Lecture 1-1Physics 241: Electricity and Optics

Lecture 0201: 10:30 – 11:20 (T,Th) 0301: 11:30 – 12:20 (T,Th)

Prof. Rolf Scharenberg (schrnbrg@purdue.edu) Office: PHYS Room 245

Textbook: Physics for Scientists and Engineers (6th edition ), volume 2 or newer by Paul A. Tipler and Gene MoscaExams: 2 evening one-hour exams and a two-hour final examQuizzes: during lectures using iClickers.Homework: CHIP (26 assignments)

Course Web Page:: http://www.physics.purdue.edu/phys241/CHIP website: http://chip.physics.purdue.edu/public/241/spring2010/

All Lectures take place in Rm. PHYS112 Lecture 0101: 9:30 – 10:20 (T,Th) Prof. Wei Xie (wxie@purdue.edu) Office: PHYS Room 246

Lecture 1-2

An Old (and Fundamental!) Question:

What is the Structure of Matter in the Universe?

Lecture 1-3Structure of matter in the Universe

scale ~ 10-15 m

scale ~ 10-10 m

Huge scale Iron

Wood

UniverseGravitational

Electromagnetic

Strong

Leptons: electron, muon, etc

Current building block

Lecture 1-4

Quantization of Charge

• Fundamental unit: elementary charge e,3,2,1, nneq

• An electron carries a charge of –e ; a proton carries a charge of +e

It is typically the electrons that move between objects.

19 191.602177 10 1.60 10e C C

• Coulomb (C): one coulomb is the amount of charge that is transferred through the cross section of a wire in 1 second when there is a current of 1 ampere in the wire.

Lecture 1-5

Conductor vs. Insulator

• Conductors: material in which electric charges can move around “freely”– Metals, tap water, human body, …

• Insulators: material in which electric charges are “frozen” in place– Air, glass, plastic, …

• Semi-conductor: material in which electric charges can move around but not as freely as in conductors– Silicon, germanium, …

Cu 1029 / m3

Ge 1019 / m3

Lecture 1-6Conservation of Charge

The net electric charge is conserved in any physical process. But …

• Charge can be transferred from one object to another.

• Individual charges can also, in fact, be “destroyed” or “created”, but not net charges

ee( annihilation)

( pair production)

24

2 24 2

( ) 2 ( ) ( )

( ) ( )

Zn s H aq SO aq

Zn SO aq H g

Lecture 1-7Coulomb’s Law

• Charges with the same sign repel each other, and charges with opposite signs attract each other.

• The electrostatic force between two particles is proportional to the amount of electric charge that each possesses and is inversely proportional to the distance between the two squared.

1 22

ˆq q

F k rr

q1 q2

r1,2

1,2

1,2r

229

0/1099.8

41

CmNk

where is called the permittivity constant.

• Coulomb constant:

1,2by 1 on 2

Lecture 1-8Warm-up Quiz

• One known charge Q1 = Q > 0 and the other unknown positive charge Q2 > 0 are held fixed at a separation d = R as shown.

• Another (non-zero) charge Q3 is introduced somewhere along the line connecting Q1 and Q2.

– Which of the following statements is true?

1. The force on Q3 can be zero only if Q3 is placed to the left of Q1.

5. The force on Q3 can never be zero, no matter what the (non-zero!) charge Q3 is.

2. The force on Q3 can be zero only if Q3 is placed between Q1 and Q2.

3. The answer to above depends on the sign of Q3.

4. The answer to above depends on the magnitudes of Q1 and Q2 .

Lecture 1-9

friction can cause electrons to move from one object to another.

Lecture 1-10

Charging by rubbing

Lecture 1-11Charging by induction

grounding

polarization by induction

Lecture 1-12

How strong are Coulomb forces?

• Electron and proton in a hydrogen atom

• Compare electric and gravitational forces2

2 2

p ee g

m meF k F G

r r electron and

proton

2392.27 10e

g p e

F ke

F Gm m

9 2 2 197

2

2

1

2

1 2

(8.99 10 / )(1.60 10 )(10 )

(5.3 10 )

eF k

Nm C CO N

mr

7

23 230

(10 )(10 ) /

(10 )/ ea F

O NO m s

O kgm

me = 9.11x10-31 kg,

mp =1.67x10-27 kg

Lecture 1-13Question

• A Human weight 120 lb, which of the following is correct?

a) A large fraction of the weight come from the attraction force between the charges on human body and earth.

b) All the weight comes the attraction force between the charges on human body and earth.

c) All the weight come from the gravitational forces. The electric forces are negligible.

Lecture 1-14

Principle of Superposition

1413121 FFFF

q1

q2

q3F13

F12

F1

• Add by components or

• Magnitude and direction separately by using trigonometry

Lecture 1-15Physics 241 – Sample Quiz A – Jan. 8, 2008

Two point charges are separated by distance d as shown. Where can you put a third charge of +1 C so that there is no net electric force acting on it? (Take Q > 0.)

e = 1.610-19 Ck = 8.99109 Nm2/C2

a) to the right of charge -Qb) to the left of charge 2Qc) between the two chargesd) some other placee) nowhere

2Q - Qd

Lecture 1-16Physics 241 – Sample Quiz B – Jan. 8, 2008

Two point charges are separated by distance d as shown. Where can you put a third charge of +1 C so that there is no net electric force acting on it? (Take Q > 0.)

e = 1.610-19 Ck = 8.99109 Nm2/C2

a) to the right of charge -2Qb) to the left of charge Qc) between the two chargesd) some other placee) nowhere

Q - 2Qd

Lecture 1-17Physics 241 – Sample Quiz C – Jan. 8, 2008

Two point charges are separated by distance d as shown. Where can you put a third charge of 1 C so that there is no net electric force acting on it? (Take Q > 0.)

e = 1.610-19 Ck = 8.99109 Nm2/C2

a) to the right of charge Qb) to the left of charge 2Qc) between the two chargesd) some other placee) nowhere

2Q Qd