Transcript ppt

General Physics II. Spring 2013
Dr Laycock
Course Elements
Lectures
Recitation
Homework
Monthly Exams(3)
Final Exam
Team Research Challenge
(with Clicker 10%)
(with Quiz 10%)
(Mastering Physics 20%)
(25% total)
(25% total)
(10%)
Mastering Physics: www.MasteringPhysics.com Course Code: GPHYS2SP2012
Policies: Attend Class!!! No Clicker make-ups or exceptions
No Late HW or Make-ups without prior arrangement
Office Hrs: Wed 1-3pm and most days by Appointment (send email)
Tutoring: Centers for Learning, and Physics Dept.
Syllabus
We have 26 Lecture slots, after taking 3 for exams, leaves 23 to
cover the following 4 subjects, roughly 6 lectures apiece.
Section 1:
Section 2:
Section 3:
Section 4:
Electricity and Magnetism
Light and Optics
Relativity and Quanta
Nuclei and Radiation
See the course Webpage for more information
http://faculty.uml.edu/slaycock
www.masteringphysics.com
• General Physics II (Spring 2012)
• GPHYS2SP2013
• ALL weekly homework assignments are
posted on Mastering Physics
• You must register for mastering physics in
order to do the weekly homework
assignments
• Most of you already have an account from
last semester (they last 2 years)
• Course webpage gives DETAILED
instructions on how to register and add this
course to your account
Clickers “iclicker”
• We will be using iclicker this
semester.
• You will be issued with an
iclicker to use this semester,
free of charge.
• Multiple choice questions to
keep you thinking during
lectures.
• Provides feedback to me, so I
know if you’re “getting it” or
not!
Forces of Nature
Which are FUNDAMENTAL forces of nature?
1. Nuclear, Atomic, Molecular, Magnetic
2. Gravity, Strong, Weak, Electromagnetism
3. Electricity, Magnetism, Friction, Gravity
4. Electricity, Gravity, Tension, Strong
Chapter 16
Electric Charge and
Electric Field
Electricity & Magnetism - Chapters 16-21
Ch 16,17 are short, Ch 18 long, and Ch 19 long also
Key learning points for E&M Course Section
1.
2.
3.
4.
5.
6.
7.
What is Electricity?
What is a Magnet?
And what do they have to do with each other?
What does Light have to do with E&M ?
Role in the fundamental workings of nature?
How are E&M manipulated to empower humanity?
Historical background on how we know these things.
Today’s Lecture: Electricity and Charge
• Static Electricity and the nature of Electric
Charge
• Electric Charge in the Atom
• Insulators and Conductors
• Coulomb’s Law
• The Electric Field
• Gauss’s Law
Static Electricity and Electric Charge
Objects can be charged by rubbing,
usually the charge is small, but can be
enormous -Why?
Do all materials respond in the same way?
Electric Charges:
Positive and Negative
Charge comes in two
types, like charges repel
and opposite charges
attract
Benjamin Franklin coined the terms
Positive and Negative, in line with his
theory that there is single “electrical
fluid”, that flows in response to some
kind of pressure, from positive to
negative.
Franklin’s terms were arbitrary, and in
fact electricity (electrons) flow in the
opposite direction to his “guess”.
Conservation of Electric Charge
Electric charge is conserved – the
arithmetic sum of the total charge cannot
change in any interaction.
What other Conservation Laws are there?
The Electric Force Originates with
subatomic particles - an explanation for
the conservation of charge?
Atom:
Nucleus (small, massive,
positive charge)
Electron cloud (large, very
low density, negative
charge)
An overly simplistic model
for several reasons. -What
might they be?
Electric Charge and its Origin
Atom is electrically neutral.
But in some elements the outer electrons are
only loosely bound to the nucleus, and can be
easily dislodged.
Rubbing charges objects by moving electrons
from one to the other.
Electric Charge and Molecules
Polar molecule: neutral overall, but charge not
evenly distributed
Insulators and Conductors
Conductor:
Insulator:
Charge flows freely
Almost no charge flows
Metals
Most other materials
Some materials are semiconductors.
Induced Charge
Metal objects can be charged by
conduction
or
induction:
Induced Charge and non-conductors
Nonconductors won’t become charged by conduction or induction,
but will experience charge separation:
This is why dust sticks to your furniture, screen, glasses etc.
Note how the insulator becomes POLARIZED, while a conductor
does not
Fully explore this in the online Homework Questions 11 and 12.
The Electroscope
The electroscope
can be used for
detecting charge:
An electroscope can be charged either by
conduction or by induction.
The charged electroscope can then be used to
determine the sign of an unknown charge.
Another Sort of Electroscope…..
Electric Forces: Coulomb’s Law
Experiment shows that the electric force
between two charges is proportional to the
product of the charges and inversely
proportional to the distance between them.
Compare this with the law of
Gravitation F = GM1M2
r2
Like Charges Repel, Opposites Attract:
They also Obey Newton’s 3rd Law….
The forces act along the line connecting the
charges, and are equal and opposite
Coulomb’s Law and the Strength of the
Electric Force
Unit of charge: coulomb, C
The proportionality constant in Coulomb’s law is:
Charges produced by rubbing are typically around a
microcoulomb:
One Coulomb may seem huge, but is only the amount of
charge passing through a household lightbulb in about one
second. Can any of you prove this for next time?
Charge on the electron
Electric charge is quantized in units of
the electron’s charge.
Robert Millikan's oil-drop experiment (1908-13) demonstrated this fact directly,
and measured the elementary charge. His experiment measured the force on
tiny charged droplets of oil suspended against gravity between two metal
electrodes. Knowing the electric field, the charge on the droplet could be
determined. Repeating the experiment for many droplets, Millikan showed that
the results could be explained as integer multiples of a common value (1.6x
10−19 coulomb), the charge on a single electron.
This was a key piece of evidence for the existence of electrons, atoms and
molecules
Calculate the number of electrons in a bucket of
water, and thus the resulting force between two
buckets of water
Lets do this on the Document Camera, where I can
write….
Coulomb’s Law
The proportionality constant k can also be
written in terms of
, the permittivity of free
space:
(16-2)
Using Coulomb’s Law for Multiple
Coulomb’s law strictly
Charges
describes point charges.
Superposition: for multiple point charges, the forces
on each charge from every other charge can be
calculated and then added as vectors.
The net force on a charge is the vector
sum of all the forces acting on it.
Where would this sort of thing be important?
Wait a minute……Where do Electric
Forces Come From?
Before we get bogged down with math, what’s all this
about charges attracting and repelling each other?
How do they do it?
Notice that the charges do not need to touch.
The Electric Field
Early scientists and philosophers struggled with the idea
of “action at a distance”.
How was the electric force propagated?
Michael Faraday proposed that a “field” extended
outwards from all charged objects, and that these fields
interacted with one another.
Fields are also a great mathematical convenience
Definition: The electric field E
is the force exerted per unit
charge, assuming the “test
charge” is extremely small, so
it doesn’t add its own field to
the mix!
Visualizing Electric Fields: A Single Point-Charge
The number of field lines starting
(ending) on a positive (negative)
charge is proportional to the
magnitude of the charge.
The electric field is stronger
where the field lines are closer
together.
Visualizing Electric Fields: Two Charges
The lines emanating from two equal charges, opposite in
sign will connect to form a Dipole (two poles).
While if the charges are the same, the lines will avoid each
other, and the charges repel
Calculating the Electric Field
For a point charge Q, we calculate its
Electric Field using an imaginary (minute)
test charge “q”:
Since the force between 2 charges is given
by Coulomb's law, the force felt by our tiny
test charge q would be F= k Qq/r2
Thus the force per unit charge (Electric field)
would be
E= F/Q
We can work that out: E = k Qq/r2
q
the q’s cancel, leaving ->
If we know the Field, its easy to find the
force exerted on charges anywhere in it!
Force on a point charge q in an electric field:
For a complex distribution of Charges, we
just add up the contribution of each’s field
at the point in question:
This is called the Superposition principle
for electric fields
Vector Electric Field Calculation - on Doc-Cam
Similar to Example 16.9 in the book
Find the Direction and Magnitude of the Electric Field
due to a a pair of unequal Charges.
(takes 10 min, so do next time if not today)
More Complex Field Lines and Symmetry
The electric field between two
closely spaced, oppositely charged
parallel plates is constant. -where
might this configuration occur?
Electric Fields and Conductors
The static electric field inside a conductor is zero. The free
charges “instantly” align themselves to totally cancel the
external field.
The net charge on a conductor is all on its surface. -Charges
want to be as far apart as possible.
Faraday Cage, Car in Thunderstorm etc…
Gauss’s Law
The net number of field lines through the
surface is proportional to the charge
enclosed, and also to the flux, giving
Gauss’s law:
This can be used to find the electric field
in situations with a high degree of
symmetry.
For example the total charge on a
thundercloud. NEXT TIME!
Photocopy Machines and Computer
Printers Use Electrostatics
Photocopy Machines and Computer
Printers Use Electrostatics
Laser printer is similar, except a computer
controls the laser intensity to form the image
on the drum
Summary of Chapter 16
• Two kinds of electric charge – positive and
negative
• Charge is conserved
• Charge on electron:
• Conductors: electrons free to move
• Insulators: nonconductors
Summary of Chapter 16
• Charge is quantized in units of e
• Objects can be charged by conduction or
induction
• Coulomb’s law:
• Electric field is force per unit charge:
Summary of Chapter 16
• Electric field of a point charge:
• Electric field can be represented by electric
field lines
• Static electric field inside conductor is zero;
surface field is perpendicular to surface
• Electric flux:
• Gauss’s law: