SUMMARY OF CLASSICAL PHYSICS MECHANICS OPTICS ELECTRICITY HEAT SEEMS TO WORK FOR THE VERY BIG...
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Transcript of SUMMARY OF CLASSICAL PHYSICS MECHANICS OPTICS ELECTRICITY HEAT SEEMS TO WORK FOR THE VERY BIG...
SUMMARY OF CLASSICAL PHYSICS
MECHANICS
OPTICS
ELECTRICITY
HEAT
SEEMS TO WORK FOR THE VERY BIG(GALAXIES) AND HUMAN SCALE,BUT WHAT ABOUT AT ATOMIC SCALE?
LINE SPECTRA
IN 1817, FRAUNHOFER OBSERVED THATTHE SUN’S SPECTRUM HAD DARK LINES
THESE FREQUECIES WERE THE SAME AS THOSE ABSORBED BY HOT GASES
THESE LINES CAME IN SEVERAL SERIES
GASES ABSORB AND EMIT LIGHT AT CERTAIN FREQUENCIES (SPECTRAL LINES)
PHOTO-ELECTRIC EFFECT
IN 1887 HERTZ OBSERVED THAT ELECTRONS WERE EMITTED FROM A METAL WHENLIGHT WAS SHONE ON IT
THE ENERGY OF THE ELECTRONS DID NOTDEPEND ON THE INTENSITY OF THE LIGHT
ELECTRON ENERGY WAS DEPENDENT ON THE FREQUENCY OR WAVELENGTH OF THE LIGHT, ABOVE A CRITICAL FREQUENCY.DIFFERENT FOR EACH METAL
DISCOVERY OF THE ELECTRON
1899 THOMSON MEASURED THE CHARGE/MASS RATIO OF AN ELECTRON
1909 MILLIKEN MEASURED THE ELECTRON CHARGE q= -1.6 x 10-19 COULOMBSEASY TO CALCULATE THE MASS 9.1x10-31 kg
IT WAS ASSUMED THE ELECTRONS WERE IMBEDDED IN A POSITIVE MATRIX
RADIOACTIVITY
1896 BECQUEREL STUDIED THE RADIATIONEMITTED FROM URANIUM SALTS
HE FOUND THREE TYPES OF RADIATION
BETA RAYS - ELECTRONS (-VE CHARGE)
GAMMA RAYS - LIKE X-RAYS
ALPHA RAYS - He NUCLEI - +2 CHARGE
DISCOVERY OF THE NUCLEUS
RUTHERFORD (AT McGILL) STUDIED HOWALPHA PARTICLES INTERACT WITH MATTER
Lead sheet
Gold foil
scintillator
Source ofalpha particles
LIKE BOUNCING A CANNONBALL OF A KLEENEX!!
Small scatterexpected
High angle scattering
NUCLEUS
MASS OF ALPHA PARTICLE = 8000 ELECRONS
MATTER MUST HAVE SMALL DENSE NUCLEI
THESE NUCLEI HAVE POSITIVE CHARGE
ELECTRONS ARE IN ORBIT AROUND THE NUCLEI
BUT! ACCELERATING CHARGES LOSE ENERGY!!!
BLACK BODY RADIATION
ALL HOT BODIES EMIT RADIATIONTHE EMISSION SPECTRUM IS THE SAME
Peak wavelength = B/TB = 2.9 x 10-3 mK
Total energy H = sT4 , s = 5.67 x 10-8 Wm-2 K-4
BLACKBODY RADIATION
WHAT IS THE TEMPERATURE OF THE SURFACE OF THE SUN IF THE PEAKWAVELENGTH IS 4 x 10-7 m (GREEN)
PEAK WAVELENGTH = b/T= 2.9 x 10-3
4 x 10-7
= 7 x 104 or 70,000 K
PLANCK’S RADIATION LAW
E = 1/2 mv2 the average velocity of atoms
SUPPOSE LIGHT IS A STREAM OF PARTICLESCALLED PHOTONS
FREQ. X WAVELENGTH = SPEED OF LIGHTENERGY = h x FREQ. (h is a constant = 6.6 x 10-34 Joules
=4.1 x 10-15 eVE (at room temp) = 1/30 Ev = hf = hc/wavelengthwavelength = 50 x 10-6 m, 50 micrometres = IR
PHOTO-ELECTRIC EFFECT
WHY EINSTEIN WON A NOBEL PRIZE
LIGHT IS ABSORBED IN QUANTA (PHOTONS)PHOTONS HAVE DISCRETE ENERGIES
Frequency of light
Energyof releasedelectrons
Sodium Magnesium
E/f = h
PHOTO-ELECTRIC EFFECT
ELECTRONS ARE EMITTED IMMEDIATELYWHEN STRUCK BY THE LIGHTNOT AFTER A CERTAIN AMOUNT OF ENERGY ID ACCUMULATED
THEREFORE THE ENERGY NEEDED TO CAUSE THE EMISSION OF AN ELECTRONIS IN ONE PHOTON.
ENERGY LEVELS IN ATOMS
E0
E1
E2
E3
E4
etc
When electrons jump betweenallow energy states, thereis a characteristic photonemitted, with correspondingenergy.
These emissions are differentfor different atoms
LASERS
• HOW DOES A LASER WORK?• PUMPING ATOMS TO AN EXCITED STATE• ELECTRONS IN HIGHER ENERGY BANDS• LIGHT CAIUSED WHEN SOME ELECTRONS
FALL TO LOWER ENERGY STATE, TRIGGERS OTHERS TO FALL, EMITTING MORE PHOTONS, AND SO ON.
• ALL THESE PHOTONS ARE IN LOCK STEP (COHERENT EMISSION)
20th CENTURY QUESTIONS
SPECTRA: WHY CERTAIN WAVELENGTHS ONLY?
PHOTO-ELECTRIC EFFECT: WHY ENERGY OF ELECTRON?INDEPENDENT OF INTENSITY OF LIGHT?
RADIOACTIVITY: WHERE IS THE POSITIVE CHARGE?WHY ARE SOME ATOMS UNSTABLE?
BLACKBODY CURVE: WHY DOES IT DEPEND ON TEMP?
20th CENTURY QUESTIONS
PLANCK-EINSTEIN: HOW CAN PHOTONS BE WAVES AND PARTICLES?
X-RAYS: WHY DO DIFFERENT METALS EMIT DIFFERENT WAVELENGTHS?
NUCLEUS: WHY DON’T ELECTRONS FALL INTO THE NUCLEUS?
WAVE OR PARTICLE?
Light behaves like a wave in interferenceexperiments,
but
like a particle in the photo-electric effect
WAVE-PARTICLE DUALITY
LIGHT TRAVELS AS A WAVE --- BUT
WHEN IT INTERACTS WITH MATTERIT ACTS LIKE A STREAM OF PARTICLES
WAVELENGTH FREQ. ENERGYGamma 10-13 m 3x1021 12 MeVUV 10-7 m 3x1015 12eVVisible 6 x 10-7 m 5x1014 2eVRadio 300 m 106 4x10-9 eV
WAVE PARTICLE DUALITY
ALL SMALL PARTICLES ACT LIKE WAVES
ALL WAVES ACT LIKE PARTICLES
IS LIGHT A WAVE OR A PARTICLE? - YES!
A STREAM OF ELECTRONS IS ALSO A WAVE!
WHAT IS THE WAVELENGTH OF AN ELECTRON? OR A PHOTON?
E= hf = hc/wavelength, so wavelength= hc/E but mass is related to Energy by E = mc2
Photon Momentum is pph = mv = mc
But a photon has no mass! How can it have momentum?
and photon wavelength = hc/E = h/ pph
So m=E/c2 and therefore pph =Ec/c2 = E/c
WAVELENGTH OF ELECTRONS
• Electron momentum pe = mev;
• wavelength = h/pe = h/mev
• if v=10,000 m/s, me = 9x10-31 kg
• and h= 6.6 x 10-34 Joules; • the wavelength of the electron is 7 nanometres; • the higher the velocity, the shorter the wavelength,
so electron microscopes can see things smaller than optical microscopes (wavelength 400-900 nm)
Two slit - typical interference diffraction pattern
electrons
These patterns exist even if only one electron is emitted at a time
SCHOEDINGER’S EQUATION
SUPPOSE ELECTRONS ARE LIKE STANDING WAVES IN A CLOSED BOX
E=1/2 mv2 = p2/2m and wavelength = h/p
Standing wavelength = 2L/n; En = h2 n2 so energy is in levels 8mL2 depending on n and L
L
HEISENBERG’SUNCERTAINTY PRINCIPLE
IF THE ELECTRON ACTS LIKE A STANDING WAVE, WHERE IS IT AT ANY GIVEN TIME?
SUPPOSE WE CONFINE THE ELECTRON TO A BOX OF LENGTH “L”
Wavelength = 2L
UNCERTAINTY PRINCIPLE
UNCERTAINTY IN POSITION “x” IS RELATED TO “L”
UNCERTAINTY IN MOMENTUM “p”
WAVELENTH = h/p; uncertainty in p~ h/L
xp = L h/L > h Planck’s constant!!!
If we try to squeeze the walls of the box to more accurately determine x, we increaseuncertainty in momentum.
UNCERTAINTY
WE CANNOT KNOW PRECISELY WHERE AN ELECTRON IS AND KNOW, AT THE SAME TIME WHAT ITS MOMENTUM (SPEED) IS.
xp > h
WE CANNOT KNOW THE ENERGY OF A PARTICLE IN A SHORT PERIOD OF TIME
Et > h
HOW CAN THIS BE?
IF AN ELECTRON IS A WAVE ..IT MUST EXHIBIT THE PROPERTIES OF A WAVE
Incident electron
Reflected electron
Transmitted electron
Can an electron be subdivided???
PROBABILITY FUNCTIONTHE ELECTRON AS A WAVE FUNCTION
THE WAVE FUNCTION REPRESENTS THE PROBABILITY THAT THE ELECTRON WILL BE IN A GIVEN POSITION
UNTIL IT HAS BEEN DETECTED, IT COULD BE ANYWHERE !!!!
WHEN IT HAS BEEN DETECTED, IT IS ONLY ONE PLACE
SCHROEDINGER’S CATTHE PROBABILITY FUNCTION STATES THATTHE ACTUAL STATE OF MATTER IS AMBIGUOUS UNTIL IT IS OBSERVED!!!
PUT A CAT IN A BOX WITH A RADIOACTIVE ATOMTHERE IS A PROBABILITY THAT THE ATOM WILL DECAY, EMITTING AN ALPHA PARTICLE. THE ALPHA WILL TRIGGER THE RELEASE OF A DEADLY GASTHAT WILL KILL THE CAT.
SO IS SCHOEDINGER’S CAT ALIVE OR DEAD RIGHT NOW? YOU CANNOT KNOW UNTIL YOU CHECK.