Transcript Lect01

WELCOME TO
PHYSICS 1302
Lectures+
in class
questions
Textbook
Read it!
Discussion sections
Talk it over; ask questions
Homework
Do it!
Lab sections
Real World!
office hours
exams
Self study
Lectures
• Lectures M,T,W 08.00-08.50. The Friday 08.00 spot is
for quizes or if I need to move a lecture.
– Watch for announcements, in lectures and on the web
• Read the text BEFORE the lecture.
– I will tell you what you need to read for the next lecture.
• Listen and think during the lecture.
• The slides will be on the web after the lecture.
– You don’t need to take notes unless it helps you to concentrate.
• Read the slides and text again after the lecture.
– Now is the time to make sure you understand
• Don’t get behind. If you don’t understand ASK
– In the lecture.
– In the discussion sessions.
– In my or the TA’s office hours.
In class Questions
• There will be in-class questions from time to time.
– You need a transmitter (buy it from the book store).
• They will make up 5% of your final grade.
– 2 points for a correct answer.
– 1 point for any other answer .
• You may need a calculator and/or pencil and paper.
• They are an opportunity for you to ask if you don’t
understand and for me to see whether or not you do
understand.
Labs and Discussion groups
• The labs are compulsory, they make up 15% of your
final grade.
• YOU MUST GET 60% ON YOUR LAB GRADE TO
PASS THE WHOLE COURSE.
• This is a writing intensive course, your lab report is
the writing part. It must be in reasonably
grammatical and well spelt English.
• The discussion groups are compulsory. They are
your opportunity to consolidate what you have learnt
from the course and/or ask if you don’t understand.
– A group quiz question will be done in your discussion group.
– If you have missed one discussion session in the previous three
weeks without permission you will lose 50% of the group question
points.
– If you have missed two sessions you will get zero points.
Homework
• Usually you will be given two homework questions in
you discussion session, to be handed in to your TA
before the next session.
– One question will be marked for 4 points.
– Handing in the other question will get 1 point.
• The homework will count for 10% of your final grade.
• To become fluent with the problem solving you need
to do more problems than the homework.
– There is a list of suggested problems from the text book, do them
and more if you find them difficult
– The answers to the problems will be on the web
– Try them first, then look at the answers.
Office Hours
• If you need help contact;
1. The TA office
2. Make an individual appointment with your TA
3. Come and see me
 My office hours are on the web but I am fairly
flexible as long as I am not overwhelmed
Quizes and Final
• There will be four quizes on Thursdays (group) and
Fridays (individual)
–
–
–
–
1st-2nd February
22nd-23rd February
22nd-23rd March
12th-13th April.
• The final will be on Wednesday 10th May at 18.30
• You can drop one quiz
• Points
– 15% for each of 3 quizes + 25% for the final or
– 15% for each of 4 quizes + 10% for the final
You will need
• Fishbane, Gasiorowicz and Thornton
– Chapters 12, 21-34
• Lab Manual II, Electricity and Magnetism
• Competent Problem Solver, Calculus version
• An interwrite PRSRF transmitter for in-class
questions
• Read the syllabus on the class web page at
www.physics.umn.edu/classes
The Challenges of Physics 1302
1. Nearly zero direct experience with E&M in everyday life
- for scientists and engineers, though, E&M is
everywhere
2. The course is full of NEW concepts: a few per lecture!!
- each concept has many different consequences
- these concepts are interrelated
3. The course “builds upon itself” sequentially
- if you miss a lecture or discussion session you
MUST make it up or you will be LOST
4. Maths is the language of physics and here you need to
learn to work with it
- calculus: line integrals, surface integrals, gradients
- vectors: addition, dot-product, cross-product,
decomposition
All of Physics 1302 in 5 lines
1.
2.
F  q( E  v  B)

E  dA 
closed surface
3.
4.
5.

Q
0
B  dA  0
closed surface
d
 E  ds   dt surface
 B  dA
d
 B  ds  0 I  0 0 dt surface
 E  dA
Outline of physics 1302
• Coulomb’s Law gives force acting
1 Q1Q2
F12 
rˆ12
on charge Q1 due to another
2
4 o r12
charge, Q2.
– superposition of forces from many
charges
– Electric field is a function defined
throughout space
Ftotal
1 Qn
 Q1 
rˆ
2 1n
n 4 o r1n


Ftotal  Q1 Etotal
r̂12
– here, Electric field is a shortcut to Force
– later, Electric field takes on a life of it’s
own!
Q1
r̂13
Q2
Q3
Outline of physics 1302 (cont.)
• Gauss’ Law
–
–
–
–
–
an integral formulation of Coulomb’s law
very useful in problems with geometrical symmetry
vector calculus:
symmetry -> integral easy
Like the gravitational force
• Potential Energy
– charges subject to electric forces
– work is done to “raise a charge to a location with higher
potential energy”
– electric potential energy versus electric potential -> U = QV
– voltage on a battery is an example of electric potential value
+Q ++++++++++
– charge on a capacitor is charge
stored at an electric potential
-Q - - - - - - - - - -
+
volts
‘R
us
-
Outline of physics 1302
• Magnetism – permanent magnets and magnetic
fields
• Moving charges (currents) give rise to magnetic
fields Ampere’s Law and Law of Biot and Savart
– unchanging currents give
static magnetic fields that
“ring” around the current
• Magnetic fields give a force that acts on moving
charges
– not a central force
• Magnetic fields and electric fields store energy
– in capacitors E-field stores energy
– in “inductors” magnetic field (B) stores energy
– along with resistors, capacitors and inductors form elements
of simple circuits (passive elements)
Outline of physics 1302
• Simple circuits
– DC circuits
– AC circuits
• Faraday’s Law
–
–
–
–
time varying magnetic fields generate electric fields
transformers, inductors in AC circuits
ignition coils in cars
electromagnetic cannon
• Time varying electric fields generate magnetic
fields
– repaired Ampere’s Law
• Maxwell’s equations describe all of
electromagnetism
– Electromagnetic waves, light, x-rays, microwaves etc
The World According to
Physics 1301
 dp

• Motion and interactions
F
 ma
dt
• Specified by geometry, mass and
forces
• Forces
• Gravity:

m1m2
F12  G 2 r̂12
r12
r̂12
m1
• Others: Tension, Normal, Friction
m2
The World According to
Physics 1302
 
• Fields and Forces ( E , B )
• Specified by geometry and mass and charge
• Forces
•
•
•
Gravity:
Electric:
Magnetic:

m1m2
F12  G 2 r̂12
r12

F12 
1 Q1Q2
2
4 o r12
m2
r
m1
r̂12
rˆ12

 
F1 mag  Q1v  B
Q1
v
Q1
Q2
Electric Charge
• Charge is an intrinsic property of matter
–
–
–
–
Protons have positive (+ve) charge
Neutrons have no charge
Electrons have negative (-ve) charge
The charge on an electron EXACTLY equals the charge on a proton
• Atoms normally have an equal number of protons
and electrons and thus have zero charge
• Normal matter thus has equal numbers of protons
and electrons and is electrically neutral
• If the number of electrons is not equal to the number
of protons a body will have a net charge
– Electrons are easier to detach from a body than protons thus
usually
» a negatively charged body has an excess of electrons
» a positively charged body has a deficit of electrons
Electric Charge
• Electric Charge
• Like charges repel
• Unlike charges attract
• Gravity only attracts
• There is only one type of mass
• There are two types of charge
Electrostatics in Matter
Force in
Atom
Model
–
Basis of Attraction Example
Opposite Charges H
+
Ionic
Crystal
Opposite Charges NaCl
Covalent
Bond
Molecules –
shared e- pair
H-H
Metal
Metal cations and
delocalized
electrons
Au
Electrostatics holds the universe together!!!
Conductors and Insulators
• Consider how charge is carried on
macroscopic objects.
– Insulators: In these materials the charges
(electrons) ARE NOT FREE TO MOVE. Plastics,
glass, and other “bad conductors of electricity” are
good examples of insulators.
– Conductors: In these materials, the electrons ARE
FREE TO MOVE. Metals are good examples of
conductors.
– Semi-conductors: the charges are not free to move
normally but small changes can free them and turn
them into conductors. Silicon is a semi-conductor
and is the basis of all modern electronics
Qualitative: Electroscope
• The Phenomena
• Charge electroscope with a PVC rod which has been rubbed with
a cloth. Leaves separate.
» Bring acrylic rod (rubbed with celophane) close to top of
electroscope. Observe leaves approach each other.
• Now repeat experiment, but charge with acrylic rod. Leaves still
separate.
» Now PVC rod causes leaves to approach each other.
» Acrylic rod causes leaves to separate.
• Explanation?
•
•
•
•
There exist two kinds of charge
PVC rod gains electrons from the cloth, negative charge
Acrylic rod loses electrons to the cellophane, positive charge
Unlike charges attract; like charges repel.
Charging electroscopes
• Inside a conductor charges (electrons) are free to move
• The electroscope is made
out of conductors
– conducting main electrode
– 2 conducting gold leaves
• Add some negative
charge (electrons)
– the leaves repel each
other
-
-
More about conductors and charging electroscopes
(continued)
• Add some positive
charge to negatively
charged leaves
(subtract electrons)
– the leaves no longer
repel
-
-
• Add some more positive
charge (subtract more
electrons
– Leaves now positively
charged, repel each other
+
+
Summary and Next Lecture
• Apart from gravity all the forces in the normal world
are produced by electrical charges
• Charges come in two varieties
– negative and positive (electrons and protons)
– negative charge on a normal body means extra electrons
– positive charge means a deficit of electrons
• Next Lecture - Forces between charges
• Read Fishbane Chapter 21
• Don’t forget you need a transmitter