Download Electric Field - Purdue Physics

PHYSICS 272
Electric & Magnetic
Interactions
Prof. Yulia Pushkar
[email protected]
Room: 70, Phone: 63279
Course Content
This course deals with electric and magnetic
interactions, which are central to the structure of matter,
to chemical and biological phenomena, and to the
design and operation of most modern technology.
The main goal of this course is to have you engage in a
process central to science: the attempt to model a
broad range of physical phenomena using a small set of
powerful fundamental principles. The specific focus of
the course is an introduction to field theory, in terms of
the classical theory of electricity and magnetism (E&M).
The course also emphasizes the atomic structure of
matter, especially the role of electrons and protons in
matter.
Textbook
The textbook is Matter & Interactions, vol II:
Electric & Magnetic Interactions by R. Chabay &
B. Sherwood (John Wiley & Sons 2007). We will
cover almost all of the topics in this volume.
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.
Lectures slides will be available shortly after lecture concludes.
• Homework
– Homework and lab assignments will be posted on the web. See
WebAssign (and Calendar section) for due dates.
• 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. We will start counting
clicker questions towards your grade at lecture #4.
– 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 two 1.5-hour exams and a 2-hour final exam. All exams are closedbook, but relevant formulas and constants will be provided.
• Grades:
– The final grade will be determined on the following basis (Course Total = 700
points):
• 200 points - final exam
• 100 points each - two 1.5-hour exams (8-9:30 pm Feb 13, April 3)
• 75 points - WebAssign homework
• 100 points – Labs
• 25 points - Clicker Questions & Attendance
• 100 points – Recitation Problems
Argonne National Laboratory
Synchrotron
Clicker Question 1
Set your clicker frequency to AB
My major is –
A)Engineering
B)Science
C)Liberal Arts
D)Education
E)Other
Clicker Question 2
Set your clicker frequency to AB
The origin of word “electron” comes from
A) current caring wire
B) fossilized tree resin
C) spring flower
D) attraction between objects
The word electron was coined in 1894 by Johnstone
Stoney (an Irish physicist) and is derived from the Latin
electrum or the Greek elektron meaning amber (fossilized
tree resin).
Structure of Atom
Matter consists of atoms
1 cm3 : ~1024 atoms
.
Nucleus
Proton is positively charged
neutron
0
mp mn  1.7*10-27 kg
10-14m
electron is negatively charged
Electron cloud size is about
10-10m = 1Å
Point Charges
• Two types: positive and negative
• Like charges: repel
• Opposite charges: attract
• Charge is quantized in units of e
Millikan’s oil drop experiment (1910-1913)
• Point charge: Size is small compared to the distance between
it and other objects of interest
• Electric charge is an intrinsic property of the fundamental
particles that everything is made of
The Coulomb Force Law
Q1Q2
F =F=
2
4pe 0 r
1
"The magnitude of the electrostatic force between two
point charges is directly proportional to the
magnitudes of each charge and inversely proportional
to the square of the distance between the charges."
Q1
Q2
F
F
Charles-Augustin de Coulomb
(1736 - 1806)
The Coulomb Force Law
1 Q1Q2
F=
r̂
2
4pe 0 r
r
+
+
2
0 = permittivity constant
F21
Force on “2” by “1”
1
Force repulsive
+
r
-
F21
2
1
Force attractive
• The force exerted by one point charge on another acts along line
joining the charges.
• The force is repulsive if the charges have the same sign and
attractive if the charges have opposite signs.
Units and Constants
SI units of electric charge: Coulomb, C
1 Q1Q2
F =F=
Constants:
2
4
pe
r
0
1/40 = 9x109 N.m2/C2
0 = 8.85x10-12 C2/N.m2 permittivity constant
e = 1.602x10-19 C
1 C = 6.24x1018 elementary charges
Particle
electron
positron
proton
antiproton
muon
pion
neutron
Charge
-e
+e
+e
-e
+e or –e
+e or –e or 0
0
How Strong is the Coulomb Force
Definition of Electric Field
q1q2
F=
4pe 0 r 2
1
æ 1 q1 ö
÷
F = q2 çç
2 ÷
è 4pe 0 r ø
F2 = q2 E1
E1 = F2 / q2
Electric Field
E = F /q
If our probe charge is positive or
negative – F due to given E will point in
correct direction.
Electric field has units of Newtons per Coulomb:
E º E (x, y, z, t )
[N/C]
The Electric Field of a Point Charge

E
1
Q
rˆ
2
40 r
+
Direction and magnitude
Electric Field
There is something in space
waiting for a charged particle to
interact with it!
This virtual force is called
electric field.
An electric field created by charge is present throughout
space at all times, whether or not there is another charge
around to feel its effect.
The Physical Concept of
‘Field’
Field: physical quantity, can be scalar or vector
Examples:
Temperature T(x,y,z,t)
Air flow, gravitational field
m
F = mg
F = qE
q
Example Problem
A particle with charge +2 nC (1 nanoCoulomb=10-9 C) is located
at the origin. What is the electric field due to this particle at a
location <-0.2,-0.2,-0.2> m?
q1
E1 =
rˆ
2
4pe 0 r
Solution:
1
1. Distance and direction:
r
r = observed _ location - source _ location
r = -0.2,-0.2, -0.2 - 0, 0, 0 = -0.2,-0.2, -0.2
r =
( -0.2 )
2
+ ( -0.2 ) + ( -0.2 ) = 0.35 m
2
2
-0.2, -0.2, -0.2
r
r̂ =
=
= -0.57, -0.57,-0.57
r
0.35
Example Problem
q1
E1 =
rˆ
2
4pe 0 r
2
-9
ö
æ
1 q æ
Nm
2
´
10
Cö
N
9
֍
÷ = 147
E=
= çç 9 ´ 10
2
2 ֍
2
2 ÷
4pe 0 r
C øè 0.35 m ø
C
è
2. The magnitude of the electric field:
3. The electric field in vector form:
Nö
æ
E = Erˆ = ç147 ÷ - 0.57,-0.57,-0.57
Cø
è
N
E = - 84,-84,-84
C
1
Forces due to an Electric Field
Example: The electric field at a particular location
is <-300,0,0> N/C. What force would an electron experience
if it were placed in this location?
Y
E
e
F
X
Solution: F = -eE = -1.6 ´ 10-19 C - 300,0,0 N/C
F = 4.8 ´ 10-17 ,0,0 N
No ‘self-force’!
q
E
2
40 r
1
r  0, E  
Point charge does not exert field on itself!
The Superposition Principle
The net electric field at a location in space is a vector sum
of the individual electric fields contributed by all charged
particles located elsewhere.
The electric field contributed by a charged particle is
unaffected by the presence of other charged particles.
The Superposition Principle