Advanced physics
= physics II
= electricity and magnetism
Dr. Fisher, ISM
Aug 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 charge
Two 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.
b.
c.
d.
No horseplay in the lab
No students may stand on chairs or tables
Students may not move chemicals from the lab
Eye goggles or safety glasses are required by all people in the lab for the
following conditions
i. Any heat source is in use
ii. Anyone is using any material (liquid, dry, or gas) that would be harmful
if put directly into an eye
iii.A pressure source (higher than a blown up balloon) is in use
iv.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 map
All 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.
2.
3.
4.
What is the process of science?
What is physics?
Define electricity.
Name something with mass but no charge.
1.
2.
3.
4.
ISM Lab Safety Rules
No horseplay in the lab
No students may stand on chairs or tables
Eye goggles or safety glasses are required by all people in the
lab for the following conditions
Any heat source is in use
Anyone is using any material (liquid, dry, or gas) that would
not be harmful if put directly into an eye
A pressure source is in use
Activities are in the lab that could result in chips, shards, or
any flying objects, such as hammering, launching
projectiles, etc.
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 to
1. Set up and try labs
2. Teach concepts
3. Show solutions to problems
Commitment 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 charge
name them + and –
+ is glass rod rubbed with fur
– is plastic (amber) rubbed with cloth
Triboelectric
series
• 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/ballo
ons
– Would this work if the balloon were condutive?
The wall?
• physlet:
http://phet.colorado.edu/en/simulation/elect
ric-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
separated
1. Strips have opposite charges
2. How fast they lose charge in air
3. 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/chargesand-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/m
ain/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 wire
a. 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 field
then, quantify the field
• http://vnatsci.ltu.edu/s_schneider/physlets/m
ain/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-andfields/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/m
ain/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 Lines
Summary 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 Conductors
1. The static electric field inside a conductor is zero –
if it were not, the charges would move.
2. Net charge on a conductor
will distribute to the outer
surface.
3. E is always perpendicular
to 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 chargecharge attractions are central to DNA
replication.
• The “lock and key” mechanisms of DNA,
enzymes, viruses, etc depend strongly on ionion or charge-charge attractions.
• Check: do they even depend on charge
repulsion?
• End of chapter 16….
Sept 10: Lab 1 for electricity
 Measure q of a pith ball by measuring force on the pith
ball
 Hypothesis: 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 group
 See below for report criteria
Pith ball
repulsion
 The 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 conducts
 Group two takes pictures
 Group three analyzes pictures
 All 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 duties
 Group 1: conduct lab, take data
 Group 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 1
 Analyzing the lab
 Let’s agree on the data
 Any 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 name
 Hypothesis
 Data
 Analysis, include the sketch of the two charges and




LABEL the lengths you used
Conclusions
This doesn’t require a lot of text
Should be about 3 pages
Due Thursday, 17 Sept, in class.
Grading
rubric
for
informal
presentation
 Rephrase the question to you are sure you clarify it and






so everyone can hear it. 2pt
Draw a picture 3pt
Derive or relate to theory that we have learned 3pt
Show on experimental set up 3pt
Address the question 5pt
Make sure your answer is clear to everyone in the
room: loud, good pictures 2pt
Was your answer interesting? 1pt
**A quiz for your comprehension
• Two charges, +4C and -2C, separated by 300 m.
a) Draw field lines
b) What is force between them
c) 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 induction
1. Electroscope conduction
2. draw fields for 4 cases
3. Calc F 1-d
4. Calc F in 2-d
5. Determine direction of F in a field
6. Field through insulator
7. Field through conductor
8. Find acceleration of charge in field
9. Balloon and wall
10. 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
•
•
1
3
2
4
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
4m
3m
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
2q
L
-2q
L
q
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….