Download Physics 272: Electricity and Magnetism

Physics 272: Electricity and
Magnetism
Instructor: Mark Palenik
Office hours: by appointment
Monday June 11th 2012
Course Info
 Syllabus and schedule posted on course website
 Homework through webassign, due Thursday and
Sunday
 Bring iclickers to class
 Help center available for homework assistance
(room 12)
 I will be teaching weeks 1-4 (Mark Palenik)
Office hours available by appointment
 Jonathan Nistor will teach weeks 5-8
Dr. Hirsch Says. . .
Dr. Hirsch
Textbook
The textbook is Matter & Interactions,
vol III: Electric & Magnetic Interactions
by R. Chabay & B. Sherwood (John
Wiley & Sons 2007). We will cover
almost all of the topics in this volume.
See the table of contents at the front of
the book (which also includes the
contents of Volume I).
Make sure it is the Third Edition. The new book comes with a free coupon for
WebAssign, the on-line homework service. Follow the instructions and get yourself
registered.
General Information
Room PHYS 144: Undergraduate office
Room PHYS 12: Help center
Room PHYS 290: Physics Library
We will use WebAssign for homework and lab
assignments.
You will be able to access your scores in CHIP.
See “Important Links” on course web page for
details!
For questions concerning WebAssign contact:
V.K. Saxena: Office: PHYS 176, Phone: 49575
Activities and Responsibilities
• In-class activities and responsibilities
– You are responsible for attending all classes, and attendance will
count toward your grade.
– Bring a scientific calculator to class.
– If you miss class, it is your responsibility to find out what you missed.
• Homework
– Homework and lab assignments will be posted on the web.
Assignments are due on Thursdays and Sundays.
• Outside class
– Study assigned textbook sections.
– An assignment to study sections of the textbook means:
• Read the assigned textbook sections thoughtfully.
• Do the "stop and think" activities.
• Write brief solutions to the in-line "exercises" and keep them in a
notebook.
Quizzes, Exams, Grades
• Clicker Questions in Lecture:
– Short multiple choice questions will be posed in lecture. The purpose is to assess
your understanding. It will also be used to check attendance.
– You have to purchase an iClicker ( http://www.iclicker.com ) from the bookstore.
– You must register your clicker ID in CHIP!!!!
• Exams:
– There will be a two hour midterm and a 2-hour final exam. Exams will be during lab.
Exams are closed-book, but relevant formulas and constants will be provided.
• Grades:
– This was how the course was graded last semester—possibly still.
– The final grade will be determined on the following basis (Course Total = 700 points):
• 200 points - final exam
• 200 points midterm
• 75 points - WebAssign homework
• 100 points – Labs – lab quizzes need to be added to CHIP
• 25 points - Clicker Questions & Attendance
• 100 points – Recitation Problems
Lectures don’t work??
• Say hello to the people next to you
• Form small groups with your surrounding
people. You’ll be discussing with them.
• Don’t worry about sitting next to the same
people from lecture to lecture, but you can if
you want
Today
•
•
•
•
Charges
Review vectors and force
Force between two charged objects
Electric field of a point charge
Electric Charge
•
•
•
•
Two kinds of charges, positive (+) and negative (-)
Protons have positive charge (1.602x10-19 C)
Electrons have negative charge (-1.602x10-19 C)
Two positive charges repel
– Proton/proton
• Two negative charges repel
– Electron/electron
• Opposite charges attract
– Proton/electron
Protons and Electrons
• Protons and electrons have equal and opposite charges
• Protons are much more massive (~2000x) than
electrons
e = 1.602x10-19 C
Charge (C)
Mass (kg)
Proton
1.602x10-19
1.7x10-27
Electron
-1.602x10-19
9x10-31
• Electrons are fundamental particles, protons are
composed of 3 particles (quarks)
U
P
D
U
E
Review: Force
• From Newton’s laws (II):
𝐹=
𝑑𝑃
𝑑𝑡
𝑑
𝑚𝑣
𝑑𝑡
m is constant
=
𝑑𝑣
=𝑚
= 𝑚𝑎
𝑑𝑡
• Also recall (III): For every action, there is an
equal and opposite reaction*
*There is a case you will learn about in magnetism where this is not quite true,
although we still have conservation of momentum which usually follows from the
3rd law.
Review: Vectors
• Vectors have magnitude and direction
– e.g. 4 meters east, <4, 0, 0>, or 4𝑥
?
Direction
Magnitude
Point in the 𝑟 direction
𝑟=<x,y,z>
1
𝑟
𝑟 = 2 2 2< x , y , z > =
𝑥 +𝑦 +𝑧
𝑟
More with vectors
• Charge q1 is located at <x1,y1,z1>
• Charge q2 is located at <x2,y2,z2>
1. Write an expression for the position of q2 relative to q1
2. Write an expression for the position of q1 relative to q2
q1
q2
1. <x2 – x1, y2 – y1, z2 - z1> = 𝑟2 − 𝑟1 = 𝑟21
2. <x1 – x2, y1 – y2, z1 - z2> = 𝑟1 − 𝑟2 = 𝑟12
Conceptual question
• Given two charges, q1 and q2, is this a
possible expression for force:
• The force on q2 due to q1:
𝐹=
1
𝑞1
𝑟21
2
4𝜋𝜖0 𝑟21
The force on q1 due to q2
1 𝑞2
𝐹=
𝑟12
2
4𝜋𝜖0 𝑟12
Conceptual question 2
• Given two charges, q1 and q2, is this NEW
expression a possible expression for force:
• The force on q2 due to q1:
−1 𝑞1
𝐹=
𝑟21
2
4𝜋𝜖0 𝑟21
The force on q1 due to q2
−1 𝑞2
𝐹=
𝑟12
2
4𝜋𝜖0 𝑟12
Force on charges
• Two charges, q1 and q2
• Force on q1 due to q2
𝐹=
1 𝑞1𝑞2
𝑟
4𝜋𝜖0 𝑟12 2 12
• Force on q2 due to q1
𝐹=
Equal magnitude, opposite sign
Satisfies Newton’s 3rd law
1 𝑞1𝑞2
𝑟21
2
4𝜋𝜖0 𝑟21
We will see there is a relationship between this force and
something called the electric field
“Clicker” question
• Two positively charged particles start from rest at point A)
• Which of the following is true about the particles at B) and
C)
A)
B)
C)
a) The particles are moving faster at B) than at C)
b) The particles are moving faster at C) than at B)
c) The particles are moving the same speed at B) and C)
Recall: 𝐹 =
1 𝑞1𝑞2
𝑟12
2
4𝜋𝜖0 𝑟12
Electric fields
• We could rewrite the previous equation as 𝐹 = 𝑞𝐸
• Force acting on a particle = charge of the particle * the electric field
it is sitting in
• Electric fields point away from negative charges and toward positive
charges
• Point in direction that positive charge would be pushed.
Electric Fields (Coulomb’s law)
• The electric field produced by a particle with charge q is
1 𝑞
𝑟
2
4𝜋𝜖0 𝑟
• Where r is the distance from the particle, and 𝑟 is a unit vector
pointing away from the particle.
• Pointing stuff?
• If a particle at <-1, -1, -1> has a charge of 2C, what is the electric
field at <1, 1, 1>?
• What is the force on a particle at <1, 1, 1> with a charge of 3C?
• What is the force on the first particle, with a charge of 2C?
Structure of atoms
• Atoms are made up of protons, neutrons, and electrons
• The nucleus is a tiny object at the center of the atom
• An electron cloud surrounds the nucleus
• Question: If the nucleus is made up
of protons and neutrons, which are
Larger than electrons, why is the
electron cloud so big and the nucleus
So small?
nucleus
Electron cloud
Charge of atoms
• Atoms are usually neutral (equal number of
protons and electrons)
• Charged atoms are called ions
• Electrons can be pulled from the nucleus, but it
requires work
– Must remove an integer number of electrons
• The protons at the center attract the electrons,
but the electrons repel each other.
• The attractive force must be slightly larger than
the repulsive force for electrons to remain bound
Extra: Electric fields in other
dimensions
• Later on you will learn about flux, which explains why E
goes as 1/r2.
• In the mean time, let’s see if we can figure out how E would
behave in 1 or 2 dimensions.
• Maxwell’s equations say
• In 1d, this becomes
𝜕𝐸𝑥
𝜕𝑥
𝜕𝐸𝑥
𝜕𝑥
=
𝑞
𝜖0
+
𝜕𝐸𝑦
𝜕𝑦
+
𝜕𝐸𝑧
𝜕𝑧
=
𝑞
𝜖0
Extra: 1d Electric field
• Consider a 1 dimensional world, where everything is on a
line (x axis).
Point charge Q at x=0
X’
• What is the charge at x=x’? X’ can take on any value except
zero.
𝜕𝐸𝑥
𝑞
• Using Maxwell’s equation
= what is the derivative of
𝜕𝑥
𝜖0
the electric field at x’?
• What does this tell us about the electric field?
Extra: Electric field in n dimensions
•
•
•
•
In 1d: Electric field goes as 1/r0
In 2d: Electric field goes as 1/r
In 3d: Electric field goes as 1/r2
In nd: Electric field goes as 1/rn-1