Download Lecture 1 - The Local Group

Physics 7D
Classical Physics
Prof. Michael Cooper
[email protected]
FRH 2123
Administration
Instructor: Prof. Michael Cooper
- Office Hours: Th 9:30-10:30am in FRH 2111 (or by appointment)
- Office Location: 2123 Frederick Reines Hall (FRH)
Course Website: https://eee.uci.edu/17w/47340 => Syllabus, Lectures, etc.
TAs: Alexis Romero, Cody Combs, Michael Fitzgerald
- Office Hours: by appointment only
- Tutor center: see Schedule in course website contacts
Contact: [email protected], [email protected], [email protected], [email protected]
- When you email us w/ questions include:
- 1) your name, 2) student ID, 3) discussion session #, 4) TA name
Registration Qs: All add/drop questions should be addressed to Physics Student
Affairs Office (FRH 4109): [email protected]
Grading
■
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■
■
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■
Final Exam: 40%
Midterm: 25%
Homework: 15% (lowest score dropped)
Quizzes: 15% (lowest score dropped)
Clickers: 5%
Midterm Exam: Thursday, Feb 23
Final Exam: Tuesday, Mar 21
Textbook & Mastering Physics
Textbook: University Physics with Modern Physics, 13th
Edition, Young & Freedman (Pearson / Addison Wesley
publishers).
Modified Mastering Physics for online homework.
Info on how to register & enroll available on course
website. See a how-to video here:
https://www.youtube.com/watch?v=ZojLfmTArb4
Must pay for access unless you’ve already paid when
you took a prior P7 course at UCI.
Self-enroll: cooper46886
Self enroll with
this code:
cooper46886
Title: Classical
Physics:
Electromagnetism
[Physics 7D,
W17, Cooper]
Clickers
Clickers will be used during lecture. They help test your level of understanding and
encourage you to engage in class.
• 3/4 of your score is participation; 1/4 performance (i.e. getting the right answer).
• You must register your iclicker at http://iclicker.com
• Even if you have registered your iclicker in the past, DO IT AGAIN.
• Be sure to register with your 8-digit UCI student ID exactly as it appears in
WEBREG.
• An iclicker can be used by only one student during a given quarter. If you have
borrowed someone else’s, make sure you register that iclicker in your name/ID #.
Typical Discussion Section
1. ~25 minutes: questions/discussion regarding homework
problems (or previous week’s quiz)
2. ~25 minutes: short quiz
Discussion sections will NOT meet tomorrow;
first discussion sections will be on Jan 18th.
Chapter 21
Electric Charge and Electric Field
Goals for Chapter 21
̣To study electric charge and charge conservation
̣To see if/how objects become charged (conductors and insulators)
̣To calculate the electric force between objects using Coulomb’s Law
̣Learn the distinction between electric force and electric field
̣To calculate the electric field due to many charges (E-field superposition)
̣To visualize and interpret electric fields
History
Electromagnetic phenomena known by the ancients
- static electricity (~600 BCE, Thales)
- magnets (~100 BCE China, Greeks, Central America(?))
Observation: Static electricity exhibits both attraction and repulsion. Many early
scientists thought that there were two types of forces or “fluids”.
B. Franklin: No, there are two charges! (he was right)
BTW: Franklin coined several electrical terms that we still
use today: battery, charge, conductor, plus, minus, positively,
negatively, condenser (= capacitor).
Convention that electrons are (-) goes back to Franklin
History
1600 - William Gilbert - Saw effects of static electricity in many materials
(e.g. rubbed glass). Identified differences among substances we now know as
conductors / insulators.
1660-1750 - Early electric generators (based on friction)
c. 1750 - B. Franklin: lightning=electricity, same as rubbed glass. A single kind
of charge “fluid” (not two; now we know that electrons are “flowing”).
1785 - Charles Coulomb => electric force follows inverse square law
1819 - Hans Oersted: compass needles deflected when near a wire carrying an
electric current (i.e. current / changing electric field produces a magnetic field).
1831 - Michael Faraday and Joseph Henry: when a wire is moved near a magnet,
an electric current flows in the wire (i.e. changing magnetic field creates an
electric field).
History
1873 - James Clerk Maxwell formulated laws of electromagnetism.
This course will culminate with Maxwell’s equations.
Modern view of the atom
Atoms are neutral (0 charge)
Equal numbers of equal magnitude,
opposite-sign charged particles
Electrons (-)
Protons (+)
Neutron (neutral)
Standard unit of charge is the
Coulomb
e = 1.6 ×10-19 Coulombs (C)
Need 6.24×1018 e for 1 C
Typical charge will be in µC
where µC=10-6 C
13
Static electricity
+
What makes plastic wrap
cling? Static electricity.
Some materials (like your hair or wool) give electrons
off more easily than others.
← When you rub a balloon on your sweater, you’re
transferring electrons from sweater to balloon.
Other materials (like silk) hold on to their
electrons more tightly. When you rub a glass rod
on silk, the silk steals electrons from the glass
rod. The glass becomes +ve.
Static electricity
1st record of static electricity dates to
ancient Greeks (~600 BCE) - noticed
that polishing amber with fur allowed
it to attract bits of straw.
Amber
(The ancients didn’t have
plastic combs, balloons,
or glass!)
- The word electric derives from the Greek word for
amber (elektron).
Conservation of electric charge.
Electric charge is conserved.
Charge is not created
in the process of
rubbing two objects
together – the
electrification is due
to a transfer of charge
from one object to
another
Electric charge
̣
Materials that elucidate the basics of electrostatics: glass rods,
plastic tubes, silk, and fur - how people figured out that there were
2 types of charges and that they attracted/repelled.
Quiz: Electric Charge I
Two charged balls are repelling each other as they hang from the
ceiling. What can you say about their charges?
A.one is positive, the other is negative
B.both are positive
C.both are negative
D.both are positive or both are negative
Quiz: Electric Charge I
Two charged balls are repelling each other as they hang from the
ceiling. What can you say about their charges?
A.one is positive, the other is negative
B.both are positive
C.both are negative
D.both are positive or both are negative
Quiz: Electric Charge II
From the picture, what can you conclude about the charges?
A.
have opposite charges
B.
have the same charge
C.
all have the same charge
D. one ball must be neutral (no charge)
Quiz: Electric Charge II
From the picture, what can you conclude about the charges?
=
=
=
=
Quiz: Electric Charge II
From the picture, what can you conclude about the charges?
A.
have opposite charges
B.
have the same charge
C.
all have the same charge
D. one ball must be neutral (no charge)
Insulators and Conductors
Insulators: electrons are somewhat tightly bound to atomic nuclei
̣Electron cannot move relatively freely through material
✓
✓
When a good insulator is charged (by doing work: e.g. rubbing) in a small
region, the charge is unable to move to another region
Most non-metals are insulators (e.g. glass, rubber, wood)
Conductors: electrons easily flow (like a liquid) from one nucleus to
another
̣Electrons can move relatively freely through the material
✓
✓
When a good conductor is charged in a small region, the charge readily
distributes itself over the entire surface of the material
Most metals are conductors (e.g. copper, aluminum, silver)
Charging by induction
The metal ball
is now “polarized”
Quiz: Conductors I
Two neutral conductors are connected by a wire
and a charged rod is brought near, but does not
touch. The wire is taken away, and then the
charged rod is removed. What are the charges
on the conductors?
A
B
C
D
E
0
0
0
?
?
?
?
?
?
0
+
-
-
+
+
+
-
-
Quiz: Conductors I
Two neutral conductors are connected by a wire
and a charged rod is brought near, but does not
touch. The wire is taken away, and then the
charged rod is removed. What are the charges
on the conductors?
A
B
C
D
E
0
0
+
-
-
+
+
+
-
-
0
0
-+
-
-
+
-?
+?
Coulomb’s law
q1q2
F12 = ke 2 rˆ12
r
1736-1806
€
Charles Coulomb measured
the magnitudes &
directions of electric
forces between two
small charged spheres
Coulomb’s law
■
■
q1q2
F12 = ke 2 rˆ12
r
The electrical force between two stationary point
charges is given by Coulomb’s Law
€
Force is
● inversely proportional to the square of the separation
between the charges
● proportional to the product of the charges, q1 and q2,
on the two particles
● directed along the line joining them
● attractive if charges have opposite signs
● repulsive if they are the same sign
Coulomb’s law: Units
& Constants
q1q2
F12 = ke 2 rˆ12
r
The SI unit of charge is the coulomb (C)
ke is called the Coulomb constant
€.
109
ke = 8.9876 x
N m2/C2 = 1/(4πεo) (~ 9x109 N.m2/C2)
εo is the permittivity of free space
εo = 8.8542 x 10-12 C2 / N.m2
●
q1q2
1 q1q2
|F2 on 1| =|F1 on 2| = k 2 =
r12
4πε 0 r122
usually use: ke = 9 x 109 N.m2/C2
●
●
e = 1.6 x 10-19 C
Typical charges ~ µC range
1 µC needs 6.24 x 1012
electrons or protons
Electric forces are
vectors
Force of 1 on 2
q1q2
F12 = ke 2 rˆ12
r
r̂12 is a unit vector directed from
q1 to q2
Like charges produce a repulsive
force between them.
Obeys Newton’s third law
!
!
F21 = −F12
Electric forces are much stronger than gravity
Compare gravitational and electric forces between an electron and a
proton for a given separation r:
mpme
Fg = G 2
r
2
Fe = k
qeqp
r2
ke2
= 2
r
2
(9 ×10 9 )(1.6 ×10 −19 )
Fe
ke
=
=
Fg Gmpme (6.67 ×10 −11 )(1.67 ×10 −27 )(9.11×10 −31 )
Fe
≈ 2 ×10 39
Fg
The electric force is almost 40 orders of magnitude stronger
than the gravitational force.
So why don’t we usually notice electric forces ?
See you Next Tuesday
No discussion sections tomorrow and no lecture Thursday.