Welcome…

…to Physics 24.

PHYSICS 24Engineering Physics II

 

Winter/Spring 2009 

Dr. Allan PringleCourse Instructor

Room 122 Physics, 341‑4031http://www.mst.edu/~pringle

pringle@mst.edu

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

Today’s agenda:

Course overview.

Physics 23: a reminder.

Coulomb’s Law (electrical force between charged particles).After today, you must be able to calculate the electrical forces between two or more charged particles.

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 notes and 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!

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 UMR catalog reads: “An introduction to electricity, magnetism, and light, with emphasis on topics needed by engineering students. Prerequisites: Physics 23, Math 21 or 15.”

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.

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.

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 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.

The three hour exams are from 5:00-6:15 pm on exam days.

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 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

The Final Exam will be Wednesday, May 13, from 8:00-10:00 am.

There are NO MAKEUPS for the final exam. I am not allowed to 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 calendar TODAY, and make sure that time remains free!

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.

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 50Ten Quizzes 50Homework 50Boardwork 100Labs 150

Total 1000

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.

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.

Homework and the PLC Homework help will be available at the Physics Learning Center (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. The PLC 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.)

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 exam or 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 work which 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 grade cutoffs be lowered by 1 point so you can get the next higher grade; it won’t happen. However, scoring mistakes (points added up wrong, score recorded incorrectly) can be corrected at any time.

No Labs This Week! Don’t go to lab this week! Labs start on Tuesday of next week.

No Labs This Week! Don’t forget about lab this week.

Your First Line of Defense Your recitation instructor is your first line of defense.

He/she is there to help you succeed. Everything—homework, exams, labs, regrade requests, etc., will be channeled through your recitation instructor.

your recitation instructor

E-Mail 

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 be resolved, you may take them up with the Physics Department Chairman, Dr. Dan Waddill (waddill@mst.edu ).

We don’t ask much outof you in this class…

Announcements

Make sure you pick up the handouts containing:Course HandbookSyllabusStarting EquationsSpecial 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, a blank handout problem sheet, and the starting equation sheet. Nothing else. Boardwork for points begins Thursday.

Today’s agenda:

Course overview.

Physics 23: a reminder.

Coulomb’s Law (electrical force between charged particles).After today, you must be able to calculate the electrical forces between two or more charged particles.

Prologue

Tell me some things you recall from “last”* semester…

*or whenever you took your previous physics class

Newton’s 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

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” aren’t going to go away!

in out U Q W

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.

Today’s agenda:

Course overview.

Physics 23: a reminder.

Coulomb’s Law (electrical force between charged particles).After today, you must be able to calculate the electrical forces between one or more charged particles.

like charges repel

unlike charges attract

charges can move but charge is conserved

Law of conservation of charge: the net amount of electric charge produced in any process is zero.

There are two kinds of charge. + -

Electric Charge

Static Electricity

Properties of charges

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).

+ -

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 were the ones that moved easily. So who’s the clown who decided electrons have negative charges?

Why is the fundamental unit of charge e = 1.602x10-19 C. Why not just 1?

And yes, he really flew the kite in the thunderstorm. See here.

Franklin’s 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 associated with electrons.

Oh, and the next two people who tried the kite experiment were killed in the process.

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

Coulomb’s Law

1 22

12

q qF k12 r

Coulomb’s law gives the force (in newtons) between charges q1

and q2, where r12 is the distance in meters between the charges,

and k=9x109 N·m2/C2.

We’ve seen attractive and repulsive electrical forces at work.

Coulomb’s law quantifies the magnitude of the electrostatic force.

To make this into a “really good” starting equation I should specify “repulsive for like,” but that makes it too wordy. You’ll just have to remember how to find the direction.

1 22

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/N·m2.

Remember, a vector has a magnitude and a direction.

I could write Coulomb’s Law like this…

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

++

+-

--

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 22

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 to make you hear little voices in your head.

A sample Coulomb’s 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.

Solving Problems Involving Coulomb’s Law and Vectors

You may wish to review vectors (on your own).

x

y

Q2=+50C

Q3=+65C

Q1=-86C

52 cm

60 cm

30

cm

=30º

Example: Calculate the net electrostatic force on charge Q3 due to the charges Q1 and Q2.

Finish, thenSkip to slide 43.

Step 0: Think!

This is a Coulomb’s Law problem (all we have to work with, so far).

We only want the forces on Q3. Don’t 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.

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

60 cm

30

cm

=30º

F31

F32Draw a representative sketch—done.

Draw and label relevant quantities—done.

Draw axes, showing origin and directions—done.

Step 1: Diagram

1 22

12

q qF k12 r

<complaining> “Do I have to put in the absolute value signs?”

x

y

Q2=+50C

Q3=+65C

Q1=-86C

52 cm

60 cm

30

cm

=30º

F31

F32

Yes. Unless you like losing points.

Step 2: Starting Equation

3 2232

Q QF k ,32 r

repulsive

3 2232

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

r31 =60 cm

r 32=

30

cm

=30º

F31

F32

Step 3: Replace Generic Quantities by Specifics

3 1231

Q QF k ,31 r

attractive

3 1231

Q QF 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 1231

Q QF k sin31, y r

(+ sign comes from diagram)

x

y

Q2=+50C

Q3=+65C

Q1=-86C

=30º

F31

F32

Step 3 (continued)

r 32=

30

cm r

31 =60 cm

52 cm

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=+50C

Q3=+65C

Q1=-86C

52 cm

60 cm

30

cm

=30º

F31

F32

Step 3: Complete the Math

I did a sample Coulomb’s law calculation using three point charges.

How do you apply Coulomb’s law to objects that contain distributions of charges?

We’ll use another tool to do that…