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Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, &...

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Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo [email protected]
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Page 1: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Physics 7C, Lecture 7Physics 7C, Lecture 7

Winter Quarter -- 2007Winter Quarter -- 2007

Magnetic Flux, Induced EMFs, Electromagnetic

Waves, & Light

Professor Robin Erbacher343 Phy/Geo

[email protected]

Page 2: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

AnnouncementsAnnouncements

• Course syllabus (policy, philosophy) on the web: http://physics7.ucdavis.edu

• Unit 9 finishes today: DLMs 8 -14.

• Quiz #4 today, Demos on induction and EMFs. • We will have a guest lecturer next week (2/28), no quiz next week.

• Turn off cell phones and pagers during lecture.

Page 3: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Faraday’s Law of InductionFaraday’s Law of Induction

Left: A close-up view of an electric guitar pickup. The pop-jazz guitarist Les Paul was the first person to use the law of induction to create an “electric guitar”. We will learn today how this device “picks up” the mechanical vibrations to produce an electric signal.Right: Shows the Prius for 2007, with the “Hybrid Synergy Drive” system. A major component of the most fuel-efficient hybrids is the use of regenerative braking, a technology that uses the law of induction.

Page 4: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Interaction of Loops, Wires, Currents, B fields

Interaction of Loops, Wires, Currents, B fields

Page 5: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

B Field force on wireB Field force on wireA wire in an external B – field will feel a force on it.

with current

Page 6: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Force on a wire…Force on a wire…

I

F

x

x

x

x

x

x

x

An external B-field

There is a force on each charged particle which pulls them against the side of the wire pulling the whole wire with it.

What sign are the moving charges in a current?

F = ILBsin

Which is the same as before, since

qv = qL/t = Lq/t = LI

Page 7: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Homework: Prove to yourself…

Homework: Prove to yourself…

Page 8: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

RHR1 for current loopsRHR1 for current loops

Direction of B field created from current in loop.

Page 9: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

A Loop in an External FieldA Loop in an External Field

We see that a current loop will experience a torque in the presence of an external magnetic field.

S N

FI

B N

I

IS

F

F

Recall PRS question last week:

Which direction will the current loop move due to the force?

1) Up2) Down3) Flip sideways to right4) Flip sideways to left

Page 10: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Magnetic torque to get powerMagnetic torque to get power

Since a current loop will experience a torque in the presence of a magnetic field, this torque can be used to produce mechanical work to the outside world: a motor.

Page 11: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electrical MotorsElectrical Motors

commutatorarmature

An electrical motor will rotate the wire loop. Reverse current after each half turn and it will keep turning.

motor: input is electric energy the output is mechanical energy

generator: has mechanical energy as the input and electrical energy as the output

Page 12: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Michael FaradayMichael Faraday

o poor English child (1 of 10) of a blacksmith

o Self-educated while working as a bookbinder

o Bound his ornate notes of the public lectures of famous chemist Humphrey Davy

o Worked as Davy’s servant and eventually became the greater scientist by far

o Very poorly understood math, so created a pictorial method (e.g. the Field)

o Invented electric transformer and generator

Michael Faraday

1791-1867

Page 13: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Faraday’s ExperimentFaraday’s Experiment

Magnetic Induction

Page 14: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Moving a magnet close to a loop

Moving a magnet close to a loop

• Moving it to the right, current suddenly flows through the loop in one direction (e.g. positive).• Holding it in place, no current.• Moving it to the left, current flows. Furthermore, it flows in opposite direction to that of (a).

Page 15: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Magnetic FluxMagnetic Flux•Flux is maximum: all the area is presented to the field lines:

= BA

•Flux is zero: none of the field lines pass through the loop.

=0

•Flux is reduced: only the component of the area in the direction of the field lines contributes to flux

= B A cos ()

Page 16: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Faraday’s Law of InductionFaraday’s Law of Induction

• An induced mf (i.e. an induced voltage) in a coil of wire will be produced when there is a changing magnetic flux through the wire coil. (Changing # field lines.)

nduced Nt

Δ=−

ΔiE

• The induced current (i.e. an induced voltage) in a coil of wire will be produced in a direction that opposes the change that caused it.

Lenz’s Law.

mf: stands for electromotive force

N = number of turns in coil.

Page 17: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Faraday’s Law of InductionFaraday’s Law of Induction

Faraday’s Law of Induction… Put another way:

Changing the number of B field lines through a loop in a circuit causes voltage (mf), and therefore, current!

Page 18: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

An Example: Lenz’s LawAn Example: Lenz’s Law

As the magnetic field decreases with time, the current in the loop flows such that it produces a magnetic field that tries to oppose the decrease.

Page 19: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Alternating Currents (AC)Alternating Currents (AC)As you see in DL, you induce a current by movinga loop through a B field. The current acts to oppose a change in B field through the loop.

Lens’ Law! The current changes direction as you move in and out.

B field

wire loop

(a)

B field

(b)

B field

(c)

Our regular household power is 110V AC. (Can get 220V, multi-phasic, etc). What does DC mean?

Page 20: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Pick-up Coil: Electric GuitarPick-up Coil: Electric Guitar

• The Pickup coil is wrapped around a magnet.• Metal string becomes magnetized.• Plucking the string makes the string oscillate.• Oscillations cause a change in flux in the coil, creating an induced current.

Page 21: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electrical GeneratorsElectrical Generators

• As the coil is rotated by an external source of mechanical work, it produces an emf that can be used to power an electric circuit.

• Actual generators used in hydroelectric power plants are pictured on the right.

Page 22: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Regenerative Braking: Electric Cars

Regenerative Braking: Electric Cars

nduced Nt

Δ=−

ΔiE

•During braking, the axle turns the motor gears, producing an induced mf that recharges the battery.

•Electric generator

•During acceleration, the battery gives power to the electric motor to help turn the axle.

•Electric Motor, producing torque•Torque =IAB sin()

Page 23: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Magnetism: Important PointsMagnetism: Important Points

o Both ends of a magnet can attract an object, e.g. steel

True or False?

Page 24: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Magnetism: Important PointsMagnetism: Important Points

o Both ends of a magnet can attract an object, e.g. steel.

o Magnetic poles are the same as electric charges.

True or False?

Page 25: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Magnetism: Important PointsMagnetism: Important Points

o Both ends of a magnet can attract an object, e.g. steel

o Magnetic poles are the same as electric charges

o Magnetic forces are distinct from electric forces.

True or False?

Page 26: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Magnetism: Important PointsMagnetism: Important Points

o Both ends of a magnet can attract an object, e.g. steel.

o Magnetic poles are the same as electric charges.

o Magnetic forces are distinct from electric forces.

o A compass needle is itself a small magnet.

True or False?

Page 27: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Magnetism: Important PointsMagnetism: Important Points

o Both ends of a magnet can attract an object, e.g. steel.

o Magnetic poles are the same as electric charges.

o Magnetic forces are distinct from electric forces.

o A compass needle is itself a small magnet.

o A magnetic field is defined to point perpendicular to the direction of the compass needle’s north pole.

True or False?

Page 28: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Magnetism: Important PointsMagnetism: Important Points

o Both ends of a magnet can attract an object, e.g. steel.

o Magnetic poles are the same as electric charges.

o Magnetic forces are distinct from electric forces.

o A compass needle is itself a small magnet.

o A magnetic field is defined to point perpendicular to the direction of the compass needle’s north pole.

o A magnetic field comes out of the north pole and goes into the south pole- it doesn’t die inside.

True or False?

Page 29: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Magnetism: Important PointsMagnetism: Important Points

o Both ends of a magnet can attract an object, e.g. steel

o Magnetic poles are the same as electric charges.

o Magnetic forces are distinct from electric forces.

o A compass needle is itself a small magnet.

o A magnetic field is defined to point perpendicular to the direction of the compass needle’s north pole.

o A magnetic field comes out of the north pole and goes into the south pole- it doesn’t die inside.

o A magnet can be either a dipole with a north and a south, or it can be a single magnetic charge.

True or False?

Page 30: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Important Points: The Answers

Important Points: The Answers

o Both ends of a magnet can attract an object, e.g. steel.

o Magnetic poles are not the same as electric charges.

o Magnetic forces are distinct from electric forces.

o A compass needle is itself a small magnet.

o A magnetic field is defined to point in the direction of the compass needles north pole.

o A magnetic field comes out of the north pole and goes into the south pole- it doesn’t die inside.

o A magnet is always a dipole of north and south. There is no monopole (an object of containing only a north pole or only a south pole). At least, none found yet.

Page 31: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electromagnetic WavesElectromagnetic Waves

We now combine two concepts we have learned in the course: electric and magnetic fields, and waves. The discovery of electromagnetic waves led us to the understanding of many phenomena, such as: visible light (the light we see in all the colors of the rainbow), radio waves such as the music in your car, or those we see from the far reaches of the cosmos (top right). Infrared waves are emitted by warm blooded animals. A “thermogram” (bottom right), which uses infrared radiation, can help us identify areas with irregular temperature variations, as in the inflammation near the abdomen seen in the image.

Page 32: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Producing an electromagnetic wave

Producing an electromagnetic wave

A travelling electromagnetic wave produced by an A/C generator attached to an antenna.

Page 33: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

An electromagnetic waveAn electromagnetic wave

• Electric and magnetic fields are perpendicular to each other.• Wave propagates perpendicular to the E and B fields, too!• Oscillating E field produces a B field, and• Oscillating B field produces an E field….

• EM wave only needs E and B fields, can travel in a vacuum!• Speed of electromagnetic waves:

8

0 0

13.0 10 /c m s

μ= = ×

Page 34: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Light as an electromagnetic wave

Light as an electromagnetic wave

• c = constant c = f, so larger frequency means smaller wavelength

• Visible light wavelengths Roses are 700 nm, Violets are 400 nm What are the frequencies for these?

Page 35: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electromagnetic SpectrumElectromagnetic Spectrum

Radio Waves:• f = 106-109 Hz• Alternating Current (AC) circuits in metal antennas.• Molecules and accelerated electrons in space.

Page 36: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electromagnetic SpectrumElectromagnetic Spectrum

Microwaves• f = 109-1012 Hz• Long distance phone calls (cell phones)• Cooking (absorption of microwaves by water)• RADAR• Highest frequency produced by purely electronic devices.

Page 37: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electromagnetic SpectrumElectromagnetic Spectrum

Infrared• f = 1012 - 4.3x1014 Hz• Feel as heat on skin.• Some animals, such as the pit viper, can detect IR light

• Allows to see warm blooded animals in dark

• Emitted by warm bodies: produced by vibrations and rotations of molecules.

Page 38: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electromagnetic SpectrumElectromagnetic Spectrum

Visible:• f = 4.3x1014-7.5x1014 Hz• Light that we can see.• Each frequency is interpreted by our nervous system as a different color.• Produced by electrons changing their positions within an atom.

• Quantum mechanics, atomic level, such as emission spectra

Page 39: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electromagnetic SpectrumElectromagnetic Spectrum

Ultraviolet (UV):• f = 7.5x1014 - 1017 Hz• Skin is sensitive to UV

• suntans (low doses)• skin cancer (high doses)

• Sun produces UV light, ozone in stratosphere absorbs most

• reducing ozone can have unwelcome consequences!

• Bees are sensitive to UV: flowers help bees “bullseye” pollen• UV absorbing chemicals in plants help bees and fend off herbivores.

Page 40: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electromagnetic SpectrumElectromagnetic Spectrum

X-Rays:• f = 1017 - 1020 Hz• Generated by rapid deceleration of high-speed electrons.

• x-ray machines but also particle accelerators

• Can pass through soft tissue.• Do not pass through dense material such as bone.• Can be used as “anti-smuggling diagnostic tool.

Page 41: Physics 7C, Lecture 7 Winter Quarter -- 2007 Magnetic Flux, Induced EMFs, Electromagnetic Waves, & Light Professor Robin Erbacher 343 Phy/Geo erbacher@physics.ucdavis.edu.

Electromagnetic SpectrumElectromagnetic Spectrum

-Rays:• f = > 1020 Hz• Generated by rearrangement of protons, neutrons in nucleus of atom •Also, generated by collisions of particle matter with anti-matter

• annihilation! Produces radiation, no particle with mass is left.

• Supernova explosions.• Gamma-ray bursts

•Some of most violent in universe, still don’t know the source!

• Highly penetrating, destructive to cells: •used to kill cancer & microorganisms in food.


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