magnetism note any history buffs in the house?jharlow/teaching/every... · 2013-06-17 ·...
TRANSCRIPT
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PHY205H1F Summer
Physics of Everyday Life
Class 9: Electromagnetic Induction, Light
• Faraday’s Law
• Generators and Alternating Current
• Power Production
• Self-Induction
• Power Transmission
• Field Induction
• Electromagnetic
Waves
• The Electromagnetic
Spectrum
• Transparent
Materials
• Opaque Materials
• The Human Eye
Clicker Accuracy Leaderboard
(just for fun)
UTORid Number of correct
clicker answers so far 1 (1) chenyuz2 94
2 (↑105) kuoyu 89 2 (2) tombergs 89 2 (2) zhuhongz 89 5 (↓4) zhangx89 88 6 (6) liyi20 86 6 (6) wanggua8 86 8 (↓6) lichaor1 85 8 (↓4) zhang294 85 10 (10) halekath 84
Chs. 25 & 26 Pre-Class Quiz Results
• 97 students wrote the pre-class quiz; the average was
3.1/4.
• When and electric current is induced in a wire due to a
changing magnetic field, this is called electromagnetic
induction.
• When a wave in 3-D encounters an interface between two
regions where the wave-speed is different, the direction of
the wave bends.
• Some comments:
• “Electromagnetic Induction” x 3
• “How does a pencil placed in a glass of water appear bent
or broken?” “Refraction of Light” x 3
Problem Set 4 is due
Wednesday by 1:10pm.
• An electronic version is available on the course web site.
• If you need a paper copy, I have some on the front table.
Magnetism Note
• Stainless steel is mostly iron, mixed
with chromium and nickel so that it
doesn’t rust.
• Not all stainless steels are magnetic!
• The two main types of stainless steel
are ferritic and austenitic, and
magnets won’t stick to the austenitic
type. • http://www.scientificamerican.com/article.cfm?id=why-dont-magnets-work-on
• Who discovered electromagnetic induction?
A.Faraday.
B.Henry.
C.Maxwell.
D.Both Faraday and Henry.
E.All three.
Any History Buffs in the House?
Faraday’s and Henry’s Discovery of 1831
Two physicists working on opposite sides of the Atlantic independently discovered and described electromagnetic induction.
Michael Faraday was the
son of a blacksmith, born in
1791 in London, England.
Joseph Henry was born in 1797 in
upstate New York to very poor parents.
Schematic of Faraday’s original experiment, which
he first tried unsuccessfully in 1825.
Faraday’s and Henry’s Discovery of 1831
When one coil is placed directly above another, there is no current in the lower circuit while the switch is in the closed position.
A momentary current appears whenever the switch is opened or closed.
When a bar magnet is pushed into a coil of wire, it causes a momentary deflection of the current-meter needle.
Holding the magnet inside the coil has no effect.
A quick withdrawal of the magnet deflects the needle in the other direction.
Faraday’s and Henry’s Discovery of 1831
2
A momentary current is produced by rapidly pulling a coil of wire out of a magnetic field.
Pushing the coil into the magnet causes the needle to deflect in the opposite direction.
Faraday’s and Henry’s Discovery of 1831 Changing the Number of “Loops”
When a magnet is plunged into a coil with twice as
many loops as another, twice as much voltage is
induced. If the magnet is plunged into a coil with 3
times as many loops, 3 times as much voltage is
induced.
Electromagnetic Induction
• It is more difficult to push the
magnet into a coil with many loops.
• This is because the induced
voltage makes a current, which
makes an electromagnet, which
repels the magnet in our hand.
• More loops mean more voltage,
which means we do more work to
induce it.
Which of the following explains the resistance you feel
when pushing a piece of iron into a coil?
A. repulsion by the magnetic field you produce.
B. energy transfer between the iron and coil.
C. Newton’s third law.
D. resistance to domain alignment in the iron.
Faraday’s Law
CHECK YOUR NEIGHBOR Electric Guitar Pick-ups
Voltage is induced in a coil of wire by changing
the magnetic field passing through the coil.
Electrical Generator
A generator is a device that transforms mechanical
energy into electric energy.
A generator inside a hydroelectric dam uses
electromagnetic induction to convert the mechanical
energy of a spinning turbine into electric energy.
Electrical Generator • Opposite of a motor
• Converts mechanical energy into electrical energy via coil
motion
• Produces alternating voltage and current
Electrical Generator
The frequency of alternating voltage induced
in a loop is equal to the frequency of the
changing magnetic field within the loop.
If you push a magnet into a coil of wire, a voltage is
produced.
If you increase the speed with which you push the magnet
into the coil, how does this change the voltage you
produce?
A. Voltage is increased
B. Voltage is decreased
C. No change in voltage.
Faraday’s Law
CHECK YOUR NEIGHBOR
3
If you push a magnet into a coil of wire, a voltage is
produced.
What else is created in the coil?
A. charge
B. current
C. energy
D. force
E. power
Faraday’s Law
CHECK YOUR NEIGHBOR
If you push a magnet into a coil of wire, voltage and current
is produced, so electric power is consumed in the coil.
Where does this energy come from?
A. Atoms in the coil decay and release the energy,
originally stored in their nuclei.
B. Electric potential energy, originally stored in the coil.
C. It is created by the electric forces involved.
D. The work you do pushing the magnet into the coil.
Faraday’s Law
CHECK YOUR NEIGHBOR Metal Detectors
• Activation of traffic lights by a car moving over
underground coils of wire
• Triggering security system at the airport by altering
magnetic field in the coils as one walks through
Inductive
Charging
No metal contacts
between toothbrush
and base!
Transformers
• Input coil of wire—the primary powered by ac
voltage source
• Output coil of wire—the secondary connected to
an external circuit
output
Transformers • Step-up transformer
– produces a greater voltage in the secondary than
supplied by the primary
– secondary has more turns in coil than the primary
• Step-down transformer
– produces a smaller voltage in the secondary than
supplied by the primary
– secondary has less turns in coil than the primary
Transformers
Transformer relationship:
Primary voltage Number of primary turns =
secondary voltage number of secondary turns
A common
neighbourhood
transformer typically
steps 2400 volts
down to 240 volts for
houses and small
businesses.
• A step-down transformer outside your house takes 2400
Volt input AC from the sub-station, and outputs 240 Volts
into your house (which is then further split to 120 V to
your plugs).
If the primary coil in this transformer contains 300 turns
of wire, how many turns of wire should be in the
secondary coil?
A. 3
B. 30
C. 300
D. 3000
E. 30,000
Power Production
CHECK YOUR NEIGHBOR Transformers
Transformer transfers energy from one coil to
another.
• Rate of energy transfer is power.
• Power into primary ≥ power out of secondary
or, neglecting small heat losses:
• (Voltage current)primary = (voltage current)secondary
4
A step-up transformer in an electrical circuit can step up
A. voltage.
B. energy.
C. Both A and B.
D. Neither A nor B.
Power Production
CHECK YOUR NEIGHBOR Power Transmission • Almost all electric energy sold today is in the form of ac
because of the ease with which it can be transformed from one voltage to another.
• Large currents in wires produce heat and energy losses, so power is transmitted great distances at high voltages and low currents.
• Power is generated at 25,000 V or less and is stepped up near the power station to as much as 750,000 V for long-distance transmission.
• It is then stepped down in stages at substations and distribution points to voltages needed in industrial applications (often 440 V or more) and for the home (240 and 120 V).
Power Transmission; Calculation
• Power is generated in Niagara Falls, and the current is sent down a 6-cm diameter copper wire that is 120 km long, which ends in Toronto.
• Each wire has a resistance of 0.5 Ω.
• In Toronto, you have house which wants to use 100 Watts of electricity.
• What is the rate of heat loss in the long-distance wire if the power is delivered at 120 V?
• What is the rate of heat loss in the long-distance wire if the power is delivered at 750,000 V?
Power Transmission; Calculation
• What is the rate of heat loss in the long-distance wire if the power is delivered at 120 V?
+120 V
0.5 Ω
0 V 0.5 Ω
P = 100 W
𝐼 =𝑃
𝑉= 0.83 𝐴
• Total current through wires needed to deliver 100 Watts to Toronto is 0.83 Amps.
• Voltage drop along each wire is 𝑉 = 𝐼𝑅 = 0.83 0.5 = 0.4 𝑉
• Heat loss rate in each wire is P = IV = 0.83 0.4 = 0.3 Watts
• 0.3% inefficiency for a single light-bulb in Toronto
Power Transmission; Calculation
• What is the rate of heat loss in the long-distance wire if the power is delivered at 120 V 750,000 V?
+120 V 750,000 V
0.5 Ω
0 V 0.5 Ω
P = 100 W
𝐼 =𝑃
𝑉= 0.83 𝐴 0.0001 𝐴
• Total current through wires needed to deliver 100 Watts to Toronto is 0.83 0.0001 Amps.
• Voltage drop along each wire is 𝑉 = 𝐼𝑅 = 0.83 0.0001 0.5 =0.4 𝑉 0.00005 V
• Heat loss rate in each wire is P = IV = 0.83 0.0001 0.4 0.00005 = 0.3 5×10−9 Watts
• Efficiency increased by a factor of 40 million (~V2)
The current through the coil is increasing
Electric current in a coil creates a magnetic field.
As the current is increasing, the magnetic field increasing, so it must induce an electric field.
Self Induction
As the magnetic field changes, it creates an electric field, which then can “self-induce” a current in the coil.
This is a direct consequence of Faraday’s Law.
Looking down the coil.. The Induced Magnetic Field
As we know, changing the magnetic field induces a circular electric field.
Symmetrically, changing the electric field induces a circular magnetic field!
The induced magnetic field was first suggested as a possibility by James Clerk Maxwell in 1855.
Field Induction
Electromagnetic induction is a “two-way street.”
• Faraday’s law:
– An electric field is induced in any region of
space in which a magnetic field is changing
with time
• Maxwell’s counterpart to Faraday’s law:
– A magnetic field is induced in any region of
space in which an electric field is changing
with time
5
The mutual induction of electric and magnetic fields can
produce
A. light.
B. energy.
C. sound.
D. None of the above.
Field Induction
CHECK YOUR NEIGHBOR
Maxwell’s Theory of Electromagnetic Waves
A changing electric field creates a magnetic field, which then changes in just the right way to recreate the electric field, which then changes in just the right way to again recreate the magnetic field, and so on.
This is an electromagnetic wave.
E
M
An electromagnetic wave (light) is traveling from left to
right, as shown.
At point P, what does the upward-pointing red arrow mean?
A. The electric wave passes above the point P.
B. A maximum amount of light is at point P.
C. Photons are traveling upward at the point P.
D. A positive electric charge at P would be pushed
upward by the electric force.
Field Induction
CHECK YOUR NEIGHBOR
P
An electromagnetic wave (light) is traveling from left to
right, as shown.
At point P, what does the blue arrow mean, which is
pointing out of the page?
A. A compass at P would point out of the page.
B. The magnetic wave passes in front of point P.
C. A minimum amount of light is at point P.
D. Photons are traveling out of the page at the point P.
Field Induction
CHECK YOUR NEIGHBOR
P
Electromagnetic Waves
Any time you shake an electrically charged object back and forth, you produce an electromagnetic wave.
Consider the following three directions associated with a
particular electromagnetic wave:
– 𝐸 is the direction of electric field oscillations
– 𝑀 is the direction of magnetic field oscillations
– 𝑣 is the direction of wave motion
A. All 3 of 𝐸 , 𝑀 and 𝑣 are parallel
B. All 3 of 𝐸 , 𝑀 and 𝑣 are perpendicular
C. 𝑀 and 𝑣 are parallel to each other, but both are
perpendicular to 𝐸
D. 𝐸 and 𝑀 are parallel to each other, but both are
perpendicular to 𝑣
E. 𝐸 and 𝑣 are parallel to each other, but both are
perpendicular to 𝑀
Electromagnetic Waves
CHECK YOUR NEIGHBOUR
Electromagnetic Waves
The electric and magnetic fields of an
electromagnetic wave are perpendicular to each
other and to the direction of motion of the wave.
Electromagnetic Spectrum • In a vacuum, all electromagnetic waves move at the
same speed
• We classify electromagnetic waves according to their frequency (or wavelength)
• Light is one kind of electromagnetic wave
If a certain material is “transparent” (ie, not
opaque), what does this mean?
A. Electromagnetic waves of all frequencies can
pass straight through it
B. Electromagnetic waves of all frequencies are
reflected from its surface
C. Electromagnetic waves of all frequencies are
absorbed throughout its volume
D. Electromagnetic waves of a certain frequency
can pass straight through it
Electromagnetic Waves
CHECK YOUR NEIGHBOUR
6
Transparent Materials
Glass blocks both infrared and ultraviolet, but it is transparent to visible light.
• When light passes from water into air or vice-versa, it
can bend its direction (making the spoon look broken
below).
• What causes this bending of the light rays?
A. Absorption due to resonance
B. Change in wave speed
C. Reflection
D. Scattering from small particles
E. Selective transmission
Refraction of Light
CHECK YOUR NEIGHBOUR • Photons can only travel at exactly the speed of
light.
• Each photon is absorbed an re-emitted each time it
encounters an atom in a transparent material.
• Averaged over many molecules, light travels more
slowly through a transparent material than through
a vacuum.
Transparent Materials Average speed of light through different
materials
• Vacuum: c = 300,000,000 m/s
• Atmosphere: slightly less than c (but
rounded off to c)
• Water: 0.75 c
• Glass: 0.67 c, depending on material
• Diamond: 0.41 c
Compared with the frequency of illuminating light on a
piece of clear glass, the frequency of light that is
transmitted into the glass
A. is less.
B. is the same.
C. is higher.
Transparent Materials
CHECK YOUR NEIGHBOUR Opaque Materials • Most things around us are opaque—they absorb
light without re-emitting it.
• Vibrations given by
light to their atoms
and molecules are
turned into random
kinetic energy—into
internal energy.
• These materials
become slightly
warmer.
Opaque Materials
Metals • Light shining on metal forces free electrons in
the metal into vibrations that emit their own light
as reflection.
Which reflects more light, a white piece of paper or a black
piece of paper?
A. Black
B. White
C. About the same
Reflection
CHECK YOUR NEIGHBOUR
Which reflects more light, a white piece of paper or a
mirror?
A. White Paper
B. Mirror
C. About the same
Reflection
CHECK YOUR NEIGHBOUR
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© 2010 Pearson Education, Inc.
Mirror versus White Paper
Mirrors
• The surface is flat at distance scales near
or above the wavelength of light
• It looks “shiny”, and you can see images
in it. © 2010 Pearson Education, Inc.
Mirror versus White Paper
White Paper
• The surface is rough at distance scales
near or above the wavelength of light
• Almost all surfaces reflect in this way!
Harlow is looking at his daughter, Zainab. In terms of what
physically allows him to see her, which arrow is best?
Reflection
CHECK YOUR NEIGHBOUR
A B
Rays and Shadows
• A very distant or small light source will
produce a sharp shadow.
• A larger or more nearby light source
produces a blurry shadow.
Shadows
The dark part inside a shadow where the light is
totally blocked is called an umbra.
The penumbra is a lighter part around the edges of a
shadow, where light from a broad source is only
partially blocked.
The photo shows a heavily filtered
image of the sun during a partial
solar eclipse. What is physically
happening to cause this eclipse?
A. Only the penumbra of the Earth’s shadow is falling on
the moon.
B. Part of the umbra of the Earth’s shadow is falling on
the moon.
C. The photographer is standing in the penumbra of the
shadow of the moon which is falling on the Earth.
D. The photographer is standing in the umbra of the
shadow of the moon which is falling on the Earth.
Reflection
CHECK YOUR NEIGHBOUR
The photo shows an image of the
moon during a partial lunar eclipse.
What is physically happening to
cause this eclipse?
A. Only the penumbra of the Earth’s shadow is falling on
the moon.
B. Part of the umbra of the Earth’s shadow is falling on
the moon.
C. The photographer is standing in the penumbra of the
shadow of the moon which is falling on the Earth.
D. The photographer is standing in the umbra of the
shadow of the moon which is falling on the Earth.
Reflection
CHECK YOUR NEIGHBOUR Total Solar Eclipse of August 21, 2017.
Do NOT miss it!
…where will I
be in 4 years
from this
summer?
Driving to
Nashville,
Tennesee!
Total Solar Eclipse of August 21, 2017.
Do NOT miss it!
8
Seeing Light – The Eye
The retina is composed of tiny antennae that
resonate to the incoming light.
• Rods handle vision in low light.
– They predominate toward the
periphery of the retina.
• Cones handle color vision
and detail.
– They are denser toward the
fovea.
– There are three types of cones,
stimulated by low, intermediate
and high frequencies of light.
Retina The retina is filled with rods and cones
The spot where the optic nerve exits contains no receptors and is insensitive to light: blind spot (we don’t notice it because our brain fills in the gap with what it expects)
At the centre of the retina is the macula, which contains twice as many cones as rods
At the centre of the macula is the fovea centralis. It contains no rods, and the cones are very densely packed.
We constantly move our eyeballs to cause the light coming from the object of primary interest to fall on the fovea centralis.
Use right eye only (close left eye)…focus only on the target for this test!
Lock head in position…hold one finger up at arm’s length to cover view of target
Move arm slowly to the right, away from the target
Find your blind spot for that eye
Slide courtesy of Ross Koning, Biology Department, Eastern Connecticut State University
http://plantphys.info/sciencematters/vision.ppt
Use right eye only (close left eye)…our target is a row of numbers
Focus on each number in turn, until the break in the blue lines is in your
blind spot. What is different when the blind spot holds a blank area?
1 2 3 4 5 6 7 8 9
Slide courtesy of Ross Koning, Biology Department, Eastern Connecticut State University
http://plantphys.info/sciencematters/vision.ppt
Optical Illusions
CHECK YOUR NEIGHBOUR
Which half of this box is a
lighter shade of gray?
A. The left half
B. The right half
C. Both halves are
exactly the same
shade of gray
D. I cannot tell!
Before Class 10 on Wednesday
- - the last class!! • Please read Chapters 27 and 28, or at least watch
the 20-minute pre-class video for class 10
• Pre-class reading quiz on chapters 27 and 28 is due
Wednesday June 19 by 10:00am
• Something to think about:
• Why are there exactly 3 “primary colours”? What
physical property of the universe causes this?