1/7/2019
Physics 24100
Electricity & Optics
Spring 2019 Semester
Prof. Dan Elliott
Physics 24100 ‐ Syllabus
This is me.
Questions about grades, grade
checks, absences, etc… please
see Prof. Pyrak‐Nolte
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Physics 24100 – Electricity & Optics
Preliminary Information
• Physics Department home page:
– http://www.physics.purdue.edu
• Course home page:
– http://www.physics.purdue.edu/phys241
• CHIP home page:
– http://chip.physics.purdue.edu/public/241/spring2019/
• Piazza:
• https://piazza.com/purdue/spring2019/phys241/home
• Rooms:
–
–
–
–
–
Physics 112: Lecture theater
Physics 144: Undergraduate Office
Physics 11: Help center
MSEE 258: My office
Wilmeth Active Learning Center (WALC) : Library of Engineering and Science
(LOES)
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Physics 24100 – Course home page
Course
information:
• Schedule/
calendar
• Syllabus
• CHIP
instructions
• i‐Clicker
registration
• Gradebook
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Calendar
• Read the assigned chapter
sections prior to attending
class.
• Note exam dates
• Homework is due on most
Mondays (some exceptions)
at 11:59 PM. Homework is
done through CHIP.
CHIP Home page:
http://chip.physics.purdue.edu/
public/241/spring2019/
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Physics 24100 ‐ Syllabus
Register your iClicker on CHIP
Always bring your iClicker!
5% of your grade is based on
lecture quizzes
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Physics 24100 ‐ Grading
Exams:
There will be two 75‐minute evening exams and a two‐hour final exam. The evening
exams are multiple‐choice and should be able to be completed within 75 minutes by a
well‐prepared student; note that we’re giving you 120 minutes. The times and locations
of the evening exams are as follows:
Exam 1: Tuesday, February 5, 2019 @ 8‐10 PM in Elliott Hall of Music
Exam 2 Tuesday, March 19, 2019 @ 8‐10 PM. Location to be announced later.
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Physics 24100 – Lecture notes
Lectures will be
posted on‐line.
(possibly a day
before the
lecture)
Generally not a
good substitute
for coming to
class…
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Online forum:
We will use Piazza for class discussions to assist you in getting help
fast and efficiently from your classmates and the teaching assistants.
Rather than emailing questions to the teaching staff, you are
encouraged to post your questions on Piazza:
You can sign up for a Piazza account and enroll in our class page at:
https://piazza.com/purdue/spring2019/phys241/home
Supplemental Instruction (SI):
• Extra Study Sessions for specific courses
• Organized through the University
• Our SI instructor is Hemanth Aroumougam.
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EMERGENCY PREPAREDNESS – A MESSAGE FROM PURDUE
To report an emergency, call 911. To obtain updates regarding an ongoing
emergency, sign up for Purdue Alert text messages, view www.purdue.edu/ea.
There are nearly 300 Emergency Telephones outdoors across campus and in
parking garages that connect directly to the PUPD. If you feel threatened or need
help, push the button and you will be connected immediately.
If we hear a fire alarm during class we will immediately suspend class, evacuate
the building, and proceed outdoors. Do not use the elevator.
If we are notified during class of a Shelter in Place requirement for a tornado
warning, we will suspend class and shelter in the basement.
If we are notified during class of a Shelter in Place requirement for a hazardous
materials release, or a civil disturbance, including a shooting or other use of
weapons, we will suspend class and shelter in the classroom, shutting the door
and turning off the lights.
Please review the Emergency Preparedness website for additional information.
http://www.purdue.edu/ehps/emergency_preparedness/index.html
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Electricity & Optics
• Classical Electrodynamics:
– Formulated by many pioneers in the mid 1800’s.
James Clerk Maxwell
Michael Faraday
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Electricity & Optics
• Maxwell’s Equations:
·
·
0
• Frequently, the goal is to solve for or as a
function of …
• and also exert forces on charged particles:
http://dx.doi.org/10.1109/.2002.995632
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Lecture 1 – Electric charges & Coulomb’s Law
• Electric charge is an intrinsic
property of fundamental
particles that make up
objects.
• Fundamental particles can
be negatively charged,
positively charged or neutral.
‐
(electron)
(photon)
+
(proton, positron)
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The electron charge is –e, where e = 1.602 x 10‐19 C.
The proton charge is +e.
The neutron charge is 0.
The net charge of a system is the algebraic sum of all
the charges of its constituents.
– An object is electrically neutral when it contains
equal numbers of positively and negatively
charged particles.
• Fundamental law of nature (charge conservation):
– Electrical charge of a closed system never
changes.
•
•
•
•
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Clicker Question: (just for fun)
• Given that:
–
–
–
–
An electron has a charge of –e.
A proton has a charge of +e.
A neutron has a charge of 0.
A 12C atom (the most abundant isotope of carbon) contains
6 protons and 6 neutrons.
– What is the charge Q of a (completely ionized)
carbon nucleus?
(a)
(b)
(c)
(d)
(e)
Q = ‐e
Q = +e
Q = 6e
Q=0
Q = 1.602 x 10‐19 coulombs
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Another Clicker Question:
• Given that:
–
–
–
–
A proton has charge
A neutron has no electric charge
A electron has charge
A Uranium‐238 nucleus has 92 protons and 146 neutrons
• What is the charge of an unionized U‐238 atom?
(a)
(b)
(c)
(d)
(e)
Q = ‐e
Q = +e
Q = +238 e
Q=0
Q = 1.602 x 10‐19 coulombs
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Electric Charge
• We will usually work with macroscopic objects which
contain many, many fundamental particles…
– Like Avagadro’s number:
6.02
10
• Unit of electric charge is the coulomb (C):
1.602 10
coulombs
– Defined (indirectly) in terms of magnetic forces on current
carrying wires.
– One Coulomb is the charge flowing through the cross
section of a wire carrying one Ampere each second
1 coulomb = 1 ampere · second
1 ampere = 1 coulomb / second
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Observing Electric Charge
• Electric charges exert forces on
each other.
• Ben Franklin proposed that there
was only one type of charge but
that objects could have too much
(+) or too little (‐).
• Thought of charge as a fluid and
electric forces cause it to move…
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Conductors and Insulators
• In some materials the electric forces cause
charges to move (conductors)
• In other materials the electric forces are balanced
by other forces (eg, atomic bonds) and the
charges can’t move (insulators)
• In some materials, the charges move, but
currents are more complicated than in a simple
conductor or dielectric (semiconductors)
• In other materials, charges move with no
resistance at all (superconductors)
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Charge Distributions in Insulators
• Individual charges are attached to atoms or
molecules that cannot move
– But the charges can be locally redistributed
Is there a force between
the glass rod and the
neutral insulator?
• 2 x 4 demo
Neutral insulator
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Forces on Charges in Insulators
Repulsive force
Charges with the same sign
repel each other.
Attractive force
Charges with the opposite
sign repel each other.
Sign convention is historical but arbitrary nonetheless.
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Forces on Charges in Conductors
• Charges are easily redistributed over large distances
in a conductor – they move “freely.”
A neutral conducting
rod will always be
attracted to a charged
insulating rod.
The charges easily
redistribute themselves.
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Demonstration
Pie tins,
Conducting spheres
• Similarly charged
• One charged, one grounded
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The Useful Concept of “Ground”
• The earth is a (relatively poor) conductor
– Dissolved mineral salts are good conductor.
• The earth is very large…
– Macroscopic charges can flow
into or out of the earth without
changing its net charge by any
significant degree
• This property can be quite
useful!
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Charging by Induction
1. Bring a charged rod close
to conductor.
3. Break connection to
ground, keeping the
charged rod in place.
2. Ground the conductor.
4. Remove the rod. The
sphere is charged.
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Forces on Charges
• Coulomb’s law of electrostatic force:
Charles‐Augustin de
Coulomb
(1736 ‐ 1806)
• The force on Q1 by charge Q2 is observed to be:
– Directly along the line joining the two charges
– Repulsive (if Q1 and Q2 are the same sign) or attractive
(if Q1 and Q2 are opposite in sign).
– Decreasing as 1/r2.
– The same magnitude, but opposite in direction, as the
force on Q2 by the charge Q1.
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Forces on Charges
• Coulomb’s law of electrostatic force:
• The magnitude of the attractive/repulsive force is
where
1
4
8.99
10
·
·
and therefore
8.85 10
·
·
(This constant is called the “permittivity of free space”)
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Coulomb’s Law of Electrostatic Force
1
But ̂
̂
4
/ so we can also write this as:
1
4
If
0, then the force
is in the same direction as
. The force is repulsive.
is the force exerted by
on .
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Coulomb’s Law of Electrostatic Force
•
exerts a force on
force on …
but
also exerts a
If
0, then the force
is in the same direction as
. The force is repulsive.
is the force exerted by
on .
• The magnitudes of the two forces are equal.
• The forces form an action‐reaction pair
– consistent with Newton’s laws.
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Example: Force on an Electron
• What is the magnitude and direction of the
force on an electron exerted by the nucleus of
a lithium (Z=3) atom of the mean atomic
?
radius is
1.77 10
3
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Principle of Superposition
• When several point charges are present, the
total force on any one charge is the vector
sum of each of the separate forces acting on
the charge.
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What’s the net force
acting on ?
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Example
• Calculate the magnitude and direction of the
force on :
1
y
1
x
4
1
0
2
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