ISP209s10 Lecture 13 -1-
Today
• HW and Extra Credit due Thursday 8am
• Exam #2 is nominally on 3/18. I don’t want
to rush it though. I will let you know before
spring break if I need to push it back.
• Electromagnetism
• Waves
ISP209s10 Lecture 13 -2-
Clicker question
What will happen if
you cut this magnet
In half at the midpoint?
A) You will get two smaller magnets, one with a North pole only
and the other with a South pole only
B) You will get two smaller magnets, each one having both a North
and a South pole
ISP209s10 Lecture 13 -3-
Clicker question
What will happen if
you cut this magnet
In half at the midpoint?
A) You will get two smaller magnets, one with a North pole only
and the other with a South pole only
B) You will get two smaller magnets, each one having both a North
and a South pole
No isolated north or south pole (“monopole”) has ever been
seen in nature or manufactured in the laboratoryISP209s10 Lecture 13 -4-
Clicker question
True or False. You can make your own magnet from
Scratch with some wire and a battery to make electrical
Current flow thru the wire. A)True B)False
ISP209s10 Lecture 13 -5-
Clicker question
. A)True B)False
Moving charge creates a magnetic
field
At the microscopic
Level, “permanent”
Magnets (e.g.,
ISP209s10 Lecture 13 -6-
• The earth’s gravitational field surrounds the earth; it
would be there even if the moon wasn’t.
• The existence of a field in a region of space means that
if an appropriate object is in this region, it will
experience a force.
• An electric field will exert a force on a charge
• a magnetic field will exert a force on a magnet or a
moving charge.
Force Fields: A disturbance of space
ISP209s10 Lecture 13 -7-
Electric Field example
• If we move a test charge, q, in the vicinity of anothercharge we can make a map of the force (magnitude &direction)
• Define: Electric field E = F/q• Electric field is a vector. Its units are N/C or V/m
(volts/meter). It points in the direction of the force.
• Once we know the electric field we can calculate theforce everywhere in space: F=qE
F = electric field times charge of the object in the field
Which is underlying reality, the force or the field?ISP209s10 Lecture 13 -8-
Samples
• Electric field lines point away from positive
charge and toward negative charge.
• Charge generates an electric field. F = qE
ISP209s10 Lecture 13 -9-
Maxwell’s Equations Unify Electricity/Magnetism
Weird.The equations predict the existence ofsomething (a wave) that travels with speed c, thespeed of light!
Charge makes an electric field.
Moving charge makes a magnetic
field.
Changing magnetic field makes an
electric field (“Faraday’s Law”)
Magnets always have a north and a
south pole0
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ISP209s10 Lecture 13 -10-
Faraday’s law is behind
the whole industry of
electric power
generation.
If a power source can be
made to turn a shaft in a
magnetic field,
electricity will be
produced.
Electricity generation
ISP209s10 Lecture 13 -11-
The Strength of the Electric Field
• Electric potential – SI unit is the Volt (V)
• Electric field is rate of change of potential
• The minus sign means that electric fields
point from + to – charge.
x
VE
!
!"=
ISP209s10 Lecture 13 -12-
A visual picture – Electric Potential
-
+
+q
Steep slope means a larger force.
Positive charge makes a hill
Negative charge makes a
valley.
ISP209s10 Lecture 13 -13-
Example: Lightning
Potential difference of 100 MV is developed between cloud and
ground. In the bolt about 5 C of charge are transferred (on average).
1 electron = 1.602E-19 C ISP209s10 Lecture 13 -14-
Sample Problem
What is the magnitude
of the electric field at:
• 0.5 m?
• 1.5 m?
• 3.0 m?
The field is 0 V/m at 0.5 m and 3.0 m since the slope is zero.
( )( ) m
V
mm
VVE 100
12
0100
x
V 1.5m)at ( =
!
!=
"
"=
ISP209s10 Lecture 13 -15-
Electric fields and potential
• In equilibrium the electric field in a metalconductor is zero.
• This means that inside a metal the electricpotential is flat, like the flat top of a table.
• Sitting inside a metal cage is like sitting on top ofa large, flat table. As long as you are in the center,there is no danger of falling off.
• This is why being in a car during a thunder stormis relatively safe.
ISP209s10 Lecture 13 -16-
Electric Field Example
Electric Field
E = -!V/!x =-(50V-90V)/1m= 40 V/m
+ means to the right in this case
Q = 0.5 µC = 0.5x10-6 C What is the magnitude of the electric
force on Q?
F = qE
F = 0.5E-6C x 40 N/C = 20E-6 N
Not asked for, but the direction is +,
to the right.
ISP209s10 Lecture 13 -17-
Flow of Charge - Current
• Current is the rate of flow of charge. SI
units is Ampere = 1 Coulomb/second
• Batteries are like pumps that lift charge to a
higher potential. The charge flows down the
hill to the other side of the battery.
A battery
is like a
pump.
Moving Charge
does work on the
way downV
ISP209s10 Lecture 13 -18-
4 Types of electric materials in Nature
• Conductor – electrons in the conduction band;electrons relatively free to flow (copper, aluminum,gold, silver)
• Insulator – no electrons is the conduction band;electrons can not flow (wood, most rubber, mostglass, most plastic)
• Semiconductor – at finite temperature, someelectrons are in the conduction band (used in mostelectronics; silicon, germanium)
• Superconductor – at very low temperature electronspair and can move freely without resistance(Niobium, Titanium, Lead)
ISP209s10 Lecture 13 -19-
Conductor
electrons
energy
Conduction
band
V
Electrons hit bumps, but are free to roll.
e.g., copper, iron, aluminum, silver, gold, …
ISP209s10 Lecture 13 -20-
Insulator
electrons
energy
Conduction
band
V
Electrons are not free to roll.
e.g., rubber, plastic, glass, wood,…
ISP209s10 Lecture 13 -21-
Semiconductors
V
electrons
energy
Conduction
band
light LED – light emitting diode
e.g., Silicon in computer chips
ISP209s10 Lecture 13 -22-
Superconductor
electrons
energy
Conduction
band
V
No resistance to flow (also no use of energy)
The magic behind magnetically levitating trains
ISP209s10 Lecture 13 -23-
Have a friend hold 1 end of a spring while
you “snap” the other end up and down.
A wave travels along a Slinky. But what is
traveling?
It’s the shape that travels, and it carries
energy with it.
The shape – or disturbance – is the wave, and
the Slinky is the medium through which the
wave travels.
Waves: Something else that travels
ISP209s10 Lecture 13 -24-
Wavelength " is the distance from any point on
the wave to the next similar point.
Frequency # (1 Hertz = 1/s) is the number of
vibrations made by any point per second.
Waves: Something else that travels
ISP209s10 Lecture 13 -25-
Amplitude is the maximum height or depth of
the wave (measured from the midpoint).
Waves: Something else that travels
ISP209s10 Lecture 13 -26-
Finally, the wavespeed is the speed at which the
disturbance travels through the medium.
Waves: Something else that travels
ISP209s10 Lecture 13 -27-
So, what enters your eyes when you see light? In
particular, is it a particle or a wave?
Light: Particle or Wave?
Answer: It depends. More later on quantum mechanics and the
Wave-particle duality of Nature.
For now, we take the pre-quantum view that light is a wave.
We’ll see why light has wave-like features next class…
What is “waving”?
ISP209s10 Lecture 13 -28-
Wavelength and Frequency
" = 1.0 m
Wavelength Frequency = 1/period
Distance over which the wave
repeats
Number of cycles (repeats) per
second.
period = 2.0 s