[PPT]PowerPoint Presentation - UC San Diego | Department of …tmurphy/phys10/lectures/18... · Web viewJames Clerk Maxwell Michael Faraday Electromagnetism Electromagnetic Induction

UCSD Physics 10

ElectromagnetismElectromagnetism

Electromagnetic InductionElectromagnetic InductionElectromagnetic WavesElectromagnetic Waves

James Clerk Maxwell Michael Faraday

Spring 2008 2

UCSD Physics 10

ElectromagnetismElectromagnetismbull Electricity and magnetism are different facets of Electricity and magnetism are different facets of

electromagnetismelectromagnetismndash a moving electric charge produces magnetic fieldsndash changing magnetic fields move electric charges

bull This connection first elucidated by Faraday MaxwellThis connection first elucidated by Faraday Maxwellbull Einstein saw electricity and magnetism as frame-Einstein saw electricity and magnetism as frame-

dependent facets of dependent facets of unifiedunified electromagneticelectromagnetic force force

Spring 2008 3

UCSD Physics 10

Magnetic fields from electricityMagnetic fields from electricitybull A static distribution of charges produces an electric A static distribution of charges produces an electric

fieldfieldbull Charges in Charges in motionmotion (an electrical current) produce a (an electrical current) produce a

magnetic fieldmagnetic fieldndash electric current is an example of charges (electrons) in motion

Spring 2008 4

UCSD Physics 10

ElectromagnetsElectromagnetsbull Arranging wire in a coil and running a current Arranging wire in a coil and running a current

through produces a magnetic field that looks a lot through produces a magnetic field that looks a lot like a bar magnetlike a bar magnetndash called an electromagnetndash putting a real magnet inside can shove the magnet back

and forth depending on current direction called a solenoid

Spring 2008 5

UCSD Physics 10

Induced CurrentInduced Currentbull The next part of the story is that a The next part of the story is that a changingchanging

magnetic fieldmagnetic field produces an electric current in a produces an electric current in a loop surrounding the fieldloop surrounding the fieldndash called electromagnetic induction or Faradayrsquos Law

Spring 2008 6

UCSD Physics 10

The Electromagnetic ConnectionThe Electromagnetic Connection

bull A A changingchanging magnetic field produces an electric field and a magnetic field produces an electric field and a changing electric field produces a magnetic field changing electric field produces a magnetic field

bull Electric and Magnetic fields can produce forces on chargesElectric and Magnetic fields can produce forces on chargesbull An An acceleratingaccelerating charge produces electromagnetic waves charge produces electromagnetic waves

(radiation)(radiation)bull Both electric and magnetic fields can transport energyBoth electric and magnetic fields can transport energy

ndash Electric field energy used in electrical circuits eg released in lightning

ndash Magnetic field carries energy through transformer for example

Spring 2008 7

UCSD Physics 10

Electromagnetic RadiationElectromagnetic Radiation

bull Interrelated electric and magnetic fields traveling through spaceInterrelated electric and magnetic fields traveling through spacebull All electromagnetic radiation travels at All electromagnetic radiation travels at cc = 3 = 3101088 ms ms in in

vacuum ndash vacuum ndash thethe cosmic speed limit cosmic speed limitndash real number is 2997924580 ms exactly

Spring 2008 8

UCSD Physics 10

Whatrsquos ldquoWavingrdquo in EM wavesWhatrsquos ldquoWavingrdquo in EM wavesbull What medium transports sound wavesWhat medium transports sound waves

ndash Can there be sound waves in the vacuum of outer space

bull What medium transports water wavesWhat medium transports water wavesbull What medium transports radio wavesWhat medium transports radio wavesbull A topic of considerable debate in the late 1800rsquos and early A topic of considerable debate in the late 1800rsquos and early

1900rsquos1900rsquosbull Led to the concept of the ldquoluminiferous etherrdquo ndash an invisible Led to the concept of the ldquoluminiferous etherrdquo ndash an invisible

ldquojellordquo that was thought to vibrate electromagnetically ldquojellordquo that was thought to vibrate electromagnetically bull Experiments that sought this ether didnrsquot find it Experiments that sought this ether didnrsquot find it bull This was quite a surpriseThis was quite a surprise

Electromagnetic waves travel through empty space

Spring 2008 9

UCSD Physics 10

Examples of Electromagnetic RadiationExamples of Electromagnetic Radiation

bull AM and FM radio waves (including TV signals)AM and FM radio waves (including TV signals)bull Cell phone communication linksCell phone communication linksbull MicrowavesMicrowavesbull Infrared radiationInfrared radiationbull Light Light bull X-raysX-raysbull Gamma raysGamma raysbull What distinguishes these from one anotherWhat distinguishes these from one another

Spring 2008 10

UCSD Physics 10

Uses of Electromagnetic WavesUses of Electromagnetic Wavesbull Communication systems Communication systems

ndash One-way and two-way

bull Radar Radar bull Cooking (with microwaves)Cooking (with microwaves)bull Medical Imaging (X rays)Medical Imaging (X rays)bull ldquoldquoNight Visionrdquo (infrared)Night Visionrdquo (infrared)bull Astronomy (radio Astronomy (radio wave IR visible UV gamma)wave IR visible UV gamma)

All that we experience through our eyes is conveyed byelectromagnetic radiationhellip

Spring 2008 11

UCSD Physics 10

The Electromagnetic SpectrumThe Electromagnetic Spectrumbull Relationship between frequency speed and Relationship between frequency speed and

wavelengthwavelength f f = c = c ff is frequency is frequency is wavelength is wavelength cc is speed of light is speed of lightbull Different frequencies of electromagnetic radiation Different frequencies of electromagnetic radiation

are better suited to different purposesare better suited to different purposesbull The frequency of a radio wave determines its The frequency of a radio wave determines its

propagation characteristics through various mediapropagation characteristics through various media

Spring 2008 12

UCSD Physics 10

US Frequency Allocation ndash the FCCUS Frequency Allocation ndash the FCC

(300 MHz has a wavelength of 1 meter)

ldquoRadiordquo frequency-space is allocated to the hiltHerersquos a sample region from 300ndash600 MHz

International allocation gets tricky

Spring 2008 13

UCSD Physics 10

Generation of Radio WavesGeneration of Radio Waves

bull Accelerating charges radiate EM energyAccelerating charges radiate EM energybull If charges oscillate back and forth get time-varying fieldsIf charges oscillate back and forth get time-varying fields

E

+++

+

Spring 2008 14

UCSD Physics 10

Generation of Radio WavesGeneration of Radio WavesIf charges oscillate back and forth get time-varying magnetic fields tooIf charges oscillate back and forth get time-varying magnetic fields tooNote that the magnetic fields are perpendicular to the electric field vectorsNote that the magnetic fields are perpendicular to the electric field vectors

B

+++

+

Spring 2008 15

UCSD Physics 10

Polarization of Radio WavesPolarization of Radio Waves

B

ETransmitting antenna

Spring 2008 16

UCSD Physics 10

Reception of Radio WavesReception of Radio Waves

Receiving antenna works best when lsquotunedrsquo to the

wavelength of the signal and has proper polarization

Electrons in antenna are ldquojiggledrdquoby passage of electromagnetic wave

B

E

Optimal antenna length is one quarter-wavelength (4)

Spring 2008 17

UCSD Physics 10

QuestionsQuestions

Why are car radio antennas verticalWhy are car radio antennas vertical

Why are cell phone antennas so shortWhy are cell phone antennas so short

How do polarizing sunglasses workHow do polarizing sunglasses work

Spring 2008 18

UCSD Physics 10

AssignmentsAssignmentsbull Read Chapter 31 for FridayRead Chapter 31 for Friday

bull QO 4 due 523 by midnightQO 4 due 523 by midnight

bull HW 6 due 523 22E1 22E5 22E11 22E16 HW 6 due 523 22E1 22E5 22E11 22E16 22E20 22E30 22E33 22P1 23E3 26E7 22E20 22E30 22E33 22P1 23E3 26E7 26E9 26E1126E9 26E11