1Lecture 11Reminders:Reading for next lecture TZD Chapter 3Homework #4 assignment posted on the web pagedue next Wednesday September 23, 2009 at 9:50 amExam #1 in class on Friday, September 25, 2009.See practice exam and information on the web page.Today’s topic: History of the AtomHow will the electron bedeflected in the B-field?A. Into the pageB. Out of the pageC. UpwardsD. DownwardsE. No deflection at allJ.J. Thomson discovered the electron in 1897VXBThomson observes the particle bending downward which means negative electric charge (from the right hand rule).BvqFvvr×=Clicker questionelectronHint: RememberIf we knew the electron velocity we could find q/m.VXBESolution: Add an external downward E-field and adjust until the particle goes straight.0=+×= EqBvqFnetvvvv||/|||| BEvvvv=The force by a magnetic field is perpendicular to the velocity so it causes centripetal acceleration.Bvqrmvvv×=2rBvmq=Measuring q/m for the electron (e/m)Thus, Thomson knew it was a negatively charged particle, and the e/m ratio (but not the charge or mass separately).Oil is ionized by the atomizer. Can adjust the electric field so the drop is stationary. ThenDrop 1: Qdrop1= ndrop1x eDrop 2: Qdrop2= ndrop2x eAfter many trials, one can determine the fundamental smallest charge e = 1.6 x 10-19Coulombs.Millikan Oil Drop Experiment (1911)0dropdrop=−=gmEqFnetEgmq /dropdrop=soFind mass by measuring the terminal velocity in air.Enter the Millikan oil drop experiment in 1911MMMmmmm, plum pudding (aka raisin cake)What was known about atoms in 1900:1. Atoms are heavy and electrically neutral2. Electrons are light and negatively chargedWith this knowledge and knowing that like charges repel and opposite charges attract, Thompson proposes the Plum Pudding model of the atom.The electrons are like little negative raisins inside the heavy positive pudding.In 1911, Rutherford (with his assistants Geiger and Marsden) did scattering experiments to test the Thomson Plum Pudding atom model.Plum pudding model prediction: The alpha particles should bend very little since there is a balance of Coulomb forces between positive (pudding) and negative (plum) particles.Rutherford scattering experiment (1911)Alpha particles are Helium nuclei (2 protons + 2 neutrons)Rutherford sent alpha particles through very thin foils of metal.Most of the alpha particles behaved exactly that way…2Rutherford’s solar system model of the atomTo his great surprise, some alpha particles scattered at very large angles, some almost straight back.There must be a much larger electric field present. This led to the…Rutherford (solar system) model:1. The positive charges in the atom are concentrated in a nucleus that contains nearly all of the mass.2. The negatively charged electrons surround the nucleus in large orbits.Electrons are held in orbit around the positive nucleus by the Coulomb force.+QrmvrqkqF2221==rvmreZekFe22))((=−+=Rutherford’s solar system model of the atomJust like gravity acts on planets in the solar system:rvmrMGMFG2planet2planetsun==Orbit 1Orbit 2Which electron orbit has the smaller frequency?A. Orbit 1 has the smaller frequencyB. Orbit 2 has the smaller frequencyC. Orbit 1 and 2 both have the same frequencyD. Impossible to determine from the information give.Clicker questionJust as Mercury goes around the Sun faster than the Earth (88 days compared to 365 days).Larger radius orbit Ælonger period T Æ smaller frequency fSmaller radius orbit Æshorter period T Æ larger frequency fIssues with Rutherford’s model of the atomClassical electricity and magnetism theory combined with classical mechanics theory allow for these orbits.That’s the good news.There were two problems, both of which come from classical EM theory which states that any charged particle that experiences acceleration must radiate electromagnetic waves.The frequency of this radiation is related to the frequency of the electron revolving around the nucleus.Prediction 1: The electromagnetic radiation will cause the electron to lose energy and eventually fall into the nucleus. Prediction 2: The light given off by the electron should be a continuous spectrum with all frequencies.Classically, an electron in an atom should radiate as it is accelerated around a circle. This causes it to lose energy and to spiral inward.First problem – death spiral of the electronCalculations of the expected time for the electron to spiral in were doneThe calculated time was 10-11seconds.Poof ! There go all the atoms in the universe. That’s not good !Why don’t planets emit radiation and spiral into the sun?A. They do, but very, very slowly.B. Because planets obey quantum mechanics, not classical mechanics.C. Because gravitational forces work differently than electrical forces.D. Because planets are much bigger than electrons.Answer is A. Gravitational radiation is much, much weaker than electromagnetic radiation.Clicker question3Well before the model of the atom, people had heated up atomic matter and could study the emission of light.Experimentally it was found that certain elements gave off lightonly at specific discrete wavelengths or frequencies.The electrons were not radiating a continuous spectrum.2nd problem – atoms should emit continuous spectrumHeated matter emits radiation.Thermal random motion yields spectrum of wavelengths of EM radiation.Spectra were observed in 1800’s without knowing how or why?Atomic spectraHydrogen atom-Balmer series Balmer (1885) noticed wavelengths followed a progression 22121nm19.91n−=λwhere n = 3,4,5, 6, …. As n gets larger, what happens to wavelengths of emitted light? A. Gets larger and larger without limitB. Gets larger and larger, but approaches a limit C. Gets smaller and smaller without limitD. Get smaller and smaller, but approaches a limit656.3 nm486.1434.0410.3We will discuss this later, but there must be a stronger force than the Coulomb force holding the nucleus togetherFinal ConundrumThe nucleus was full of positive particles which should all be repelling each other.So what keeps the nucleus together?In the Plum Pudding model the positive and negative charges were distributed throughout the atom so this was not a problem.Not being too original, physicists end up calling this the strong force; also called the nuclear force or strong nuclear