1Physics 1010:The Physics of Everyday LifeTODAY• EM waves• Some exam questions2Today’s topics• Electromagnetic waves; speed of light,radio transmitters• Go over some of the exam questions3Electromagnetic radiation• Encoding sound on EM radiation AM FM• The spectrumbroadcasting antennac = 3 x 10^8 m/s4Force due to electron?60 mdirection of force on a “test” electron I stick at A. ?a. b. c. d. e. ACannot tell5Force due to electron?60 mdirection of force on a “test” electron I stick at A. ?a. b. c. d. e. ACannot tell6Force due to electron that has moved?Amove downdirection of force on a “test” electron I stick at A. ?a. b. c. d. e. Cannot tell7Force due to electron that has moved?Amove downdirection of force on a “test” electron I stick at A. ?a. b. c. d. e. Cannot tellIt can be a or c; we need to know WHEN electron moved.8Moving electrons cause a waveStationary ions give attractive force60 mmoving out with time at speed of lightc = 3 x 108 m/snotice that antenna is neutral, + and - charges, butonly the minuses (electrons) move.9Moving electrons cause a waveHow long does it take radio wave to go 60 m?a. .2 sec, b. .002 s, c. 2 x 10-6 s, d. 2 x 10-7 s, e. none of abovec = 3 x 108 m/s10Moving electrons cause a waveans d. time = distance/speed = 60 m/(3 x 108m/s) = 2 x 10-7 secafter electron in transmitter moves, receiver electron knows itmoved.How long does it take radio wave to go 60 m?a. .2 sec, b. .002 s, c. 2 x 10-6 s, d. 2 x 10-7 s, e. none of abovec = 3 x 108 m/s11--+30 mmagnifiedverticallyc = 3 x 10^8 m/scombined force- antenna neutral charge, onlymoving electrons.drawing force arrows as a wave.Combining forces gives a transverse wave12--+30mmagnifiedverticallyc = 3 x 108 m/sABCa very short time (much shorter than the period) after this “snapshot”, thestrength of the field at Awill bea. more downward, b. the same,c. down but smaller, d. zero, e. large upwardWave motion?13--+30mmagnifiedverticallyc = 3 x 108 m/sABCa very short time (much shorter than the period) after this “snapshot”, thestrength of the field at Awill bea. more downward, b. the same,c. down but smaller, d. zero, e. large upwardWave motion?ans. c. - wave just to left of A moves over to position A.14EM waves can be used to transport energy!Set up 100MHz transmitter. Hook flashlight bulb betweentwo halves of receiving antenna nearby. Bulb willa. light up if signal strong enough,b. not light up because no current through it,c. not light up because current oscillates up and down so fast. a. lights up. current goingback and forth 100 M osc./s, but always heats.15Variation with distance?ABsignal strength (electric force field)a. at A is stronger than at Bb. at A is same as at Bc. at A is weaker than at Bd. no way to tell--a) A is stronger than B. Energy in electromagneticwave spreads out. Total energy same, but field atany point weaker.Similar to sound!16Variation with distance because energyspreads outB-A-ans. A is stronger than B. Energy inelectromagnetic wave spreads out. Totalenergy same, but field at any point weaker.Amount ofenergy in EMwave spreadover sphere,area 4πr2. Sosignal getsweaker as1/(distance fromtransmitter)2r17How should we orient the receiver?signal strength (electric force field)a. parallel to broadcast antenna is stronger than when perpendicularb. parallel to broadcast antenna is same as when perpendicularc. parallel to broadcast antenna is weaker than when perpendicular18Orient the receiver like the original antennaAnswer: a) electric force field in same direction as electrons move in broadcast Antenna (up and down; “polarization”)easy to move electrons large amount up and down in receiving antenna andgo through bulb if antenna up and down. Hard if antenna vertical. same with receiving electronics of normal radio.19Demonstration• http://www.colorado.edu/physics/phet/simulations/emf/emf.jnlp• Experiment with electromagnetic fieldStatic fieldWaves20What do the arrows represent? a. The velocity of the electrons that are at each of those points, moving due tothe electromagnetic wave.b. The evenly spaced electrons (blue dots) moving up and down between thetwo antennae.c. The strength and direction of the force that would be exerted by theelectromagnetic wave on an electron.d. The position of the electrons that are at each of those points, moving due tothe electromagnetic wave.e. The force resulting from electrons moving off of the transmitting antennatowards the receiving antenna following the curved path.TRANSMITTINGANTENNARECEIVINGANTENNA21What do the arrows represent? Answer is c. The strength and direction of the force that would beexerted by the electromagnetic wave on an electron.How does this work?1) Electrons can exert a force on other electrons through electrostatic forces(Coulombs law).2) When electrons oscillate up and down the transmitting antenna, the forcethat would be exerted on other electrons is constantly changing.3) Horizontal component of force is canceled by horizontal component ofattractive force due to stationary positive charges in antenna, so resultingforce is up and down.4) It takes some time for this force (electromagnetic signal) to radiate(propagate) from the antenna outwards (velocity = speed of light).TRANSMITTINGANTENNARECEIVINGANTENNAHere curve indicating, large upward force at this timeHere curve indicating no force at this time22How to measure the propagation speed of the wave?The speed of the wave (signal) is measured as…a. how fast this peak moves towards receiver.b. how fast this peak moves up and down.c. both a and bTRANSMITTINGANTENNARECEIVINGANTENNAAnswer is a. How fastpeak moves towardreceiving antenna. Speed of light (c) = 3 x108 m/s23Radios work at a particular frequencyEach radio station broadcasts at a particularfrequency.AM stations 530 to 1600 kHz(530 kHz means 530,000 completeoscillations of electron up and downantenna per second)To broadcast radiowaves: Electrons oscillate up and down in antenna(a rapidly oscillating electric current)-Wave traveling out at speed of light.24electrons going up and down in antenna(rapidly oscillating electric current)If electrons oscillate up and down transmitting antenna more times persecond (higher frequency), the wavelength of the