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Lecture 29Chapter 25

TransformersChapter 26

Properties of Light

Back Voltage & Counter Torque in MotorAn electric motor turns because there is torque on it due to magnetic force on the current. We would expect the motor to accelerate unless there is some sort of drag torque.That drag torque exists, and is due to the induced voltage, called a back voltage. Changing magnetic field in a motor coil induces a voltage in the coil itself.

Back voltage in motor coil

Vinduced=108 V

Electric motors that slow down too much often burn out due to excessive current flow when back voltage is reduced.

Electric Power Production & TransmissionUsing Faraday and Henry’s discovery of electromagnetic induction, Nikola Tesla and George Westinghouse showed that electricity could be generated in sufficient quantities to light cities.

Problem is how to transmit the power long distances without ohmic heating losses in wires. Solution: Transformers

Transformers: Link circuits magnetically

Transformer

• Input coil of wire—the primary; powered by ac voltage source

• Output coil of wire—the secondary; connected to an external circuit that uses the power

• If same number of turns in primary & secondary, voltage in secondary same as primary.

Voltage “Step-Up” Transformer

This is a step-up transformer. There are more turns in secondary than primary, so the voltage in the secondary coil is larger than the voltage in the primary:

Transformer TypesTransformers can step-up or step-down voltage.• 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

Power Transfer with TransformersTransformer transfers energy from one coil to another. There is no energy gain or loss.Example: • voltage stepped up before leaving power station• voltage stepped down for distribution near cities by

cables that feed power to the grid• voltage stepped down again before

being supplied to businesses and consumers through substations

Transformers & Transmission of PowerTransformers work only if the current is changing; this is one reason why electricity is transmitted as ac.

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).

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 ProductionCHECK YOURSELF

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.

Explanation:Stepping up energy is a conservation of energy no-no!

Power ProductionCHECK YOURSELF

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Chapter 26Properties of Light

Part VI: Light

Field InductionElectromagnetic induction is a “two-way street.”• Faraday’s law

– States that 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– States that a magnetic field is induced in any region of

space in which an electric field is changing with time• Electromagnetic wave (light) is produced by the

mutual induction of electric and magnetic fields.• Speed of light is the speed of emanation of these

fields.

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Electromagnetic WavesMoving charges (currents) create magnetic fields.Oscillating magnetic fields create electric fields.These effects create electromagnetic waves.

Light originates from the accelerated motion of electrons

If an electron vibrates up and down 1000 times each second, it generates an electromagnetic wave with a

A. period of 1000 s.B. speed of 1000 m/s.C. wavelength of 1000 m.D. None of the above.

Electromagnetic WavesCHECK YOURSELF

If an electron vibrates up and down 1000 times each second, it generates an electromagnetic wave with a

A. period of 1000 s.B. speed of 1000 m/s.C. wavelength of 1000 m.D. None of the above.

Explanation:The vibrating electron would emit a wave with a frequency of 1000 Hz, which is not in the list above.

Electromagnetic WavesCHECK YOUR ANSWER

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Speed of EM Waves in VacuumSpeed of electromagnetic wave is 300,000,000 m/s

c = 3.0 x 108 m/s (in vacuum)“Speed of Light”

Since(Wavelength) =

then radio station at 100 Megahertz frequency has

(Wavelength) = = 3 meters

(Wave Speed)(Frequency)

300,000,000 m/s100,000,000 Hz

Electromagnetic Spectrum• Frequency f and wavelength λ related by f λ = c

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Electromagnetic Spectrum

Very broad spectrum ofelectromagnetic waves

Cell phone

Which of these is fundamentally different from the others?

A. Sound wavesB. Light wavesC. Radio wavesD. X-rays

Electromagnetic SpectrumCHECK YOURSELF

Which of these is fundamentally different from the others?

A. Sound wavesB. Light wavesC. Radio wavesD. X-rays

Explanation:B,C,D are all electromagnetic waves, which can travel through vacuum as well as through some materials. Sound, which is a mechanical wave, cannot travel through vacuum

Electromagnetic SpectrumCHECK YOUR ANSWER

Light in Matter• When light hits matter, it can

– Be transmitted through (Transparent material)– Be absorbed (Opaque material)– Be reflected (Metals; considered opaque)

• Light travels more slowly in matter than in vacuum– c’ speed of light in material; c’ < c– Index of refraction n = c/c’

• Slowing of light in materials depends on frequency (color) --- ”Dispersion”– Higher freq (violet) – more slowing– Lower freq. (red) – less slowing

Transparent Materials

How light penetrates transparent material such as glass

• Electrons or molecules in the glass are forced into vibration.

• Energy is momentarily absorbed and vibrates the electrons in the glass.

• This vibrating electron either emits a photon (a corpsucle of light) or transfers the energy as heat.

• Time delay between absorption and re-emission of energy of vibrating electrons results in a lower average speed of light through a transparent material.

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Transparent & Opaque MaterialsA material is transparent or

opaque to different wavelengths depending on its atomic properties.

Some opaque materials absorb certain waves while other materials reflect those electro-magnetic waves.

For visible light, glass is transparent, while rubber and metal are opaque (rubber absorbs, metal reflects)

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Check YourselfThe forced oscillations of electrons in glass

caused by ultraviolet light happen to match the natural frequency of electrons in glass. So what happens?

So is glass opaque to ultraviolet light?

Transparent Materials• In glass, infrared waves, with frequencies lower than

those of visible light, cause not only the electrons but entire atoms or molecules to vibrate, increasing the temperature of the structure.

• So we see that glass is transparent to visible light, but not to ultraviolet and infrared light.

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 the illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted

A. is slightly less.B. is the same.C. is slightly higher.D. depends on the type of plastic.

Transparent MaterialsCHECK YOURSELF

Compared with the frequency of illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted

A. is slightly less.B. is the same.C. is slightly higher.D. depends on the type of plastic

Explanation:Speed of light in plastic may vary, but the frequency transmitted doesn’t.

Transparent MaterialsCHECK YOUR ANSWER

Opaque Materials

• Most things around us are opaque—they absorb light without re-emitting it.– Books, desks, chairs, and people are opaque.

• 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.

Opaque MaterialsShadows• A thin beam of light is often called a ray. • When we stand in the sunlight, some of

the light is stopped while other rays continue in a straight-line path.

• We cast a shadow—a region where light rays do not reach.

Shadows• Either a large, far-away light source or a

small, nearby light source will produce a sharp shadow.

• A large, nearby light source produces a somewhat blurry shadow.

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Ray Tracing for ShadowsTrace rays from light source to wall to map out location of

deep shadow (umbra) and fuzzy shadow (penumbra).

UMBRA

Penumbra

PenumbraLight source

Object

Wal

l

Eclipse• In a solar eclipse, because of the large size of

the Sun, the rays taper to provide an umbra (total eclipse) and a surrounding penumbra (partial eclipse).

Key Points of Lecture 29Key Points of Lecture 29

Before Friday, read Hewitt Chap. 26.

Homework #20 due by 11:00 PM Friday Nov. 5.

Homework #21 due by 11:00 PM Sunday Nov. 7

• Back Voltage• Transformers and Power Transmission• Field Induction and Electromagnetic Waves (Light)• Electromagnetic Spectrum• Speed of Light• Transparent and Opaque Objects• Shadows