magnetism note any history buffs in the house?jharlow/teaching/every... · 2013-06-17 ·...

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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.


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.


http://www.scientificamerican.com/article.cfm?id=why-dont-magnets-work-on









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

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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?


• 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


• 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

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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


CHECK YOUR NEIGHBOUR


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



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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


Which reflects more light, a white piece of paper or a

mirror?

A. White Paper

B. Mirror

C. About the same

Reflection


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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


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


Reflection


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


D. The photographer is standing in the umbra of the


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!

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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


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?

magnetism note any history buffs in the house?jharlow/teaching/every... · 2013-06-17 ·...

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