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PHYSICS 632 SUMMER 20089:00 – 10:50 Room 203Electricity, Magnetism

and Light

Richard A. Lindgren, Office Room 302

Text: Halliday, Resnick, and Walker, 7’th edition, Extended Version, Starting with Chapter 21 on Charge.

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Your Goals and Interest• get a degree• crossover teaching• fill in knowledge gaps• review, learn new teaching ideas• peer learning• modeling• inquiry learning• group learning• new experiments and demos• computer technology• solidify concepts• learn how to do problems.

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Assignments• See syllabus for daily homework assignments using WebAssign.• Work on problems in recitation after lecture.• Discuss with TA’s and others in the apartments at night. Due at 8:00

am next day.• Algebra, trigonometry, unit vectors and vectors, derivatives and

integration review.• How do you improve problem-solving skills. Lots of practice and more

practice.• Quizzes on second three Tuesdays and final on WebAssign, 30%

conceptual questions, 70% problem oriented.• There will also be 4 homework assignments due in August and

September.• Final exam at the end of September on WebAssign.

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Class Organization

• Cartoon• Demonstrations• Lecture and discussion• Experiments and demos with explanations• Hand-worked problems on Elmo• Clicker/Polling problems

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References and other Texts

• Calculus-Based Physics II (download)– Schnick– http://www.anselm.edu/internet/physics/cbphysics/downloadsII.ht

ml• Physics for Scientists and Engineers, 6th Edition

– Serway, Jewett• Physics for Scientists and Engineers, 3rd Edition

– Fishbane, Gasiorowicz, Thornton• Physics for Scientists & Engineers, 4th Edition

– Giancoli• Principals of Electricity

– Page, Adams

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Lecture 1 Charge Ch. 21• Electrostatics: study of electricity when the charges are not in motion. Good place tostart studying E&M because there are lots of demonstrations.

• Cartoon - Charge is analogous to mass• Demo - Large Van de Graaff

• Math Review and Tooling up

• Topics–What is electric charge? Point objects, Size. Atomic model–Methods of charging objects. Friction,Contact, Induction, Machines–Instruments to measure charge–Quantization of charge and conservation of charge–Coulombs Law and examples–Principle of superposition and examples

• Demos• Elmo

–Two pennies–Three charges in a plane–Hanging path ball

• Polling

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Charged Hair Van de Graaff Demo

• How does this gadget produce a mini-lightning bolt?• What upward forces are keeping your hair up?• How are these forces produced?• Why do the hair strands spread out from each other?• Why do they spread out radially from the head?• Is hair a conductor or insulator? How can we find out? Does it depend if

is wet or dry.• To understand what is going on we need to know about charge and we

need a model of electricity.

• Need a female teacher to come forward.

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• “In the matter of physics, the first lessons shouldcontain nothing but what is experimental andinteresting to see. A pretty experiment is in itselfmore valuable than 20 formulae.” Albert Einstein

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Charge• What is charge?• How do we visualize it.• We only know charge exists because in experiments electric forces cause objects to move.• Charge is analogous to mass in mechanics. We know how it behaves, but we don’t know what it really is. The same is true for charge..• Need more experiments to get the point across.

• UVa Electrostatics Kit(http://people.virginia.edu/~ral5q/classes/phys632/summer08/lecture_1.html)• Rub a Balloon• 2 x 4 on glass.

• Need a model

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More on charging using the UVaElectrostatic Kit

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Bohr Model

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3 protons, 3 neutrons

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More complicated than just protons,neutrons, and electrons, but this will

suffice.• Electron: Considered a point object with radius less than 10-18 meters with electric

charge e= -1.6 x 10 -19 Coulombs (SI units) and mass me= 9.11 x 10 - 31 kg

• Proton: It has a finite size with charge +e, mass mp= 1.67 x 10-27 kg and with radius– 0.805 +/-0.011 x 10-15 m scattering experiment– 0.890 +/-0.014 x 10-15 m Lamb shift experiment

• Neutron: Similar size as proton, but with total charge = 0 and mass mn=– Positive and negative charges exists inside the neutron

• Pions: Smaller than proton. Three types: + e, - e, 0 charge and radius– 0.66 +/- 0.01 x 10-15 m

• Quarks: Point objects. Confined to the proton and neutron,– Not free– Proton (uud) charge = 2/3e + 2/3e -1/3e = +e– Neutron (udd) charge = 2/3e -1/3e -1/3e = 0

– An isolated quark has never been found

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• Normally atoms are in the lowest energy state. This means thatthe material is electrically neutral. You have the same number ofelectrons as protons in the material.

• How do we change this?

• How do we add more electrons than protons or removeelectrons?

• There are several different ways.

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More on our Model of electricity

1cm long and a radius of 0,005 cm 1cm long and a radius of 0,005 cm

Copper atom:Copper atom:Z=29(protons), N= 34(neutrons),Z=29(protons), N= 34(neutrons),29 Electrons29 Electrons

Carbon or diamond - poor conductor

Copper (Face Centered Cube) - good conductor

Consider solid material like a piece of copper wire. The proton core is fixed in position in a lattice like structure. In a conductor, the valence electrons are free to move about. How many electrons are free to move about?

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Summary Comments

•Silk(+) on teflon(-)

•Silk (-) on acrylic (+)

•Wood doesn’t charge

•Charged objects always attract neutral objects

•Show Triboelectric series

•Not only chemical composition important, structure of surface isimportant - monolayer of molecules involved, quantum effect.(nanotechnology)

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Triboelectric serieshttp://www.sciencejoywagon.com/physicszone/lesson/07elecst/static/triboele.htm

Positive (Lose electrons easily)

Air

Human Hands

Asbestos

Rabbit Fur

Glass

Mica

Acrylic

Human Hair

Nylon

Wool

Fur

Lead

Silk

Aluminum

Paper

Cotton

Steel

Wood

Amber

Sealing Wax

Hard Rubber

Nickel, Copper

Brass, Silver

Gold, Platinum

Sulfur

Acetate, Rayon

Polyester

Styrene

Orlon

Saran

Balloon

Polyurethane

Polypropylene

Vinyl (PVC)

Silicon

Teflon

Negative (Gains electrons easily)

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Electrostatics is based on 4 four empiricalfacts

• Conservation of charge

• Quantization of charge

• Coulombs Law

• The principle of superposition

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Conservation of charge

• Rubbing does not create charge, it is transferred from object toanother

• Teflon negative - silk positive• Acrylic positive - silk negative Demo: Show electronic electroscope with cage: gives magnitude and sign of charge. Use teflon and silk to show + and -.

• Nuclear reactions γ0 = e+ + e-

• Radioactive decay 238U92 = 234Th90 + 4He2

• High energy particle reactions e- + p+ = e- + π+ + n0

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What is meant by quantization of charge?• Discovered in 1911 by Robert A. Millikan in the oil drop experiment

• The unit of charge is so tiny that we will never notice it comes inindivisible lumps.

• Example: Suppose in a typical experiment we charge an object upwith a nanoCoulomb of charge (Q = 10-9 C). How manyelementary units, N, of charge is this?

N =Q

e=

10!9

C

+1.6 "10!19

C= 0.625 "10

10= 6.25 "10

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Six billion units of charge

e = +1.6 !10"19CElementary unit of charge

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Coulombs Law

In 1785 Charles Augustin Coulomb reported in the Royal Academy Memoires using atorsion balance two charged mulberry pithballs repelled each other with a force that isinversely proportional to the distance.

q2q1 r RepulsionRepulsion

RepulsionRepulsion

AttractionAttraction

++ ++

++

-- --

Lab ExperimentLab Experiment

Spheres sameSpheres same as points as points

F=kq1q2

r2 where k = 8.99 !109Nm2 /C2

--Point chargesPoint charges

Also F=1

4!"0

q1q2

r2 where "0 = 8.85 #10-12 C2 / N . m2

Electrical Permittivity of vacuum

F=Gm

1m

2

r2

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Coulombs LawTwo Positive Charges

(Equivalent to the weightof a long strand of hair)

F=kq1q2

r2

1 nC1 nC 1 cmq2q1 r

• What is the magnitude of the force between two positive charges,each 1 nanoCoulomb, and 1cm apart in a typical demo?

RepulsionRepulsion

F =8.99 !10

9 Nm2

C2( ) 10"9

c( )2

10"2m( )

2= 9 !10

"5N

• What is the direction of the force?1 nC1 nC 1 cm

What is the direction of the force in unit vector notation?

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F =8.99 !10

9 Nm2

C2( ) 10"9

c( )2

10"2m( )

2= 9 !10

"5N

1 nC1 nC 1 cm

• What is the direction of the force in unit vector notation?

x

y

j

i

F1= !9 "10

!5N i

1 2

F2= +9 !10

"5N i

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Coulombs Law in vector notation.

!F12= k

q1q2

r2

r12

where:

12F!

is the force exerted by particle 1 on particle 2,

12r is a unit vector in the direction “from 1 to 2”, and

k, q1, and q

2 are defined as before (the Coulomb constant, the charge on particle 1, and the

charge on particle 2 respectively).

!F

12=

8.99 !109 Nm

2

C2( ) 10

"9c( ) 10

"9c( )

10"2m( )

2r12= +9 !10

"5N r

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The positive sign means the force on q2 is in the same direction as the unit vector

.q1 . q2

r

r12

!F12

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How do you write the Force on q1

!F21= k

q1q2

r2

r21

where:

!F21

is the force exerted by particle 2 on particle 1,

r21

is a unit vector in the direction “from 2 to 1”, and

k, q1, and q

2 are defined as before (the Coulomb constant, the charge on particle 1, and the

charge on particle 2 respectively).

!F

21=

8.99 !109 Nm

2

C2( ) 10

"9c( ) 10

"9c( )

10"2m( )

2r12= +9 !10

"5N r

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The positive sign means the force on q1 is in the same direction as the unit vector.

.q1 .q2

r

r21

!F21

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Principle of SuperpositionThree charges In a line

ExampleThree charges lie on the x axis: q1=+25 nC at the origin, q2= -12 nC atx = 2m, q3=+18 nC at x=3 m. What is the net force on q1 and the direction ofthe force? We simply add the two forces keeping track of their directions.

1 2x

3

= 8.99 !109 Nm2

C2( )(25 !10"9

C)"12 !10"9

C

(2m)2

r21 +18 !10"9

C

(3m)2

r31#$%

&'(

= 8.99 !109 Nm

2

C2( )(25 !10

"9C) "3!10

"9C r21 + 2 !10

"9C r31( )

= !2.25 "10!7N r

21(Directed in the positive x direction)

!Fnet ,1

=!F21+!F31

!Fnet ,1 = k

q1q2

r21

2r21+ k

q1q3

r31

2r31= kq

1

q2

r21

2r21+q3

r31

2r31

!

"#$

%&

r21

r31

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Uniformly charged metal spheres of Radius R

F=kq2

(r)2

F!kq2

(r+2R)2

Demo: Show uniformity of charge around sphere using electrometer. Demo: Show charging spheres by induction using electrometer

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Coulombs LawTwo Pennies without electrons

What is the force between two 3 gm pennies one meter apart if we remove all the electrons from the copper atoms?

(Use Elmo)

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What is the force on charge q1 due to many othercharges lying in a plane. I would like to show you

another notation.

-q3

+q2+q1

r13

r12

r12

r13

-q3

+q2+q1

!F13

!F12

!Fnet

!Fnet

=!F12+!F13

!F12=kq

1q2

(r12)2r12

!F13=kq

1q3

(r13)2r13

!Fnet

=!F1i

i=2

N

!

!Fnet

!F13

!F12

In practice you find the x and y comonents of all the vectors and add them up

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Question: What is the net force on q1 and in what direction?

Hint : Find x and y components of force on q1 due to q2 and q3 and addthem up.

q1= + 1 nC q2= + 1 nC

+y

+x

5 cm

2 cm

1 cm

q3= - 2 nC

Example: Three point charges in a plane (Use Elmo)

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In an atom can we neglect the gravitational force between theelectrons and protons? What is the ratio of Coulomb’s electricforce to Newton’s gravity force for 2 electrons separated by a

distance r ?

Fc

Fg=ke2

Gm2

qq r mm r

Fg=Gmm

r2

Fc=kee

r2

Fc

Fg=

8.99 !109 Nm2

C2( ) 1.6 !10"19

C( )2

6.67 !10"11 Nm2

kg2( ) 9.1!10"31

kg( )2

= 4.2 !1042

Huge number - pure number - no units

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Why are neutral objects always attracted to positiveor negative charged objects.

For example:•Rubbed balloon is attracted to wall•Comb is attracted to small bits of paper•Clothes in the dryer stick together.

Demo: Put wood on the spinner and place charged teflon and plastic rods near it.

What is the explanation of all of these phenomena?

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Explanation: The neutral objects atoms and molecules orientthemselves in the following way so that the Coulomb forces due to attraction are greater thanthose due to repulsion because the latter are further away. (Inverse square Law)

++++++

Acrylic Rod Wooden block

++++++

------

++++++

Acrylic Rod Wooden block

AttractionRepulsion

Attractive forces >> Repulsive Forces

2

21

r

qkqF =

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Problem: Two equally charged hanging pith balls (Use Elmo)

• Show induction: Click onhttp://people.virginia.edu/~ral5q/classes/phys632/summer08/lecture_1.html

-- Demo: Using conducting spheres and electrometer– Demo: Electrophorus– Demo: Electroscope

– Demo: hanging charged/conducting pith ball- firstattraction by induction, then contact, then conduction ofcharge, then repulsion

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Chapter 21 Problem 29In crystals of a certain salt, singly charged cations form the eightcorners of a cube and a singly charged anion is at the cube's center(Figure 21-32). The edge length of the cube is x = 0.32 nm. Thecations are each deficient by one electron (and thus each has a chargeof +e), and the anion has one excess electron (and thus has a charge of-e).

Fig. 21-32

(a) What is the magnitude of the net electrostatic force exerted on theanion by the eight cations at the corners of the cube?(b) If one of the cations is missing, the crystal is said to have a defect.What is the magnitude of the net electrostatic force exerted on theanion by the seven remaining cations?

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Chapter 21 Problem 68

In Figure 21-23 particles 1 and 2 are fixed in place, but particle3 is free to move. If the net electrostatic force on particle 3 dueto particles 1 and 2 is zero and L23 =2.00 L12, what is the ratioq1/q2?

Figure 21-23