Lecture01 Course Introduction; Electric Charge; Coulomb's Law

8/3/2019 Lecture01 Course Introduction; Electric Charge; Coulomb's Law

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Welcome

to Physics 24.


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PHYSICS 24

Engineering Physics II

Fall 2011

Dr. Allan PringleCourse Instructor

Room 122 Physics, 341-4031

http://www.mst.edu/~pringle

[email protected]

http://physics.mst.edu/classes/class_24.html


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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 important

information 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.


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Text

The text is University Physics, Young and Freedman, 12th

Edition.

Course Description

Physics 24 is a 4-hour, calculus-based introductory physics

course. This is the second semester of the Engineering

Physics sequence. The S&T catalog reads: An introduction to

electricity, magnetism, and light, with emphasis on topics

needed by engineering students. Prerequisites: Physics 23,Math 21 or 15.


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Course Schedule

Physics 24 lectures are Monday and Wednesday.Recitations are Tuesday and Thursday. Labs take place

throughout the week. Homework assignments are being

handed out today and are posted on the course web site.

Section Changes

For the first week or two of class, see the office staff in the

department office (102 Physics) to make section changes.

Dont come to me this week asking to make a section

change, because Ill just ask why you werent listening

in lecture, and then send you to 102 Physics!


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Examinations

There will be four exams worth 200 points each. The first threeare during the semester; the fourth is the final exam. Exam

rooms will be announced later.

The exams will cover concepts and definitions, assigned

problems with minor numerical changes, and problems similar

to those assigned but requiring a deeper understanding of

concepts 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 first three exams are from 5:00-6:15 pm on exam days.

Tuesday, September 20

Tuesday, October 18

Tuesday, November 15

If you have a schedule conflict, you need to resolve it now. You

can drop Physics 24, drop the other class causing the scheduleconflict, or take the exam from 5:30-6:45 pm on exam day.


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There are only two circumstances under 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 nomakeups) 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


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The Final Exam will be Tuesday, December 13, from 1:30 pm

3:30 pm.

There are NOMAKEUPSfor the final exam (the lowest exam

score is dropped, so if you cannot take the final, that is your

dropped score).

So reserve that time in your calendarTODAY, and make sure

that 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, usually* at 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.

*See your recitation instructor right away if there will be issues with working your homework at the blackboard.


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Letter grades for Physics 24 will be assigned as follows:

895.0- up A (89.50%)

795.0 894.999 B (79.50%)695.0 794.999 C (69.50%)

595.0 694.999 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 600End-Material Test 50

Ten Quizzes 50

Homework 50

Boardwork 100

Labs (lowest dropped) 150

Total 1000

Grade cutoffs willnot be lowered!


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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 townon 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, October 3, 2011.

The last day to withdraw from this class is Friday, November

11, 2011.

Drop Policy

Students with inadequate attendance may* be dropped. Any

student who has inadequate attendance, as evidenced by

missing 7 classes or a total of 7 graded assignments of anykind (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.

*But do not rely on being dropped. If you need to drop, do it yourself.


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Homework and the PLC

Homework help will be available at the Physics LearningCenter (PLC). You may excel in the course without ever

setting 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 Wed., Aug. 24. (One

recitation instructor will be available Monday evening, Aug.

22.)

I had to study ten hours a week for this four-hour class!

(Student complaint on spring 2005 course evaluations.)


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Regrade Requests

If you want an assignment regraded, please write the reason for

the request on a sheet of paper, staple it to the assignment, andreturn it to your recitation instructor (see course handbook for

deadlines).

Specify which problem you want regraded, and provide adetailed written statement as to why the original workwhich

appeared 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 Monday of next week.

No Labs This Week!

Dont forget about labs!


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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.

yourrecitationinstructor


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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 be

resolved, you may take them up with the Physics Department

Chairman, Dr. Dan Waddill ([email protected] ).


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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 Handbook

Syllabus

Starting Equations

Special Homework assignments.*

Go to Blackboard orhttp://physics.mst.edu/currentcourses/labs.html to

get a lab schedule. There are no labs this week.

Your recitation instructor will call students to the board

tomorrow. You may use your calculator, a blank handoutproblem sheet, and the starting equation sheet. Nothing else.

We do understand that this is only the second day of class.

*If you lose your Special Homework assignments, you can

download them from the Physics 24 web site.


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From the syllabus:

1. Tuesday, August 2321: 5, 18, 23, 72, Special Homework #1


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Prologue

Tell me some things you recall from last* semester

*or whenever you took your previous physics class

y Newtons Laws

F ma!&

&

y energy and its conservation

21KE mv

2

!2

spring

1U ks

2

! gravU mgy!

? Af i other i fE E W p !E K U!


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y laws of thermodynamics

y 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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y like charges repel

y unlike charges attract

y charges can move but charge is conserved

Law of conservation of charge: the net amount of electric

charge produced in any process is zero. (Not on your starting equationsheet, but a fact that you can use any time.)

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 is e = 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 good

terminology 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.


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Coulombs Law

1 2

2

12

q qF k

12 r

!

Coulombs law gives the force (in newtons) between charges q1

and q2, where r12 is the distance in meters between thecharges, and k=9x109 Nm2/C2.

Coulombs law quantifies the magnitude of the electrostatic

force.


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a note on starting equations

1 2

2

12

q qF k12 r

! is on your starting equation sheet.

In general, you need to begin solutions with starting equations.

You may begin with any correct variant of a starting equation.

For example, is legal and may be used.

A B

2

Q QF

kE D!

Dont get hung up about starting a problem with an equation

which is an exact copy of one from the OSE sheet.


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To make this into a really good starting equation I should also

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/(4TI0),

where I0=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 q1and 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 and

signs 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 littlevoices 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.

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, and

techniques you learned in Physics 23. 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=+50QC

Q3=+65QC

Q1=-86QC

52 cm

30c

m

U=30

Example: Calculate the net electrostatic force on charge Q3due to the charges Q1 and Q2.

Finish, thenSkip to slide 46.


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Step 0: Think!

This is a Coulombs Law problem (all we have to work with, so

far).

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=+50QC

Q3=+65QC

Q1=-86QC

52 cm

30cm

U=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=+50QC

Q3=+65QC

Q1=-86QC

52 cm

30cm

U=30

F31

F32

Yes. Unless you like losing points.

Step 2: Starting Equation


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3 2

2

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=+50QC

Q3=+65QC

Q1=-86QC

52 cm

r32=30cm

U=30

F31

F32

Step 3: Replace Generic Quantities by Specifics


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3 1

231

Q QF k ,31 r

attractive

!&

3 1

2

31

Q Q

F k cos31, x r! U

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 12

31

Q QF k sin31, y r

! U

(+ sign comes from

diagram)

x

y

Q2=+50QC

Q3=+65QC

Q1=-86QC

U=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 the

vector sum of all the

forces on Q3.

x

y

Q2=+50QC

Q3=+65QC

Q1=-86QC

52 cm

30cm

U=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 contain

distributions of charges?

Well use another tool to do that

Documents

Lecture01 Course Introduction; Electric Charge; Coulomb's Law