PHYS 241D Electricity & Optics

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Calendar &

Syllabus are

under this link.

Lecture notes

will posted here.

Calendar/Schedule (posted under Course Information)

*Read the assigned

Chapter sections

prior to watching

the lecture.

*Note Exam Dates

*Homework is usually

due on Mondays at

11:59 PM.

Homework is done

through CHIP.

*Lecture quizzes are due

on Friday.

*Recitation quizzes are

due on Mondays.

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Syllabus

*Register your I-clicker on CHIP.

*Always bring your I-clicker to class.

*5% of your point total is based on

lecture quizzes.

CHIP: http://chip.physics.purdue.edu/public/241/spring2016/

Syllabus

Questions about grades, grade

checks, absences, etc… please see

Prof. Pyrak-Nolte

Send an e-mail to make an in-person appointment or an

audio appointment by Skype

Homework: http://chip.physics.purdue.edu/public/241/spring2016/

Syllabus (see course web page for complete syllabus)

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EMERGENCY PREPAREDNESS – A MESSAGE FROM PURDUE

Electric & Magnetic Fields

Electric and magnetic fields manifest their existence

through interactions with matter.

Maxwell’s Equations

Lorentz Force

o

div E E

0div B B

o o

Ecurl B B J

t

B

curl E Et

enclosed

oS

QE dA

0S

B dA

C S

BE dl dA

t

o o o

C S

EB dl I dA

t

Differential Form Integral Form

F qE qv B

James Clerk Maxwell

(1831–1879)

Lecture 1 Electric Charges & Coulomb’s Law

www.ehow.com/how_2180464_reduce-

static-cling.html

http://www.diyhappy.com/wp-

content/images/Lightning.bmp

http://andreacarlisle.files.wordp

ress.com/2012/02/static-

cling_dogs.jpg

Electric Charge

Electric charge is an intrinsic characteristic of the fundamental

particles that make up objects.

+

Positive Charge Negative Charge

•Electrically neutral: object contains equal amounts of

positive and negative charges

•Net charge: imbalance in charge

Electric Charge

Net charge of a system:

algebraic sum of all the charges

Law: Conservation of charge

The net charge of a closed system never changes

Electric Charge

Electric charge is quantized

•Elementary charge:

q ne, n 1,2,3,.....

e = 1.60602176462(63) x 10–19 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.

$ = n

http://scrapetv.com/News/News%20Pages/B

usiness/images/us%20penny.jpg

Charge of Particles Electron

Positron

Proton

Anti-Proton

Neutron

Photon

Up Quark

Down Quark

0

Particles Charge

Nucleus charge= +Ze, atom with Z electrons is neutral.

e

e

e

e

2

3e

1

3e

0

Proton charge: |e+ | = 1.60 x 10–19 C

Electron charge: |e- | = 1.60 x 10–19 C

Interaction of Charges

Charged objects interact by exerting forces on one another.

DEMO: Rod & Fur

Conductors versus Insulators

• Conductors: material in which electric charges can move around “freely.

• Insulators: material in which electric charges are “frozen” in place.

• Semi-conductor: material in which electric charges can move around but not as freely as in conductors.

• Super-conductor: no resistance to the movement of charge.

Interaction of Charges: Insulators

•Insulators: material in which electric charges are “frozen” in place.

Interaction of Charges: Insulators

Force of Repulsion Force of Attraction

Charges with opposite

electrical signs attract each

other.

Charges with the same

electrical sign repel each

other

Mobility of Charge

• Conductors: material in which electric charges can move around “freely.”

• Negatively charged plastic rod

will attract either end of the

electrically isolated copper rod

• Reason: charges in copper rod

can redistribute themselves.

Mobility of Charge

• Demo: Pie Tins

Charging by Induction

1. Bring a charged rod close to conductor.

2. Ground the conductor.

3. Break connection to ground, keeping the

charged rod in place

4. Remove the rod. The sphere is charged.

Charge Induction

• Demo: Chimes

Conducting

thread

Insulating

thread

Grounded

Charged

Coulomb’s Law of Electro-static Force

q1 q2

r

The electro-static force of attraction/repulsion has a

magnitude:

F kq1 q2

r2

Coulomb’s Law

k 1

4o 8.99x109Nm2 /C2

where:

and the permittivity constant is o 8.55x1012C2 / Nm2

Charles-Augustin de

Coulomb

(1736 - 1806)

Coulomb’s Law of Electro-static Force

*Each particle exerts a force of this magnitude on the other

particle.

*The two forces form an action-reaction pair.

1 2

2

1ˆ

4 o

Q QF r

r

Force repulsive

1

+

+

2

r F12

Force attractive

+

-

2

r

F12

1

Force by “1” on “2”

Coulomb’s Law of Electro-static Force

Force exerted by q1 on q2 at a distance r12

1 212 1,22

1,2

ˆ kq q

F rr

12 1,2

12 1,2

Same sign charges: is in the direction of .

Opposite sign charges: is in the direction opposite to .

F r

F r

q1, q2 in coulombs (C)

r12 in meters (m)

F12 in newtons (N)

12 F

Coulomb’s Law Analogous to Newton’s Equation of

Gravitation

F kq1 q2

r2F G

m1m2

r2

* k electro-static constant

* Inverse Square Law

* Charge

*Attractive/repulsive

depending on sign of

charges

*Two kinds of charges

*Dominates on small scale

* G gravitational constant

* Inverse Square Law

* Mass

*Always attractive

*One kind of mass

*Dominates on large scales

DIFFERS

Analogous

Electro-Static Force versus Newton’s Force of

Gravitational Attraction

DEMO: 2 x 4

Quiz 1. January 11, 2016

Two charges q = + 1 µC and Q = +10 µC are placed near each other as

shown below. Which diagram best depicts the electrostatic forces

acting on the charges?

+1 µC +10 µC

A

B

C

Quiz 1. January 11, 2016 Consider the two cases shown below. In both cases, a central

charge q has two charges of equal magnitude at equal distances

above and below it. In Case 1, the signs of the two outer

charges are opposite, and in Case 2 they are both positive. You

are not told the sign of the charge in the center.

Case 1 Case 2

+Q +Q

-Q +Q

q q

In which Case is the magnitude of

the net force on the center charge

bigger:

A.Case 1

B.Case 2

C.They are the same.

D.The answer depends on the sign of

the charge q in the center.

Problem Solving Strategies:

• Draw a clear FORCE diagram

• Use consistent units (meter, Coulomb, Newton)

• Remember that the force is a vector

• Look for (possible) symmetry

Principle of Superposition • When several point charges are put together, the total force on

any one charge is the vector sum of the each of the separate

forces acting on that charge.

F F21y F31y 2F21y

F 2 kQ1Q2

r2cos 300

F

2 9 109 N m2

C 2 (106C)2 0.866

1m 2

F 15.59 103N

F

F21

• Exercise:

Q2

Determine force on Q1

Q1=Q2=Q3=1C Q3

R=1m

600

Q1

y

x Determine force on Q1

F31

Electro-Static Force versus Newton’s Force of

Gravitational Attraction

Given such strong electrical interactions, atoms tend to remain uncharged. Matter prefers to be neutral.

Forces we experience, if not gravitational, are electrical in nature (even

though the net charge may be zero).

2

e

grav2

2

el

mF

e F

r

mG

rk

p

Fel

Fgrav

k

G

e2 / r2

memp / r2 1.35 1020 kg2

C2

(1.6 1019C)2

(1.7 1027Kg)(9.11031Kg)

Fel

Fgrav

2.3 1039 Fel 8.2 10-8N Fgrav 3.6 1047 N

Fel between the proton and the electron in a hydrogen atom in the ground state. From the Bohr model r=0.53 x 10-10 m.

Quiz 1. January 11, 2016

The nucleus of a Helium atom has a charge equal to

twice the proton’s charge. Let FN denote the magnitude

of the force the Helium nucleus exerts on one of the

electrons in a Helium atom, and Fe denote the

magnitude of the force one electron in the Helium atom

exerts on the Helium nucleus. Which of the following

statements concerning the magnitudes of FN and Fe is

true?

(A) FN < Fe

(B) FN = Fe

(C) FN > Fe

Quiz 1. January 11, 2016

Two charges q = + 1 µC and Q = +10 µC are placed near each other as

shown below. Which diagram best depicts the electrostatic forces

acting the charges?

+1 µC +10 µC

A

B

C

Conductors versus Insulators

• Demo 5A-04: Charges are more readily transferred by conductors