Advanced physics = physics II

= electricity and magnetism

Dr. Fisher, ISMAug 2015

www.ismscience.org

24 Aug 15, the start of a good year

• Points: • know what is difference between doing science

and studying science• Be able to define/organize the scope of Phys II vs

Phys I• Be able to generate positive and negative static

charge • Be able to draw the topics map and fit in electricity• Define and characterize chargeTwo atoms were walking down the street one day, when one of them exclaimed, "Oh no -

I've lost an electron!" "Are you sure?" the other one asked. "Yes," replied the first one, "I'm positive."

ISM Lab Safety Rules

a. No horseplay in the labb. No students may stand on chairs or tablesc. Students may not move chemicals from the labd. Eye goggles or safety glasses are required by all people in the lab for the

following conditionsi. Any heat source is in useii. Anyone is using any material (liquid, dry, or gas) that would be harmful

if put directly into an eyeiii.A pressure source (higher than a blown up balloon) is in useiv.Activities are in the lab that could result in chips, shards, or any flying

objects, such as hammering, launching projectiles, etc.e. Food in the labs

i. Food is not allowed in the lab (except water in bottles)ii. Food can be allowed for planned special activities and with teacher

permission, but all table tops must be disinfected and wiped down beforehand.

iii.Food or water are not allowed if a lab activity is ongoing

Demo at each table: the charge

• Generate a charge and show ability to move something: what can you observe

• Balloon• Paper and comb• Pith balls on string• Pith ball and charged sphere• Question: what do you observe?

– Force• How could we measure …

– Force?– static charge?

• Assign for Wednesday:

Review the big mapAll knowledge

physics

Our normal task as physics students

• Define the language, precisely• Understand the fundamentals – charges, forces,

fields, energy• Describe quantitatively how these all relate• Become good at problem solving• Develop a hands-on “feel” through

demo/lab/physlets• See practical applications to daily life and

technology

In class test using Loggerpro charge sensor

• Do in class lab with charge sensor and range of materials: use clean sheet for HW tomorrow.

• Check both rod and cloth• Choose one new item per table to check• rate of loss of charge in cage = _________nC/s– How many e- per second are lost?

• Note how sensitive it is to local activity

The basics• What is science? 1. A systematic process that builds and organizes new knowledge in the form of testable explanations and predictions about the universe.2. The knowledge of the physical or material world gained through observation and experimentation.

• __will we do science in this class?• __will we learn science in this class?

physics

• What is physics? The study of the behavior and structure of matter and energy

• • Matter -- any substance that has both mass and

volume• Energy – eg. Heat, light (all frequencies),

motion • Check: name something natural that isn’t

physics?

@definitions

• Electricity: physical phenomena of electrons (negative charge) delocalized away from the protons (positive charge).

• Static electricity: situations where charges are not flowing from one place to another

• Describe the charge• All matter has charge, some matter has a net charge• Charge is never created or destroyed, only separated

or combined*– *classical view, good for most of what we do

Open notes quiz

1. What is the process of science?2. What is physics?3. Define electricity.4. Name something with mass but no charge.

ISM Lab Safety Rules1. No horseplay in the lab2. No students may stand on chairs or tables3. Eye goggles or safety glasses are required by all people in the

lab for the following conditionsAny heat source is in useAnyone is using any material (liquid, dry, or gas) that would not be harmful if put directly into an eyeA pressure source is in useActivities are in the lab that could result in chips, shards, or any flying objects, such as hammering, launching projectiles, etc.

4. Food in the lab?Food is never allowed during class (except water in bottles)Food can be allowed for planned special activities and with teacher permission, but all table tops must be disinfected and wiped down beforehand. Food or water are not allowed if a lab activity is ongoing

Logistics for the class:

• The website: review• physics tools on the web:• Course handout:• Topics Map of physics; draw it• Where are all the electricity tools in the lab?

Shadow teachers needed

Wanted: 1 student per class willing to1. Set up and try labs2. Teach concepts3. Show solutions to problemsCommitment is up to 1 hr/wk

If interested, see Dr. F for an interview.

Ideas and notes for shadow teachers

• Interview• 1 per class• Email contact• Weekly grading• Choose and set up a lab.

Why E&M is cool and also not intuitive.

• ..\..\Physics I\start of year materials\how we learn physics.pptx

a little history

• History: ancient times – static charge on objects is a curiosity; charges applied to living things give motion

• : after scientific revolution, the renaissance, rapid learning through controlled experiments and observation and theory testing

•

• e.g. Ben Franklin: only two types of chargename them + and –+ is glass rod rubbed with fur– is plastic (amber) rubbed with cloth

Triboelectricseries

• From Greek– Tribo = rubbing– Elektra = amber

• How tightly a material will “hold onto” electrons when it physically contacts another material.

• Has nothing to do with conductivity

Think about the charge

• Observations/questions• Is an electron inherently negative?• Are there only 2 charges, why not 3?• Why do like repel and opposites attract?

Q: Why was the free electron so sad?A: It had nothing to be positive about!

MiniExpt 1: Loss of charge

• Find the rate of loss for e- from an object that we can agree on.

• Vary : air flow• Report: 1-page memo, typed from each person

to me. Comment on variable effects. Comment on why effect exists.

• Hypothesis, sketch, data, e-/s, comments• Report due 6 Sept• Groups:

Conductors and insulators

• Conductors: material through which e- can flow with very little resistance. List

• Insulator: material through which electrons cannot flow. List

• How does this relate to the outer shell electron?

• Can liquids be conductors? If so, list• Can gasses be conductors?, if so, list

Conductors and insulators quiz

• Conductors: • Insulator: • How does this relate to the outer shell

electron?• Can liquids be conductors? • Can gasses be conductors?

How charge moves around

• Conduction – through a conductor– Electrons jump from atom to atom

• convection – flow of a charged particle, an atom, a molecule, an electron, or a proton– Electron does not jump between atoms

• So how does the charged glass rod gain electrons?

30 aug

physlets to get us thinking

• Physlet: http://phet.colorado.edu/en/simulation/balloons – Would this work if the balloon were condutive?

The wall?• physlet:

http://phet.colorado.edu/en/simulation/electric-hockey can you score with one field charge?

Return to the Faraday cage/charge sensor demo

• Similar to this data: faraday charge mini experiment

• rate of loss of negative charge in cage = 0.007127 nC/s– How many e- per second are lost?

• e- charge = 1.60x10-19 C• So calculate 4.45x107 electrons/second!!

So, charges can be added to a body“charging by conduction”

• By pulling them from another neutral body– Rod and fur, balloon and sweater– Always a balance, charges move both ways

• By transferring them from a net charged body– The charged rod to the pith ball– The charged rod to the electroscope• Show this

And charge can be induced in a body without a net transfer“charging by induction”

• %16-4, please read• Show with electroscope– What is happening without touching?

• A force at a distance can move charges within a conductor, attracting some, repelling others

• Can even force charges into a “ground” or infinite sink of charge.

• Can I induce charge in an insulator?• Think about balloon on the wall

Memo lab on static charge

1. Work as a single team.2. Show that when two strips of scotch tape are

separated1. Strips have opposite charges2. How fast they lose charge in air3. Does rate of airflow over the faraday affect rate

3. Write one-page memo with data by Thursday. See file online for guidance.

29 aug: The magnitude of the force of the charge, Coulomb’s Law

• Start with problems from Monday. And Farnsworth birthday• The electroscope, %16-4

– Pose challenge for later solution: why does the needle wheel spin when charged?

• % 16-5: Coulomb’s Law– the quantity of the force between two charges

• Charles Coulomb, 18th c quantified it, and related to charge and distance– Consider the mechanics of his experiment– F = kq1q2/r2 and k = 9.0x109 Nm2/C2 and is pointed away for same charges

and together for opposite– What about sign or direction? Use common sense!

• Compare to another well known equation of force (find in book)– Analogies: q is like m– k is like G

The force of the charge

• Look at chapter opening question.• later experiments found the q on a single electron

e = 1.602 x 10-19C. – Why is there is no smaller charge?

• look at http://phet.colorado.edu/en/simulation/charges-and-fields for charge, charging, and coulomb’s law 1-d demos.– Do example problems 16-3, with set up on physlet above.

• Demo pith on charged sphere. Review statics from last spring. Show that:

• F = m g tanq

• Where did this force come from?

Dynamics of motion for a charge

• Charged objects have mass• We can describe direction and magnitude of

force, so we have F on a charge • and F = ma is still true, (bold is vector)• So we can describe motion of the charge• And we can make them work for us!!!!

Solving other than point charges

• Everything we do, almost, will be a point charge.

• Neat fact #1: large charges that are spherically symmetrical act as point charges IF you are outside the sphere.

• Neat fact #2: if you are far enough way, any charge looks like a point charge.

• Neat fact #3: there are only two neat facts.

Some practice on 1-d multiple charges

a. +1C charges, 1 m separation, F = ?

b. Now same, with extra charge….F on each?

• F1 = F1,2 + F1,3 where subscript is – force on first, caused by second

• Answer to a is 9x109 N• Answer to b is 11.3x109 N

: Multiple charges, 2-d

• %16-6. If you have multiple charges, each pair gives a force vector F and the net force on any point is SF. – Vector addition rears it’s ugly head again!

• Do example 16-4 for vector forces

• Look at 4 point charges in http://vnatsci.ltu.edu/s_schneider/physlets/main/es_01.shtml

– Look only at net force vectors in this situation: what are signs of these charges?

– Draw the component vectors for this case

The electric field: a radical concept

Charges exert forces on one another, without touching.

Therefore, force acts at a distance.As in gravity, we can describe an electric field, E, for

charges.E = F/q (the definition of electric field)

E will describe force F per charge q in space.q is usually a “postitive test charge”: need to keep the q low so it doesn’t make it’s own field.

Why do we care about the electric field?

• The electric field is a property of a given point in space, and depends on the charges in that vicinity. If no charge present, then no field.

• But, if we know the field, we don’t care about the charge(s) that create the field.

• The field is a useful and concise way to know what will happen to a separate charge in that region.

• Check: for given E, what is F on charge q?

A force SHIELD is not a force FIELD

And I don’t think this is real

Quiz for clickers (open notes)

1. What is the triboelectric series?2. The movement I’m demonstrating is

a. conduction b. magic c. induction d. deduction

3. how do electrons move through a wirea. Conduction b. convection c. induction d. subtraction

4. A -3pC charge is 40.m away from a 5pC charge. What is the force between them?

a. 15*9/4 something b. 15*9/20 something c. 15*9/16 something d. 40*9/15 something

Quiz for 3 sept

• I have here an object with a measured charge (show loggerpro graph)

• Across the room is another object with shown charge from loggerpro.

• They are separated by distance of 6.0 m• What is the magnitude and direction of the

force on each object?

This, September 1, was the date, in 1859, of a massive solar superstorm. It's sometimes called the "perfect space storm" or the Carrington Event, after British astronomer Richard Carrington. He reported witnessing a massive white-light solar flare: a bright spot suddenly appearing on the surface of the Sun. At the same time, the Sun produced a coronal mass ejection, or CME: a large eruption of magnetized plasma. CMEs usually take three to four days to reach Earth, but the magnetic burst from the superstorm of 1859 reached us in just under 18 hours.

While Earthlings of 1859 didn't have any cell phones, GPS units, or television signals to worry about, they were growing accustomed to rapid communication over the telegraph, which had been in use for 15 years. Within hours of the CME, telegraph wires began shorting out, starting fires and disrupting communication in North America and Europe. Compasses were useless because the Earth's magnetic field had gone haywire. The northern lights were seen as far south as Cuba and Hawaii, and the southern lights — aurora australis — were seen in Santiago, Chile. People in the northeastern United States could read the newspaper by the light of the aurora, and the Sun itself was twice as bright during the event.See http://www.youtube.com/watch?v=lT3J6a9p_o8 for good video

See the SDO for our response…….

Sep 2 :First: Visualize the fieldthen, quantify the field

• http://vnatsci.ltu.edu/s_schneider/physlets/main/efield.shtml

• Make a single point charge• Show two charges alike and opposite• What does the test charge do?

http://phet.colorado.edu/sims/charges-and-fields/charges-and-fields_en.html

Fisher’s easy™ rules to draw field lines

• + charges have lines radiate out, - radiate in• Draw line nearby the charges• Use 6-8 lines per charge, and use number of

lines as proportional to charge quantity• Draw lines very far from charges, where you can

add up charges and treat as net point charge• Connect the lines, head to tail• Lines may not cross or touch

I need your help

• To find a physlet that runs on our systems• Shows electric fields for multiple charges,

similar to http://vnatsci.ltu.edu/s_schneider/physlets/main/efield.shtml

Practice: draw field lines

The answer

Practice: draw the field lines

See the “mirror” between like charges

Practice with 2Q and Q

2q q

*16.8 Field Lines

Electric dipole: two equal charges, opposite in sign:

16.8 Field Lines

The electric field between two closely spaced, oppositely charged parallel plates is constant.

16.8 Field LinesSummary of field lines:

1. Field lines indicate the direction of the field; the field is tangent to the line.

2. The magnitude of the field is proportional to the density of the lines.

3. Field lines start on positive charges and end on negative charges; the number is proportional to the magnitude of the charge…

More complicated fields

• what is the field at a point near a line of charge or a ring of charge?

• And notice how a point charge vs. a line of charge gives same shape field if you go far away.

• Notice how a small line of charge looks like an infinitely long line if you are very close to it.

**Another look at electroscopes

You need to know both types• How they work?• Conduction charging• Induction charging• Grounding• The effect of moisture in the

air

Static Electric Fields and Conductors1. The static electric field inside a conductor is zero – if it were not, the charges would move.2. Net charge on a conductorwill distribute to the outersurface.

3. E is always perpendicularto the surface of a conductor

Just think what the charge will do…..

16.9 Static Electric Fields and Conductors

The electric field is perpendicular to the surface of a conductor – again, if it were not, charges would move.

7 sept: Now that we visualized the field

we can quantify the field• Remember E = F/q and – F = kqQ/r2 for force between a point charge Q (the

source charge) and another point charge q (let this be the test charge)

• So E = kqQ/qr2 = kQ/r2 out from the point charge Q

• Check: what is E when 2 m from a 2.0 nC charge?• Check: sketch and show the vectors for p-37a…

Gauss’ Law

• A more GENERAL form of Coulomb’s Law• The net electric flux through any closed surface is

equal to 1⁄ε times the net electric charge enclosed within that closed surface.

• Says mathematically that– Field lines must come from a + charge

or end at a – charge• Requires a new concept: flux

Flux = how much of something goes through a certain area

In integral form……

Application: the photocopy machine

• A copier uses many of these phenomena from chapter 16– Charged insulators (dark selenium, toner powder,

paper)– Conductors (charged roller, lighted selenium,

aluminum sheet)– Attractive forces (ink to drum, ink to paper)– Check: how many charge transfers does it take to

put one toner particle on the paper?...

And finally, DNA

• Just look at %16-11 to see that the charge-charge attractions are central to DNA replication.

• The “lock and key” mechanisms of DNA, enzymes, viruses, etc depend strongly on ion-ion or charge-charge attractions.

• Check: do they even depend on charge repulsion?

• End of chapter 16….

Sept 10: Lab 1 for electricityMeasure q of a pith ball by measuring force

on the pith ballHypothesis: we can measure q on a pith ball

by measuring the force of repulsion between two, and we can compare this to a direct measurement using the Vernier charge sensor.

Develop procedure as a groupSee below for report criteria

Pith ball repulsionThe balls are

conductive, but we’ll assume the charges remain evenly distributed.

A photo of the pith ball repelled by the lead ball. The top of the pith string is not shown, but is directly above the center of the lead

ball. Separation is

measured center-to-center.

Conducting pith force lab in class(an experiment in conducting an experiment)Group one conductsGroup two takes picturesGroup three analyzes picturesAll will evaluate the data and write reports

Note: I’m asking for your total focus and care for 40 minutes. If you can’t do this, please have a seat in the rear.

Groups and their dutiesGroup 1: conduct lab, take dataGroup 2: will do schematic sketch of physics,

label all variables, derive the equation due Friday and build the spreadsheet for all

Group 3: will sketch the experimental setup and take a picture and write out the procedure

Load all group contributions onto J drive

11 sept: finish Lab 1Analyzing the lab

Let’s agree on the dataAny holes left in the data?

Impromptu, extemporaneous explanation of your groups’ work in the lab

Sharing data (hand out copies)

Lab 1 report requirements Name, date, lab nameHypothesisDataAnalysis, include the sketch of the two

charges and LABEL the lengths you usedConclusionsThis doesn’t require a lot of textShould be about 3 pagesDue Thursday, 17 Sept, in class.

Grading rubric for informal presentationRephrase the question to you are sure you clarify it and so everyone can hear it. 2pt

Draw a picture 3ptDerive or relate to theory that we have

learned 3ptShow on experimental set up 3ptAddress the question 5ptMake sure your answer is clear to everyone

in the room: loud, good pictures 2ptWas your answer interesting? 1pt

**A quiz for your comprehension• Two charges, +4C and -2C, separated by 300 m.

a) Draw field linesb) What is force between themc) Put a square conductor between them and draw

the field lines and any moved charges in the conductor

d) Bonus: Put a square insulator between them and draw the field lines and any moved charges in the conductor

4C -2C

Done with Chapter 16

• Problems due today• Quiz: some words• Sketch field lines for 5 basic cases of charge• Application: understand the force between • Na and Cl. ~ 1 nN

Classics chapter 16

1. Electroscope induction1. Electroscope conduction2. draw fields for 4 cases3. Calc F 1-d4. Calc F in 2-d5. Determine direction of F in a field6. Field through insulator7. Field through conductor8. Find acceleration of charge in field9. Balloon and wall10. Gauss’s Law for point charge (we cover in next chapter’s deck of

slides)

Practice with chapter 16• Look over Quiz ; the E and e-/s parts• Classic list

– Force on one charge of 3x4m rectangle, 1,2,3,4C charge (vector problems)

– Draw vectors, be quantitative– write equations for each component

• Don’t fret about final resolved vector• This was too hairy to calculate numerically for a test

• Force on middle charge of 3 unlike charges• Charge –q in field E distance L from + wall.

– What is a of charge?– How long to hit wall?

• Electroscope problems, conduction and induction• Analogy of forces in mass and charge• Practice problems using electroscope: induction, conduction

1

42

3

3m

4m

Review for final

• Main concepts• Classics• Questions?• A few weak points– Release charged mass in various fields or near

point charges; what is path taken– Vector of net F near multiple charges

A 2-d problem; find force vector on q, find V at q

L

L

2q

q-2q

These don’t work anymore: Quiz http://webphysics.davidson.edu/physlet_resources/bu_semester2/index.html

•We’ve looked at this site.•I want you to practice with these 19 cases•The quiz will be during class, and you will each present, in one minute, a case of my choosing•So you need to be ready to talk about any of them.•I’ll demonstrate now….