1Lecture 12Reminders:Reading for this week TZD Chapter 3Homework #4 due this Wednesday September 23, 2009Exam #1 in class on Friday, September 25, 2009.See practice exam and information on the web page.Today’s topic: History of lightPre-Quantum Mechanics ViewSimple classical model of the atom with new particles just being discovered (electrons, protons, …).Also important to review the pre-quantum view of light. This is also helpful for reviewing the basic properties of waves (of all sorts).Clicker questionWhich of the following is not a problem with the Rutherford Model of the Atom?A) Experiments on the scattering of alpha particlesB) Experiments measuring emission of EM wavesC) Experiments on the lifetime and stability of atomsD) Experiments on the charge neutrality of atomsE) More than one of the abovePrevailing view from Isaac Newton was that light was a stream of individual particles.Brighter light, more light particles.That is until Maxwell’s equations…Which of the following pictures shows Maxwell?CBADUlysses S. GrantJames Clerk MaxwellWilliam Thomson,(Lord Kelvin)Thomas CrapperFamous plumberClicker questionHigh points of this lecture:22221tEcE∂∂=∇In 3-D: 222221tEcxE∂∂=∂∂In 1-D: Work through solution to 1-D differential equation. E(x,t) = Emaxcos(ax+bt)is a solution. Show that Use boundary conditions to get constants, Emax, a, b.2When does an electric field exert a force on a charge?A. AlwaysB. Sometimes (changing field)C. Sometimes (depends on the charge)D. Sometimes (depends on many things)E. NeverF. Something elseClicker questionAn electric field exerts a force on a charge ALWAYS!()qFqEvB=+×rrrrElectromagnetic, or Lorentz, ForcesAn effort to see forces as arising from particle properties and properties of the rest of the universe:Properties of the particleProperties from the rest of the universe.Properties of light Æ Interaction with matterLight is an oscillating E-field (and B-field). It interacts with matter by exerting forces on the charges – the electrons and protons in atoms.Electric fields exert forces on chargesEF=qEX_EF=qEX+(e’s and p’s in atom)Force = charge x electric fieldF= qEHow do you generate light (electromagnetic radiation)?A. Stationary charges B. Charges moving at a constant velocityC. Accelerating chargesD. B and CE. A, B, and CAccelerating charges Æchanging E-field and changing B-field (EM radiation Æ both E and B are oscillating)Stationary charges Æconstant E-field, no magnetic (B)-field+ECharges moving at a constant velocity ÆConstant current through wire creates a B-fieldBut B-field is constantIBClicker questionAns. is c. Accelerating charges create EM radiation. Surface of sun- very hot! Whole bunch of free electrons whizzing around like crazy. Equal number of protons, but heavier so moving slower, less EM waves generated. ++++The SunGo to radiowave simClicker questionHow do you generate light (electromagnetic radiation)?• A. Stationary charges • B. Charges moving at a constant velocity• C. Accelerating charges• D. B and C• E. A, B, and CAt t=0Light is an Oscillating E and B-fieldEE(x,t) = Emaxsin(ax-bt)Function of position (x) and time (t):• Oscillating ELECTRIC and magnetic field • Traveling to the right at speed of light (c)XcSnap shot of E-field in time: Electromagnetic radiationA little later in timeEmaxsin(ax+bt)3Electromagnetic Wave:ztkxBtxBytkxEtxEpeakpeakˆ)sin(),(ˆ)sin(),(ωω−=−=vvElectric and Magnetic Fields have the same wavelength, frequency and thus speed. They are both in a direction perpendicular to the motion (x direction) and perpendicular to each other as well. In fact, direction of wave propagation v = direction of E x B1. E and B fields are perpendicular to each other and to the direction of propagation (i.e. EM waves are transverse)2. E and B fields are “in phase” (i.e. that is E reaches the maximum at the same time and place as B reaches maximum, etc.)3. B(peak) = E(peak) / cNote that E field is much larger than the B field !4. EM waves carry (transport) energy!What does the curve tell you? A. The spatial extent of the E-field. At the peaks and troughs the E-field is covering a larger extent in spaceB. The E-field’s direction and strength along the center line of the curveC. The actual path of the light travelsD. More than one of these E. None of these. OREM radiation often represented by a sinusoidal curve.Clicker questionEM radiation often represented by a sinusoidal curve. What does the curve tell you? Correct answer is B. The E-field’s direction and strength along the center line of the curveXOnly know E-field, along this line. At this time, E-field at point X is strong and in the points upward.Path of EM Radiation is a straight line.? Demonstration ?Does it matter which way the antenna is oriented?EM Waves as a solution to Maxwell’s eqn. are transverse waves.E and B vector directions point transverse to the direction of propagation. Wave propagates at the speed of light.EBcIt is the electric field that directly moves the electrons in the antenna.Thus the orientation of the antenna