45429_Electric Charge & Coulombs Law

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

[email protected]

http://physics.mst.edu/classes/class_24.html
http://www.mst.edu/~pringlemailto:[email protected]:[email protected]://www.mst.edu/~pringle


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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.pdf


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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/


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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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E-Mail

You can send e-mail to me at [email protected]. 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 ([email protected] ).
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[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 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.


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.


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.html


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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

45429_Electric Charge & Coulombs Law

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