Announcements

1
Announcements

• Read chapters 10 and 11 next week we will talk
  about electricity, magnetism, and power
  generation
• HW4 has been posted and is due November 15
  Wednesday - at 200PM. No extensions, since . .
  .
• The second quiz is scheduled for Wednesday,
  November 15 and will cover chapters 5-9. The
  cheat sheet is the same as before and can be
  found here.
• Quiz 2 will have exactly the same format as quiz
  1 10 short answer questions from the text, 6
  multiple choice (one of which has numbers), and 2
  problems to be worked. There are two
  non-numerical questions on heat pumps, a few on
  passive solar, hydrocarbon resources, and
  atmospheric stuff. There are numerical questions
  on solar energy resource and fossil fuel
  resource.
• Bring a calculator (but if you dont have one,
  just set the problems up units and all, and
  leave the arithmetic undone)

2
Electrostatics
Pith balls demo Electroscope demo Tape demo
There are two kinds of electrical charge,
positive and negative, measured in Coulombs.
The electron is a fundamental particle with a
unit of negative charge of -e -1.6x10-19
C. A proton is a composite of other fundamental
particles (quarks) with a net charge of
e1.6x10-19 C. Like mass, charge is grainy
(quantized in units of e), but we normally do not
have the sensitivity to individual grains. Like
charges repel opposite charges attract. How do
we know there are only two kinds of charge? How
do we know the positive charges are really
positively charged, and negative charges are
really negatively charged?
3
Electrical Charge and Atoms
Atoms are made of positively charged nucleus
containing protons and uncharged neutrons and a
cloud of negatively charged electrons. The
number of electrons is normally equal to the
number of protons, so the net charge of a
(neutral) atom is zero. If opposite charges
attract, why dont the electrons fall into the
nucleus? If similar charges repel, why dont the
protons in the nucleus fly apart? Or do
they? Charge conservation is one of the
fundamental laws of physics net charge cannot
be created or destroyed. For example, alpha
decay U-235 (Z92) -gt Th-231 (Z90) a (helium
nucleus, Z 2) ( nucleons is also conserved)
4
Coulombs Law
The force between charges q1 and q2 separated by
distance d is given by Where k is a constant.
Normally the charges are measured in Coulombs
and d is measured in meters, in which case k
9x109 N-m2/C2. If you had taken Phys 101
before this course, you would have learned that
the gravitational force between two masses m1 and
m2 separated by d is Where G 6.67x10-11
N-m2/kg2 is Newtons gravitational
constant. What are the forces between two 1 kg
1C charges at d 1m?
5
Coulombs Law, cont.
Newtons and Coulombs laws have similar form
inverse square law. Still these forces seem to
behave very differently. k is much larger than
G, so the Coulomb force is intrinsically much
stronger that the Gravitational force. If
Coulombs Law is so strong compared to Newtons
Law of gravitation, why do we experience the
latter all the time but rarely directly
experience the former? Or do we experience
Coulombs Law regularly? Whats the minus sign
in Newtons law of gravitation mean? Why is it
not in Coulombs law? Whats the sign of the
masses in Newtons law? How about the sign of
the charges in Coulombs law? In Coulombs law,
does it matter whether we assigned the signs of
the charges correctly?
6
Conductors, Insulators, Semiconductors
Supercon demo
Somewhat conventional notions Superconductors
metals that conduct electricity without
dissipation (wires dont heat up) Conductors
have both free and bound charge facile
transport of electrical currents by free
charge. Semiconductors have bound charge and a
small amount of free charge free charge can be
added with impurities Insulators normally have
only bound charge all electrons are associated
with a given atom or molecule poor conductors
of electrical currents
Even ignoring superconductors, there is a huge
range of electrical conductivities nearly 30
orders of magnitude!
7
Charging by Friction (and Cats)
As the neutrally charged person walks across the
wool carpet, his leather soled shoes have less
desire for electrons than the wool carpet.  As a
result, electrons get stolen from the shoe by the
carpet.  With every step the person becomes more
and more positively charged.  That charge
distributes itself over the body.   When the
positively charged person gets near the metal
door he will actually attract charges from the
door which jump in the form of a spark.  Notice
how only the negative charges (electrons) are
free to move. It is important to point out that
if he was wearing rubber soled shoes on a wool
carpet, his shoes would steal electrons from the
carpet.  He would become more negatively charged
with each step.  When he gets near the door the
electrons will jump from him to the door.  From
his point of view it would look and feel the same
as it did in the first example.  He can't tell
whether charges jumped to or from him.
8
Human Hands (if very dry) Leather Rabbit Fur
Glass Human Hair Nylon Wool Fur Lead Silk
Aluminum Paper Cotton Steel (neutral) Wood
Amber Hard Rubber Nickel, Copper Brass,
Silver Gold, Platinum Polyester Styrene
(Styrofoam) Saran Wrap Polyurethane
Polyethylene (scotch tape) Polypropylene Vinyl
(PVC) Silicon Teflon 
Triboelectric Series
more positive (gives up electrons)
This should help your cat plan its attack . .
. In actual fact, at the molecular level,
charging by friction and rubbing must be
enormously complicated.
more negative (does not like to give up electrons)
9
Demo van der Graaf Generator
How it works
10
Charge Polarization
Charging by induction involves moving charge
around the surface of a conductor. A similar
phenomenon happens in insulators (and
semiconductors, where such effects are key to how
modern electrical devices work).
An atom can have its positive charge in the
nucleus displace relative to its negative charge
in the electron cloud. Such an atom is said to
be electrically polarized. A collection of atoms
or molecules in an insulator can all be polarized
by a nearby charged distribution (e.g., a rod).
This leads to a net positive charge on one side,
and net deficit if charge on the other side of
the object.
11
Charge Polarization, cont.
After electrical polarization, the closer
proximity of opposite charges leads to a net
Coulombic attraction. You have experienced this
many times we call it static charge or static
electricity. Demo pith balls again
12
Electric Fields
Electrical and gravitational forces act at a
distance, on objects that are not in contact. A
convenient and useful construct in such
situations is a force field. The electric
field at a point in space is given by the net
Coulomb force on a charge divided by the charge
itself Since forces have direction, so do
fields. For example, the field around a negative
charge must be directed inward, since a positive
charge is attracted to smaller spacing, and it
must get larger near the negative charge
Left longer arrows, larger field
Right more dense field lines, larger field
13
Electric Fields, cont.
Field lines start on positive charges and end on
negative charges the number of field lines on a
charge is proportional to the magnitude of the
charge.
Field line demo Fields and equipotentials applet
14
Electrical Shielding
Gravitational forces and fields cannot be
shielded. The availability of both positive and
negative charge means it is possible to shield an
electric field.
Field line and shielding applet
Charges on a closed conducting surface rearrange
themselves to ensure that the electric field
inside vanishes! This is possible because there
are two types of charge. For gravity, there is
only positive mass, and it always attracts other
mass, so such cancellation is not possible.
Gravity shields do not exist, unfortunately.
15
Electric Potential (Voltage)
First, recall the gravitational potential
energy The gravitational force on an object
(its weight) is weight mass x g or w mg In
physics, we define work as force x displacement.
So, to raise an object a distance h near the
earths surface requires a work W w x h
mgh. In doing this, we say we have increased the
objects potential energy by this amount. This
energy has the potential to become kinetic
energy. In other words, if we drop the object,
after it has fallen a distance h, its potential
energy has decreased by mgh, but its kinetic
energy has increased by the same
amount. Ignoring friction and air resistance,
the total energy (kinetic potential) is
constant. If we include friction, some energy is
turned into heat, but if we keep track of that,
still the total energy will be constant. These
ideas are general to all forces. How do they
work for the electrical force?
16
Electric Potential (Voltage), cont.
In cartoons There is an electric potential
energy that increases when we separate opposite
charges we do work against the electric force.
If we drop one charge, the electric potential
energy will be turned into kinetic energy.
The electric potential is defined as the electric
potential energy of a charge at a certain
position divided by its charge. Note the
similarity to field Electric field electric
force/charge Electric potential electric
potential energy/charge
17
Electric Potential (Voltage), cont. again
Units! Electric potential electric potential
energy/charge Energy -gt Joules Charge -gt
Coulombs So, the units of electric potential are
Joules/Coulomb We use this combination so much
that we give it a special name 1 J/C 1 Volt
1 V Electric potential and voltage are
used interchangeably. 12 V car battery
delivers 20 C of charge. How much electric
potential energy has it delivered?
18
Electric Potential (Voltage), cont. again and
again
The voltage has a unique value at every point in
space. Unlike the electric field, it has no
direction (energy is energy, with no particular
direction). The voltage is useful since it is
directly related to potential energy. If we
doubled the number of coulombs in a test charge
at a particular position where the voltage is V,
would the electric potential energy of the test
charge with respect to the charged sphere be the
same or would it be twice as great? Would the
electric potential of the test charge be the same
or would it be twice as great?
Field line and electric potential applet
U 2 can make sparks
19
High Voltage Down Under . . .
20
Where Do Voltages Come From?
Charge separation implies a voltage. e.g.,
transferring negative charge from the positive to
negative plates of a capacitor would require
doing work. The plates are at different electric
potentials (voltages). Other ways charges are
separated Rubbing/contact/van der
Graaf Polarization Piezoelectric and other
material effects Electric generators/induction Con
version of chemical energy/batteries
Demos lemon battery thermocouple piezoelectrici
ty
Note the zero of electric potential is
arbitrary. The voltage difference between
terminals in a 1.5V battery is 1.5 V, but the
absolute voltage is not specified. We normally
specify zero voltage by connecting something to
ground the earth is a huge reservoir of
charge, and adding a little more does not notably
change its voltage.
21
What Can Voltage Do? Electric Current
Analogies pump battery pressure
voltage reservoir ground flow
current tube wire constriction resistance
22
Electric Current
When the terminals of a voltage source (e.g., a
battery) are connected through a resistance
(e.g., a light bulb), charge will flow. We call
this a current.
The unit of current is the Ampere, or Amp, or A.
One Amp corresponds to 1 Coulomb/sec. Given that
there are 1/1.6x10-19 electrons/Coulomb, thats a
lot of electrons! Resistance quantifies a
conductors inability to support current. It is
measured in Ohms, and is given the symbol
W. Conductors have low resistance semiconductors
have moderate resistance insulators have very
high resistance.
Demos marble current analogy water flow analogy
A current of 0.5 A flows through a light bulb for
1 minute. How much charge flowed during this
time?
23
Speed of an Electric Current

• How fast do electrons travel in a wire that is
  carrying current?
• Near the speed of light
• As fast as a galloping horse
• About as fast as a snail moves
• Note that when you flick a switch, the lights
  come on very quickly, so the electrons carrying
  the current must go pretty fast, right?

The signal in a wire travels near the speed of
light, basically because the wire does not like
to accumulate charge. The electrons, on the
other hand, have a net drift velocity of
typically a few mm/hour!
24
Electric Current Ohms Law
There is a simple relationship between voltage,
resistance, and current embodied by Ohms
Law Current voltage/resistance or Amperes
Volts/Ohms In the water flow analogy, does a
tube with a large diameter have a large or small
resistance? How much current will flow through a
lamp that has a resistance of 60 O when 12 V are
impressed across it? What is the resistance of
an electric frying pan that draws 12 A when
connected to a 120-V circuit?
25
AC - DC
It is much more efficient to transport
electricity over long distances using AC
alternating current. This is because we can use
transformers to step-up and step-down AC voltages
easily, so power can be transported at higher
voltage and smaller current than would otherwise
be possible. AC voltages and currents
oscillate sinusoidally at 60 Hz (in the US) and
50 Hz (elsewhere). The standard AC voltage in
the US is 120 V, though this is the rms value
the amplitude is actually about 170 V.
There is a very readable account of this in the
book Empires of Light, focusing on efforts of
Edison, Westinghouse, and Tesla to provide cheap
power to the public. Edison got this one wrong.
26
Converting AC to DC
OK, AC is nice for electricity transmission over
long distances, but DC is required by many
appliances (but Tesla did invent the AC motor
one reason he and Westinghouse won the battle
with Edison). To convert from AC to DC current,
we need a diode and a capacitor. A diode allows
current to pass in just one direction
A water capacitor
diode
diode
capacitor
Partially rectified voltage
AC voltage
27
Examples

• What is the ratio of the currents that flow in
  these 2 circuits?

4 V 10 Ohms 8 V
20 Ohms
28
Examples

• What is the ratio of the currents that flow in
  these 2 circuits?

4 V 10 Ohms 2 V
40 Ohms
29
Examples

• What is the ratio of the currents that flow
  through the resistor in these 2 circuits?

1.5 V 10 Ohms
10 Ohms
Dual 1.5V batteries
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