Atomic & Nuclear Physics Unit 10

Discovery of Electrons Towards the end of the 19th century experiments were being performed on the discharge of electricity through rarefied (has less oxygen & pressure) gases The negative end is the cathode and the positive end is the anode

Cathode Rays From the diagram you can see that “something” traveled from the cathode to the opposite side of the tube The “somethings” were called cathode rays Scientists had much discussion over what the rays might be Resembled light Charged particles The beam could be deflected and would deflect as a negatively charged particle would To figure out the particle’s nature, scientists tried to find the e/m ratio (charge/mass)

JJ Thomson (1856-1940) Mr. Thomson used a cathode ray tube that accelerated the electrons using a high voltage and then passed the electrons through a parallel plate magnetic field Based on the interactions between the magnetic and electric field deflections he discovered e/m = 1.76 x 1011 C/kg Because of Thomson’s work, cathode rays soon came to be identified as electrons

Electrons as SUBatomic particles Thomson also believed that electrons were constituents of atoms and not ions or particles themselves It was not until Robert A. Millikan came along that the charge was developed and electrons were shown to come in discrete amounts

Oil-Drop Experiment The electric field was adjusted until the droplet was suspended in mid air. Then using F = qE and W = mg, the “q” was determined to be q = mg/E. The mass was calculated using the terminal velocity of the charge in the absence of an electric field.

Oil-Drop Experiment Millikan determined that any electric charge was a multiple of the smallest charge “e” that was attributed to an electron: 1.6 x 10-19 C Since every charge is a multiple of the electrons means that electric charge is quantized

Black Body Radiation When objects get heated to extreme temperatures, they emit a spectrum of light that cause them to “glow” This is called black body radiation A blackbody is something that would absorb all the radiation falling on it

Black Body Radiation

Wien’s Law λP T = 2.90 x 10-3 mK Question Estimate the temperature of the surface of our Sun, given that the Sun emits light whose peak is in the visible spectrum is around 500 nm.

Max Planck (1858-1947) Although scientists could predict where blackbody radiation came from and how to relate λ& T, they could not correctly predict the spectra of emitted light Planck came along and proposed a formula that depended on a radical new assumption: The energy distributed among the charges of molecules is NOT continuous, but consists instead of a finite number of very small discrete amounts related to the frequency of oscillation by

Planck’s Quantum Hypothesis E = hf H is Planck’s constant: h = 6.63 x 10-34 Js Quantum Hypothesis: E = nhf where “n” is a whole number multiple (1, 2, 3…) “Quantum” just means fixed amount The hypothesis states that E can exist as a multiple of hf, but there CANNOT be vibrations whose energy exists between these values

Electro-magnetic Spectrum January 24, 2011

The Photoelectric Effect

Real World Burglar Alarms When a person interrupts the beam of light, the sudden drop in current in the circuit activates a switch which operates the bell. Automatic door openers Smoke Detectors When even a tiny bit of smoke interrupts the beam of electrons, the alarm is triggered.

Einstein Joins Quantum Theory In the same year he introduced the theory of relativity, 1905, he boldy added to the quantum idea by proposing a new model of light He stated that since vibrations only occur at E = nhf, then when a source loses energy of this amount, the light must also be emitted in these packets or “quanta” Einstein suggested that light is transmitted as tiny particles, or photons as we now call them

Photoelectric Effect Einstein proposed this as a test to the quantum theory of light The photoelectric effect is the phenomenon that when light shines on a metal surface, electrons are emitted from the surface http://shaba9a.files.wordpress.com/2010/06/photoelectric_effect.gif

Work Function In order for electrons to actually be emitted, the incoming light must have a certain frequency as to overcome the attractive forces on the metal’s surface. The work function is symbolized by either W0 or Φ So, the incoming energy of light will equal the outgoing energy of the electron, plus the energy needed to get it out of the metal: hf = Keelectron + Φ

Photon Theory Prediction 1) An increase in intensity of the light beam means more photons are incident, so more electrons will be ejected; but since the energy of each photon is not changed, the max KE of electrons is not changed. 2) If the frequency of the light is increased, the max KE of the electrons increases linearly according to 3) If the frequency is less than the cutoff frequency, f0 , where hf0 = Φ, no electrons will be ejected at all, no matter how great the intensity.

KEY POINT Intensity of the light will ONLY change the NUMBER of electrons ejected per second It will NOT effect the level of energy ONLY increasing the FREQUENCY will change the energy of the electrons emitted

Photon Theory Prediction Question: pg 831 Calculate the energy of a photon of blue light, λ= 450 nm.

Photon Theory Prediction Question: pg 831 Estimate how many visible light photons of a100 W lightbulb emits per second.