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Slide 1Lightning ReviewReminder (for those who don’t read syllabus)21.11 Introduction: Maxwell’s equationsMaxwell’s predictionsMaxwell’s PredictionsNote: Charges and FieldsElectromagnetic WavesElectromagnetic Waves are Transverse WavesReview problem: car radioSlide 11Properties of EM WavesQuestionProperties of EM Waves, 2Properties of EM Waves, 3Properties of EM Waves, finalDetermining Radiation PressureThe Spectrum of EM WavesThe EM SpectrumNotes on The EM SpectrumNotes on the EM Spectrum, 2Notes on the EM Spectrum, 3Notes on the EM Spectrum, finalExample: talking to a submarineSlide 251101/14/1901/14/19General Physics (PHY 2140)Lecture 22Lecture 22 Electricity and MagnetismElectromagnetic wavesPropertiesSpectrum Chapter 21http://www.physics.wayne.edu/~apetrov/PHY2140/2201/14/1901/14/19Lightning ReviewLightning ReviewLast lecture: 1.1.AC circuitsAC circuitsResistors, capacitors, inductors in ac circuitsResistors, capacitors, inductors in ac circuitsPower in an AC circuitPower in an AC circuitResonance in a RLC circuitResonance in a RLC circuitReview Problem: The light bulb has a resistance R, and the emf drives the circuit with a frequency .The light bulb glows most brightly at1. very low frequencies.2. very high frequencies.3. the frequency ( )22L CZ R X X= + -tanL CX XRf-=012fLCp=1, 22C LX X fLfCpp= =1 LCw =3301/14/1901/14/19Reminder (for those who don’t read Reminder (for those who don’t read syllabussyllabus)) Reading Quizzes (bonus 5%):It is important for you to come to class prepared, i.e. be familiar with the material to be presented. To test your preparedness, a simple five-minute quiz, testing your qualitative familiarity with the material to be discussed in class, will be given at the beginning of some of the classes. No make-up reading quizzes will be given.There could be one today… … but then again…4401/14/1901/14/1921.11 Introduction: Maxwell’s equations21.11 Introduction: Maxwell’s equationsElectricity and magnetism Electricity and magnetism were originally thought to be were originally thought to be unrelatedunrelatedin 1865, James Clerk Maxwell in 1865, James Clerk Maxwell provided a mathematical provided a mathematical theory that showed a close theory that showed a close relationship between all relationship between all electric and magnetic electric and magnetic phenomenaphenomena5501/14/1901/14/19Maxwell’s predictionsMaxwell’s predictionsElectric field lines originate on positive charges and terminate on Electric field lines originate on positive charges and terminate on negative chargesnegative charges ElectricElectric field is produced by field is produced by chargeschargesMagnetic field lines always form closed loops – they do not begin or Magnetic field lines always form closed loops – they do not begin or end anywhereend anywhere MagneticMagnetic filed is produced by currents ( filed is produced by currents (moving chargesmoving charges))A varying magnetic field induces an emf and hence an electric field A varying magnetic field induces an emf and hence an electric field (Faraday’s Law)(Faraday’s Law) ElectricElectric field is also produced by field is also produced by changing magnetic fieldchanging magnetic field Magnetic Magnetic field is also produced byfield is also produced by changing electric field. changing electric field.Question:Question: is there a is there a symmetrysymmetry between electric and magnetic fields, between electric and magnetic fields, i.e. can i.e. can magnetic fieldmagnetic field be produced by be produced by changing electric fieldchanging electric field??????Maxwell: YES!!!6601/14/1901/14/19Maxwell’s PredictionsMaxwell’s PredictionsIf magnetic field can create electric field and vice versa, there is a very If magnetic field can create electric field and vice versa, there is a very interesting phenomenon to be observedinteresting phenomenon to be observedChanging electric field produces magnetic field…Changing electric field produces magnetic field…… … which in turn produces changing electric one (but: energy conservation)which in turn produces changing electric one (but: energy conservation)… … which in turn produces changing magnetic field…which in turn produces changing magnetic field…Maxwell concluded that visible light and all other electromagnetic Maxwell concluded that visible light and all other electromagnetic waves consist of fluctuating electric and magnetic fields, with each waves consist of fluctuating electric and magnetic fields, with each varying field inducing the othervarying field inducing the otherMaxwell calculated the speed of light to be 3x10Maxwell calculated the speed of light to be 3x1088 m/s m/s7701/14/1901/14/19Note: Charges and FieldsNote: Charges and FieldsStationaryStationary charges produce only charges produce only electric fieldselectric fieldsChargesCharges in in uniform motionuniform motion (constant velocity) produce (constant velocity) produce electricelectric and and magnetic fieldsmagnetic fieldsChargesCharges that are that are acceleratedaccelerated produce produce electric and electric and magnetic fields and electromagnetic wavesmagnetic fields and electromagnetic wavesThese fields are These fields are in phasein phaseAt any point, both fields reach their maximum value at the same At any point, both fields reach their maximum value at the same timetime8801/14/1901/14/19Electromagnetic WavesElectromagnetic WavesEM waves can be produced by an antenna, which is just some kind of wire that is connected to an ac source. The ac source produces oscillating + and - charges which set up electric field (due to the separation of charge) and a magnetic field (due to the current in the wire). Note that the electric and magnetic fields are perpendicular to each other. This field begins to move away from the antenna and in a little while the ac source has caused the situation to reverse.9901/14/1901/14/19Electromagnetic Waves are Transverse WavesElectromagnetic Waves are Transverse WavesTheThe E E and and BB fields are fields are perpendicularperpendicular to each other to each otherBoth fields are perpendicular to Both fields are perpendicular to the direction of motionthe direction of motionTherefore, em waves are Therefore, em waves are transverse wavestransverse waves1 LCw =101001/14/1901/14/19Review problem: car radioReview problem: car radioAn An RLC RLC


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