice end-material test given
concurrently with the final exam.
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The three hour exams are from 5:00-6:15 pm on exam days.
There are only two circumstancesunder which you may take
an hour exam at a different time:
1. You are involved in an official university event and have a
faculty sponsor who can administer the exam at some other
time on exam day and ensure exam security.
2. You have a lab or a test in another class (that offers no
makeups) during exam time. In that case you may take the
exam from 5:30-6:45 pm.
See this memo for details about exceptions 1 and 2:http://campus.mst.edu/physics/courses/24/CourseInformation/sponsor_mst.pdf
http://campus.mst.edu/physics/courses/24/CourseInformation/sponsor_mst.pdfhttp://campus.mst.edu/physics/courses/24/CourseInformation/sponsor_mst.pdf7/31/2019 45429_Electric Charge & Coulombs Law
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The Final Exam will be Wednesday, May 13, from 8:00-10:00
am.
There are NO MAKEUPSfor the final exam. I am not allowedto give the final exam to you at any other time, unless you have
secured written permission from the Provost at least a week
before the beginning of Finals week.
So reserve that time in your calendarTODAY, and make surethat time remains free!
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Course Grades
The lowest of the four exam scores will be dropped.
There will be twelve 5-point lecture quizzes during the
semester. The quizzes will cover recently-assigned reading
material and problems. Your two lowest quiz scores will be
dropped.
Recitations will be devoted in part to student presentation of
their homework, usuallyat the blackboard. A maximum of
100 points will be given for boardwork.
Homework will be collected six times in recitation, with the
lowest homework score dropped.
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Letter grades for Physics 24 will be assigned as follows:
895.0- up A (89.50%)
795.0894.999 B (79.50%)695.0794.999 C (69.50%)
595.0694.999 D (59.50%)
Below 595.0 F
There is no limit to the number of A's, B's, etc.
The following table summarizes the points available during
the course:
Three Exam Scores 600
End-Material Test 50Ten Quizzes 50
Homework 50
Boardwork 100
Labs 150
Total 1000
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Make-Up Exam Policy
There are no make-ups in this course. The dropping of the
lowest score is intended to accommodate students who
miss one exam due to hospitalization, illness, family
emergencies, mental stress, athletic events, etc.
See the Physics 24 handbook for procedures for
incompletes and for taking an exam if you are out of town
on an official university event. Also see the handbook for
notes on appealing course policies.
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Important Dates
The last day to drop this class without a withdrawal showing
on your transcript is Monday, February 23, 20098.
The last day to withdraw from this class is Friday, April 17,
2009.
Drop Policy
Students with inadequate attendance may be dropped. Any
student who has inadequate attendance, as evidenced by
missing a total of 7 graded assignments of any kind (exams,
homework, quizzes, boardwork, and labs) will be placed on
Academic Alert. Students who fail to take the recommended
action are subject to being dropped if a subsequent
assignment is missed.
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Homework and the PLC
Homework help will be available at the Physics Learning
Center (PLC). You may excel in the course without eversetting foot in the PLC. You may need to spend ten hours in
the PLC every week just to pass.
The PLC is open* from 2:00-4:30 pm and 6:00-8:30 pm. ThePLC operates in rooms 129-130 of Physics, with Physics 24
help available on Mondays and Wednesdays. Individual tutors
are also available. For up-to-date information visit the web site
http://lead.mst.edu. The first PLC will be Wednesday, Jan. 14.
*Official hours will be announced at the start of the semester.
I had to study ten hours a week for this four-hour class!
(Student complaint on spring 2005 course evaluations.)
http://lead.mst.edu/http://lead.mst.edu/7/31/2019 45429_Electric Charge & Coulombs Law
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Regrade Requests
If you want a quiz or exam problem regraded, please write the
reason for the request on a sheet of paper, staple it to the examor quiz, and return it to your recitation instructor (see course
handbook for deadlines).
Specify which problem you want regraded, and provide a
detailed written statement as to why the original workwhichappeared on the assignment deserves more points.
Don't wait until final grades are posted and ask for Exam 1 to be
regraded. Don't wait until after the final exam and ask that gradecutoffs be lowered by 1 point so you can get the next higher
grade; it wont happen. However, scoring mistakes (points
added up wrong, score recorded incorrectly) can be corrected
at any time.
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No Labs This Week!
Dont go to lab this week! Labs start on Tuesday of next week.
No Labs This Week!
Dont forget about lab this week.
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Your First Line of Defense
Your recitation instructor is your first line of defense.
He/she is there to help you succeed.
Everythinghomework, exams, labs, regrade requests, etc.,
will be channeled through your recitation instructor.
your recitationinstructor
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You can send e-mail to me at pringle@mst.edu. If you send e-
mail from an account which is not your UMR e-mail account
(e.g., a friend's computer, a Hotmail account) and want a reply
to your UMR account, be sure to include you UMR e-mail
address.
Unresolved Complaints
It is hoped that any complaints about the course can be resolved
in a collegial manner through discussions between student and
instructor. However, if there are any complaints that cannot beresolved, you may take them up with the Physics Department
Chairman, Dr. Dan Waddill (waddill@mst.edu ).
mailto:pringle@mst.edumailto:waddill@mst.edumailto:waddill@mst.edumailto:waddill@mst.edumailto:pringle@mst.edu7/31/2019 45429_Electric Charge & Coulombs Law
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We dont ask much out
of you in this class
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Announcements
Make sure you pick up the handouts containing:
Course HandbookSyllabus
Starting Equations
Special Homework (one is due tomorrow).
You may get a lab schedule from your recitation instructor. Or
go to Blackboard. There are no labs this week.
Your recitation instructor will call students to the board
tomorrow (but not for points). You may use your calculator, ablank handout problem sheet, and the starting equation sheet.
Nothing else. Boardwork for points begins Thursday.
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Todays agenda:
Course overview.
Physics 23: a reminder.
Coulombs Law (electrical force between charged particles).After today, you must be able to calculate the electrical forces between two or morecharged particles.
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Prologue
Tell me some things you recall from last* semester
*or whenever you took your previous physics class
Newtons Laws
F ma
energy and its conservation
21KE mv
2
2
spring
1U ks
2
gravU mgy
f i other i fE E W E K U
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laws of thermodynamics
momentum and its conservation (linear and angular)
p mv
z zL I
f iP P z,f z,iL L
These things arent going to go away!
in outU Q W
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This semester we study electromagnetic forces and their
consequences.
These forces are responsible for holding together living
and man-made things, as well as all things in nature, so I
suppose they are worth studying
not to mention the fact that the technology that
dominates your life depends on electromagnetic forces.
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Todays agenda:
Course overview.
Physics 23: a reminder.
Coulombs Law (electrical force between chargedparticles).After today, you must be able to calculate the electrical forces between one or morecharged particles.
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like charges repel
unlike charges attract
charges can move but charge is conserved
Law of conservation of charge: the net amount of electriccharge produced in any process is zero.
There are two kinds of charge. + -
Electric Charge
Static Electricity
Properties of charges
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Although there are two kinds of charged particles in an atom,
electrons are the charges that usually move around.
A proton is roughly 2000 times more massive than an
electron.
The charge of an electron ise =1.6x10-19 coulombs.
The charge of a proton is +e = +1.6x10-19 coulombs.
Charges are quantized(come in units of e= 1.6x10-19 C).
+ -
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Nitpicking: electric charge is a property of matter, not a thing
in itself. It is not good to say like charges repel. It is good
to say like-charged particles repel. I choose the not goodterminology here to be consistent with your text.
It would be much more convenient if + charged particles werethe ones that moved easily. So whos the clown who decided
electrons have negative charges?
Why is the fundamental unit of charge e = 1.602x10-19 C. Why
not just 1?
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And yes, he really flew the kite in the thunderstorm. See here.
Franklins experiments showed him that there were two kinds
of charge, which he named positive and negative. More than
a century later we learned that negative charges are associatedwith electrons.
Oh, and the next two people who tried the kite experiment were killed in the process.
http://scienceworld.wolfram.com/biography/FranklinBenjamin.htmlhttp://scienceworld.wolfram.com/biography/FranklinBenjamin.html7/31/2019 45429_Electric Charge & Coulombs Law
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Conductors and Insulators
There is also no such thing as a perfect conductor.
A superconductor has no resistance to the flow of
current, but is not the same as a perfect conductor.
There is no such thing as a perfect insulator.
http://www.maps.jcu.edu.au/course/CAUTscience/elec/elec02a.html
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Coulombs Law
1 2
2
12
q q
F k12 r
Coulombs law gives the force (in newtons) between charges q1
and q2, where r12 is the distance in meters between the
charges, and k=9x109 Nm2/C2.
Weve seen attractive and repulsive electrical forces at work.Coulombs law quantifies the magnitude of the electrostatic
force.
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To make this into a really good starting equation I should
specify repulsive for like, but that makes it too wordy. Youlljust have to remember how to find the direction.
1 2
2
12
q qF k ,12 r
attractive for unlike
Force is a vector quantity. The equation on the previous slide
gives the magnitude of the force. If the charges are opposite in
sign, the force is attractive; if the charges are the same in sign,
the force is repulsive. Also, the constant k is equal to 1/40,where 0=8.85x10
-12 C2/Nm2.
Remember, a vector has a magnitude and a direction.
I could write Coulombs Law like this
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The equation is valid for point charges. If the charged objects
are spherical and the charge is uniformly distributed, r12 is the
distance between the centers of the spheres.
If more than one charge is involved, the net force is the vector
sum of all forces (superposition). For objects with complex
shapes, you must add up all the forces acting on each separate
charge (turns into calculus!).
+ -
r12
+
++
-
--
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We could have agreed that in the formula for F, the symbols q1
and q2 stand for the magnitudes of the charges. In that case,the absolute value signs would be unnecessary.
1 2
2
12
q qF k ,12 r
However, in later equations the sign of the charge will be
important, so we really need to keep the magnitude part.
On your homework diagrams, show both the magnitudes andsigns of q1 and q2.
Your starting equation sheet has this version of the equation:
which gives you the magnitude F12 and tells you that you need
to figure out the direction separately.
I want this class tomake you hear little
voices in your head.
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A sample Coulombs law problem involving multiple charges is
on the following slides.
I will work the problem on the blackboard in lecture, if there is
time.
Usually vector problems are easiest if you manipulate the
whole vector at once, using unit vectors.
Sometimes it is easier to work the problem a component at a
time.
The slides use the component-at-a-time approach. At the
blackboard I will use the unit vector approach. I recommend
the unit vector approach.
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Solving Problems Involving Coulombs Law andVectors
You may wish to review vectors (on your own).
x
y
Q2=+50C
Q3=+65C
Q1=-86C
52 cm
30cm
=30
Example: Calculate the net electrostatic force on charge Q3due to the charges Q1 and Q2.
Finish, thenSkip to slide 43.
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Step 0: Think!
This is a Coulombs Law problem (all we have to work with, sofar).
We only want the forces on Q3. Dont worry about other forces.
Forces are additive, so we can calculate F32 and F31 and add
the two.
If we do our vector addition using components, we must resolve
our forces into their x- and y-components.
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Draw and label forces (only those on Q3).
Draw components of forces which are not along axes.
x
y
Q2=+50C
Q3=+65C
Q1=-86C
52 cm
30cm
=30
F31
F32Draw a representative
sketchdone.
Draw and label relevant
quantitiesdone.
Draw axes, showing
origin and directions
done.
Step 1: Diagram
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1 2
2
12
q q
F k12 r
Do I have to put in the absolute value signs?
x
y
Q2=+50C
Q3=+65C
Q1=-86C
52 cm
30cm
=30
F31
F32
Yes. Unless you like losing points.
Step 2: Starting Equation
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3 22
32
Q QF k ,32 r
repulsive
3 2
2
32
Q QF k32,y r
F 032,x (from diagram)
Can you put numbers in at this point? OK for this problem. You
would get F32,y = 330 N and F32,x = 0 N.
x
y
Q2=+50C
Q3=+65C
Q1=-86C
52 cm
r32=30cm
=30
F31
F32
Step 3: Replace Generic Quantities by Specifics
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3 1
2
31
Q QF k ,31
rattractive
3 1
231
Q Q
F k cos31,x r
Can you put numbers in at this point? OK for this problem. You
would get F31,x = +120 N and F31,y = -70 N.
(- sign comes from diagram)3 1
2
31
Q QF k sin31, y r
(+ sign comes from
diagram)
x
y
Q2=+50C
Q3=+65C
Q1=-86C
=30
F31
F32
Step 3 (continued)
r32=30cm
52 cm
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F3x = F31,x + F32,x = 120 N + 0 N = 120 N
F3y = F31,y + F32,y = -70 N + 330 N = 260 N
You know how to calculate the magnitude F3 and the angle
between F3
and the x-axis. (If not, holler!)
F3The net force is thevector sum of all the
forces on Q3.
x
y
Q2=+50C
Q3=+65C
Q1=-86C
52 cm
30cm
=30
F31
F32
Step 3: Complete the Math
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I did a sample Coulombs law calculation using three point
charges.
How do you apply Coulombs law to objects that containdistributions of charges?
Well use another tool to do that
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