1

PHYSICS 1302W.400 - Spring 2018 Lecture 1

!Chapter 22

• Electric charge • Coulomb Law

Laboratory sections *will* meet this week. Discussion sections *will* meet Thursday.

2

!Lectures: Prof. Martin Greven

!1:25 pm to 2:05 pm in Tate B20

!Teaching Assistants: • Physics Tutoring Room: TAs will staff the Physics

Tutoring Room beginning the second week of classes. The Tutoring Room is the glass-walled room off the Tate Atrium, opposite the entrances to Tate B50. The TA schedule will be posted outside the Tutoring Room.

• (ask any TA on duty ! ) • starting next week.

This week: lecture today (Tuesday) no lecture Wednesday or Friday.

Next lecture is Monday, January 22.

The course code for Mastering Physics is: PH1302400S2018

MasteringPhysics

6

Quizzes!50 minute in-class quizzes:

Friday, February 9 Friday, March 2 Friday, March 30 Friday, April 20

Format:2 long problems5 MC problemsNo early, late, or make-up quizzes

7

Final Exam

! Thursday, May 10, 6:30 P.M. – 9:30 P.M.

!Final exam schedule online: http://onestop.umn.edu/calendars/final_exams/

common.html

Careful – this is a “common final”Does not follow the usual exam scheduleYou might have a conflict.

8

Introduction

! Course Philosophy • read it (text book) • think (Conceptual Questions & Lectures) • play with it (Labs) • Do it (Homework) • Talk about it (Discussion)

Lectures!Read first (for TOMORROW, read chapter 22) !Mostly Concepts !Peer Instruction !Clicker questions / reading quizzes !Demos

Please Participate

10

Background Needed for Course

! All of 1301 ! High school algebra

• manipulation of formulas • solutions of two equations with two unknowns • solution of quadratic equation

! Trigonometry • sine, cosine, tangent • Pythagorean theorem

! Calculus • derivatives • Integrals – including 3-dimensional !

! Geometry • straight lines • circles, spheres, etc.

11

Components of 1302

! Book: Mazur ! Lecture: MTW + some Fridays ! clickers ! Discussion: Thursday ! Quizzes: four Fridays, four Thursdays ! Lab ! Homework (online) ! final exam

Physics 1302: Lecture 1

Grades!The grade for Physics 1302 will be based on in-class

questions, laboratory, four quizzes, discussions, and a final examination.• In-class problems (using PRS transponders): 5% • Laboratory participation and lab reports: 20% • homework (MP): 10%• Exam Scores: 65%

• Exam scores will be calculated by two methods and you will be given the higher score:

• Method 1: 4 Midterm Exams 10% each; Final Exam 25% Method 2: Three highest midterm exam scores: 10% each; Final Exam 35%

13

Academic Honor

http://www1.umn.edu/regents/policies/academic/Student_Conduct_Code.html

Clicker:turn in on,wait for it, “Ready”

wait for instructor to start a sessionbig green GO button

once session is runninghold down POWER press and hold power buttonchannel name flashesenter AB.

then it is set for all time.

I have my clicker here today.

A) YES

B) NOPoints : +1 incorrect answer +2 correct answer +0 no answer

17

Course Outline

!Electrostatics !Currents, DC Circuits !Magnetism !E&M

18

19

20

• Add some negative charge

– add electrons

• Inside a conductor charges (electrons) are free to move• The electroscope is made of conductors

– conducting main electrode – 2 conducting gold leaves

• Add some positive charge

– subtract electrons

- -

+ +

21

• Add some positive charge to negatively charged leaves

– subtract electrons - -

22

Electric charge is quantized.

Elementary particles have Three Intrinsic Properties:

1. Mass 2. Angular momentum 3. Electric charge

The “quantum of charge”, e, is approximately 1.6 x 10^(-19) Coulomb.

23

Examples: - neutron decay - electric current

24

Coulomb used a sensitive torsion balance to investigate the forces between charged balls.

1736 - 1806

Charles-Augustin de Coulomb

12212

2112 ˆ

41

rrqq

Foπε

= “12” convention is not standardized.

12212

2112 ˆ

41

rrqq

Foπε

=

Force on 2 due to 1

12r̂ Unit vector pointing from 1 to 2.

28

Concept test

!A charged ball Q1 is fixed to a horizontal surface.

! another massive, charged ball Q2

has an equilibrium position at a distance d directly above Q1.! gravity not switched off.

Q2

Q1

gd

A) The charge of Q2 has the same sign of the charge of Q1 B) The charge of Q2 has the opposite sign as the charge of Q1 C) Cannot determine the relative signs of the charges of Q2 & Q1

29

Concept testQ2

Q1

gd

A) The charge of Q2 has the same sign of the charge of Q1 B) The charge of Q2 has the opposite sign as the charge of Q1 C) Cannot determine the relative signs of the charges of Q2 & Q1

Thursday Discussion

The point:

All the “F=ma” stuff from

Phys 1301 applies, now

we have a new force,

the electric force.

In this problem, the electric field in a region is constant, which means the electric force on the electron is constant, and if the force is constant so is the acceleration. It is the same as 2-d ballistics!

Gravitational vs. Electrical Force

FelecFgrav

= q 1q 2m 1m 2

1

4 πε 0G

r

F Fq1

m1

q2

m2

Felec = 1

4πε0 q1q2r2

F grav = G m 1m 2

r 2

→

For two electrons: q = 1.6 × 10-19 Cm = 9.1 × 10-31 kg

→ FF

elec

grav

= × +417 10 42.

33

Two charges: q = + 1 μC and Q = +10 μC

Which of the following diagrams best describes the forces?

example:

+10 μC+1 μC

a)

b)

c)

34

+ -

r = 0.53x10-10 m

Qp=1.6x10-19 C Qe = -1.6x10-19 C

Coulomb’s Law

! Magnitude of the force between charges q1 and q2 separated a distance r:

! F = k q1q2/r2 k = 9x109 Nm2/C2

! Force on electron in Hydrogen atom

35

Concept test : Coulomb’s Law

!What is the direction of the force on the proton due to the electron?(1) Left (2) Right (3) Zero

!What is the magnitude of the force on the proton due to the electron?

+ -

r = 0.53x10-10 m

qp=1.6x10-19 C qe = -1.6x10-19 C

F= _______ NF

36

CT: Coulomb’s Law

! What is the direction of the force on the proton due to the electron?(1) Left (2) Right (3) Zero

! What is the magnitude of the force on the proton due to the electron?

+ -

r = 0.53x10-10 m

qp=1.6x10-19 C qe = -1.6x10-19 C F = 8.2 x 10-8 N F

q1

Superposition

qFtotal = F1 + F2 + ...q2

F1

F2

F

38

CT

+ --

A positive and negative charge with equal magnitude are connected by a rigid rod, and placed near a large negative charge. What is the net force on the two connected charges?

1) Left 2) Zero 3) Right

+ --

Positive charge is attracted (force to left)

Negative charge is repelled (force to right)

Positive charge is closer so force to left is larger.

A positive and negative charge with equal magnitude are connected by a rigid rod, and placed near a large negative charge. What is the net force on the two connected charges?

1) Left 2) Zero 3) Right

221

rqkqF =

40

Concept test : Induced Dipole

1) Nothing2) Attracted to charged sphere.3) Repelled from charged sphere.

An uncharged conducting sphere is hung next to a charged sphere. What happens when the uncharged sphere is released?

41

CT: Induced Dipole

Negative charge attracts + repels -

An uncharged conducting sphere is hung next to a charged sphere. What happens when the uncharged sphere is released?

42

CT: Induced Dipole

ii) Since + is closer, attractive force is strongesti) Negative charge attracts + repels -

An uncharged conducting sphere is hung next to a charged sphere. What happens when the uncharged sphere is released?

CT: Induced Dipole

1) Negative charge attracts + repels –

2) Since + is closer, attractive force is strongest

An uncharged conducting sphere is hung between a charged sphere and a grounded sphere and held midway between the two. What happens when the uncharged sphere is released?

To Do

! Work through chapter 22! tip : prepare homework notebook for the term.! HW1 due soon on Mastering Physics

Next lecture: Monday, Jan 22 Discussion: Thursday, Jan 18