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Physics 133 Electricity & Magnetism
Class #2
Today’s Concepts: A) Coulomb’s Law B) Superposi<on
Physics 133
Your instructional team
Instructor: Alex Rudolph
LAs: Bryan Flores James Munteanu
Yasmina Rousan
Zong Yu Wang
Physics 133
ENROLL IN PHY 299A RECITATION CLASSES
PHY 131 – PHY 132 – PHY 133 – PHY 235
If you are enrolled in PHY 131, PHY 132, PHY 133, or PHY 235 for Spring 2015 and need help, you are eligible to take a one unit supplementary course to assist you with concepts and problem solving skills.
PHY 299A-01 (PHY 131) Friday, 12:00PM-1:50PM Instructor: *Rosie Nawpar PHY 299A-02 (PHY 132) Friday, 10:00AM-11:50AM Instructor: *April Hankins PHY 299A-03 (PHY 133) Friday, 8:00AM-9:50AM Instructor: *Lana Stouklova PHY 299A-04 (PHY 235) Wednesday, 1:00PM-2:50PM Instructor: *Alex Small
! Recitation classes will provide students with the opportunity to discuss key concepts of the course in depth and in small groups.
! Classes are offered on a credit/no credit basis (no grades).
*Contact the Instructor listed above for a permission number to enroll in a recitation class.
Enrollment is limited!
Physics 133
This class will be taught in a way that is different from what you might have previously experienced
Very liLle <me in class will be spent on tradi<onal “lecture”
Rather, we will be engaging in “interac<ve learning strategies”: ac<vi<es designed and shown by research to improve your understanding of the basic physics concepts taught in the course
To help you prepare for this format of class, there will be on-‐line assignments due before each class to introduce the main concepts to you – more on that later
Physics 133 - Intro
Physics 133
Interactive Learning - what is it?
Interac<ve learning is a method of learning material in which you take an ac<ve role in mastering material
There has been considerable research regarding the use of ac<ve learning techniques to improve student learning of the basic concepts of physics
Some of you have already experienced this in previous Physics 130 classes or other science classes
Because this course is so interac.ve in nature, I don’t allow laptop use in class. Also, be sure to turn off or silence your cell phone before coming to class
Physics 133
60
65
70
75
80
85
90
Never 1-2 times 3-5 times Over 5 times
Fin
al
Gra
de
(per
cen
t)
Frequency of Cell Phone Use (per class)
3 Astronomy Classes (N=328) Spring 2012
Avg. Score no phone – Avg. Score phone users = 4.7 ± 1.4 %
The effect of cell phone use on grades*
*Similar results have been found for laptops. In addition, there is a “halo” effect: those around the user are affected also
Physics 133
Interactive Learning and “clickers” - a tool
Some of you may have had a chance to try out a new technology -‐ classroom response systems, or “clickers” (shown at right)
These are electronic devices that work like a remote control; I have a receiver aLached to my computer which can collect responses to ques<ons I will ask you during class
The “clickers” will also be used to take aLendance, so remember to bring them!
The use of clickers allows me to give you credit for par<cipa<ng in class, and also helps promote ac<ve learning through the use of “Think-‐Pair-‐Share” Ques<ons
Physics 133
Let’s try them out! If you have your “clicker” here today, take it out and
power it on Those of you who bought your clicker and used it today
will receive 1 point added to your overall grade for the class (out of 100)
You can s<ll receive this 1 point bonus if you get one and use it at next Tuesday’s class, so if you don’t have one yet, get one!
You can buy your clicker at the bookstore New clickers should be registered at
hLp://www.iclicker.com/registra<on Use your Bronco ID for your Student ID Used clickers can be registered on Tuesday between
5:15-‐5:25pm via “roll call” before class To avoid being dropped from class, do it by next week! Since you will be given credit for aLending and
par<cipa<ng in class, bring your clicker to class every day!
Physics 133
Attendance Quiz
Are you here today?
(a) yes
(b) no
(c) what exactly to you mean by “here”?
Here!
Physics 133
What is the best baseball team in Southern California?
(a) The Angels
(b) The Dodgers
A Simple Intelligence Test
See you in the World Series!
Physics 133
A chance to participate in an education study! To determine how instruc<onal style affects student learning, we have been conduc<ng an educa<on study of students in various PHY 133 sec<ons taught in different ways
Par<cipants in the study will take 2 assessments twice: once this week and once at the end of the quarter, plus complete short demographics and opinion surveys
All of these will be administered on-‐line via Blackboard; you should have received an e-‐mail containing instruc<ons
Par<cipa<on in the study is completely voluntary, but to encourage you to par<cipate, we are giving 2 points of extra credit to your overall grade if you complete all 4 assessments and the demographics and opinion surveys
Your iden<ty will remain confiden<al, and results will only be used in sta<s<cal aggregates
Your performance on the assessments will not affect your grade in any way There is no compensa<on for par<cipa<on in the study (other than the extra credit) but you will have the sa<sfac<on of helping us learn how to run these classes beLer
You may withdraw from the study at any <me If you have any ques<ons, contact me, Prof. Alex Rudolph, at [email protected]
Physics 133
SmartPhysics Overview
SmartPhysics includes: 1. Online PreLectures (animated lessons, completed
before class; due at 8am each class day)
2. Online CheckPoints (quizzes to check knowledge, completed before lecture; due at 8am each class day)
3. Online homework exercises (due by 5pm on Fridays)
Physics 133
How we’ll use smartPhysics
Reading:
READ the Textbook before each week and as needed for review
The reading assignments are also listed on the smartPhysics schedule for this course
If you don’t own the PHY 131-‐133 textbook by Halliday, Resnick, and Walker, you can use any other introductory physics text you like
Physics 133
How we’ll use smartPhysics
You will VIEW PreLectures before class.
Physics 133
How we’ll use smartPhysics
Next, you’ll complete a CheckPoint quiz before lecture to CHECK your understanding of the PreLecture.
Physics 133
How we’ll use smartPhysics
COME to class! The class will be more interac.ve because of the work
you’ve done before class.
Physics 133
How we’ll use smartPhysics
Next, you must complete assigned homework problems.
*A single assignment may contain mul.ple problems.
Physics 133
Immediate feedback ques<ons
Homework: Immediate Feedback
Physics 133
Purpose: Promote REFLECTION
These ques<ons serve as a test of your understanding of the
ques<ons posed as immediate feedback.
Aler first deadline Delayed feedback ques<ons turn
into immediate feedback ques<ons. 80% credit can be obtained by answering these ques<ons correctly before the
second deadline.
Homework: Delayed Feedback
Physics 133
Everything can be found online – GO THERE The syllabus can be found on the course website: you are responsible for
downloading and reading it!
The assigned reading can be found on the SmartPhysics website
You have web based pre-‐lectures and checkpoints due before every lecture You have web based homework due every Friday by 5pm
Informa<on about how SmartPhysics works can be found at: www.smartphysics.com/Content/smartPhysics/About/faq-‐students.html (or Google “smartphysics students faq”)
Be sure to complete the next prelecture and checkpoint before 8am on Tuesday
Register your clicker ASAP so you can get credit for your par<cipa<on
Two important websites: www.cpp.edu/~alrudolph/classes/phy133/
www.smartphysics.com
Physics 133 – Course structure
Physics 133
The midterm exams will be administered on-‐line via Blackboard: hLps://blackboard.cpp.edu/
There will be 4 midterm exams, one every other week, covering about two chapters of the book (the full schedule can be found on the smartPhysics course schedule)
Each exam will be available for a 24-‐hour period, and you will have one hour to complete each exam in one sipng
These exams will be open-‐book, open notes, but your work must be your own Aler each midterm exam, you will rework each exam as a homework problem set
for addi<onal credit on the exam The final exam will be given on Tuesday, June 9, 6:00-‐8:00pm in this room This will be a closed book, closed note exam This is the <me the university has alloLed for this final exam, and there will be no
makeup exams given If you have an irreconcilable conflict with this <me, please inform me ASAP
Midterm and final exams:
Physics 133 – Course structure
Physics 133
Grades will be assigned as follows: Pre-‐class ac<vi<es 10% Par<cipa<on/clickers 10% Homework (on-‐line and wriLen) 15% Midterms (on-‐line) 30% Final exam (June 9, 6:00-‐8:00pm) 35%
100%
Physics 133 – Course structure
Physics 133
In my office (8-‐220) ALR: T Th, 4:00-‐5:00pm Addi<onal <mes by appointment In the Physics tutoring room, 3-‐2011 LAs: TBD Many other LAs have office hours and you are free to aLend any of them. I will post the tutoring room (3-‐2011) schedule on the course website
Office hours:
We are here to help you
Physics 133
If you have questions, remember…
…or on one of the course websites!
Physics 133
Coulomb Law and Superposi<on
I think I found the interac<on between several (three or more) charges to be slightly confusing.
For some reason I am having a hard <me dealing with 2-‐D problems with the magnitude of forces, and would greatly appreciate if <me could be taken in
class to further explain the process.
everything was very informa<ve
I found the Force from Four Charges extremly challenging. Also, I find it very difficult to apply what I have learned from the BRIEF lecture videos. I prefer learning in the classroom sepng in which I can ask ques<ons to gain a beLer
understanding of the subject. Thank you.
Sum of the forces was a difficult topic to understand when there are many forces ac<ng on a single atom.
Stuff you asked about:
I found all of this difficult.
Physics 133
The force on a charge due to another charge is propor<onal to the product of the charges and inversely propor<onal to the separa<on squared.
r
The force is always parallel to a line connec<ng the charges, but the direc<on depends on the signs of the charges:
q2
q2 q1
q1
Like signs repel
Coulomb’s Law:
q2 q1
Opposite signs aLract q2 q1
221
rqqF ∝
Physics 133
r
q2
q2 q1
q1
Coulomb’s Law v. Gravity
q2 q1
q2 q1
221
rqqF ∝
r
m2 m1
F ∝
m1m2
r 2
Gravity always aLracts m2 m1
Electric forces depend on the rela<ve signs of the charges
Physics 133
Take two balloons and rub them both with a piece of cloth. Aler you rub them they will: A) ALract each-‐other B) Repel each-‐other C) Either – it depends on the material of the cloth
Balloons
Physics 133
Balloons
If the same thing is done to both balloons they will acquire the same sign charge. They will repel!
+ +
+ +
+ +
+ +
Physics 133
Coulomb’s Law
F1,2 =
kq1q2
r1,22 r̂1,2
2,1F
Examples:
If the charges have the same sign, the force by charge 1 on charge 2 would be in the direc<on of r12 (to the right).
q2 q1 2,1r
If the charges have opposite sign, the force by charge 1 on charge 2
would be opposite the direc<on of r12 (lel).
q1 q2 2,1F
2,1r
Our nota<on:
is the force by 1 on 2 (think “by-‐on”) is the unit vector that points from 1 to 2. 2,1F
12r̂
Physics 133
Coulomb’s Law question
Which charge on the right exerts the greatest force on the +Q charge on the lel?
A. +q only B. +2q only C. +4q only D. Two of these exert the same force E. None of the above
Physics 133
A: “The charge of right is 10 <mes greater than the charge on the lel, therefore it produces a greater force.” B: “Using Newton's Third Law, the force exerted on q by Q must be the same as the force exerted on Q by q; therefore, the two charges experience the same magnitude of force.” C: “Both charges experience repulsive forces because both charges are posi<ve. However, the charge of Q is greater than that of q thus, the force that Q exerts onto q is greater than the force of q onto Q.”
Checkpoint 1
Physics 133
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:
F2,1
F3,1
F4,1
F1
1F
...1,41,31,2 +++= FFF
Physics 133
What happens to the magnitude of the force on q1 if the sign of its charge is changed?
A) |F1| increases
B) |F1| remains the same
C) |F1| decreases
D) Need more informa<on to determine
Superposition Clicker Question
q1
q2
q3
q4
Physics 133
The direc<on 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
1F
...1,41,31,2 +++= FFF
1F
− ...1,41,31,2 −−−−= FFF
Physics 133
CheckPoint
q q
-Q +Q
+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
A: “The component forces in case 2 are equal and in opposite direc<ons, while the forces in case 1 are equal and in the same direc<on. So the net force in case 2 is zero and the net force in case one has posi<ve magnitude.”
B: “Q+ and Q-‐ basically cancel each others forces out so case two has a greater net force.”
C: “When calcula<ng the net force, the sign of the charges does not maLer. The only thing that maLers is their magnitude.”
D: “Because we don't know the sign of q we can't determine the magnitude of the net force on the center charge.”
Physics 133
CheckPoint
What is the direc<on of horizontal force on Q?
A) Fx > 0 B) Fx = 0 C) Fx < 0
x
y
3q
q q q
Q
Great job!
Four posi<ve charged par<cles are placed on a circular ring with radius 3 m as shown below. A par<cle with posi<ve charge Q is placed in the center of the ring
Physics 133
CheckPoint
Four posi<ve charged par<cles are placed on a circular ring with radius 3 m as shown below. A par<cle with posi<ve charge Q is placed in the center of the ring What is the direc<on of the ver<cal force on Q?
A) Fy > 0 B) Fy = 0 C) Fy < 0
A: “The y component of the net force will be greater than 0 because there are three charges pushing Q up, when only one is pushing it down.”
B: “The charges on the boLom act in opposi<on to the top charge of 3q. The summa<on of the boLom forces is equal to the top force. Therefore the net force is zero.”
C: “The y component of the three boLom charges are less than y component of the 3q at the top causing the net force on the par<cle to be in the nega<ve direc<on of y.”
x
y
3q
q q q
Q
Physics 133
Tutorial Question
A. B, D, A, C B. B, A, D, C
C. B, A, C, D D. D, A, C, B E. D, B, A, C
Physics 133
Coulomb’s Law question
Three charges are arranged at the corners of an equilateral triangle. Which diagram is a correct representa<on of the net Coulomb force on the charge in the lower right?
A. (1) B. (2) C . (3) D. (4) E . None of the diagrams are correct
Physics 133
Practice Problem
For each charge configura<on, draw the force vector ac<ng on the central charge on the y-‐axis. The charges on the x-‐axis are equidistant from the y-‐axis. Use the rules we have learned!
