1 / 13

Quantum Physics Part 1

Quantum Physics Part 1. AP Physics – J. Ruffolo , Ph.D. Planck’s Equation:. E = hf ( h = 6.626 x 10 -34 J s). Photon. E = hf. Plank’s Constant. In his studies of black-body radiation, Maxwell Planck discovered that electromagnetic energy is emitted or absorbed in discrete quantities.

ting
Download Presentation

Quantum Physics Part 1

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Quantum Physics Part 1 AP Physics – J. Ruffolo, Ph.D.

  2. Planck’s Equation: E = hf (h = 6.626 x 10-34 J s) Photon E = hf Plank’s Constant In his studies of black-body radiation, Maxwell Planck discovered that electromagnetic energy is emitted or absorbed in discrete quantities. Apparently, light consists of tiny bundles of energy called photons, each having a well-defined quantum of energy.

  3. 1 eV = 1.60 x 10-19 J 1 keV = 1.6 x 10-16 J 1 MeV = 1.6 x 10-13 J Energy in Electron-volts Photon energies are so small that the energy is better expressed in terms of the electron-volt. One electron-volt (eV)is the energy of an electron when accelerated through a potential difference of one volt.

  4. E = 2.24 eV Or Since 1 eV = 1.60 x 10-19 J Example 1:What is the energy of a photon of yellow-green light (l = 555 nm)? First we find ffrom wave equation: c = fl E = 3.58 x 10-19 J

  5. Useful Energy Conversion Since light is often described by its wavelength in nanometers (nm) and its energy Eis given in eV, a conversion formula is useful. (1 nm = 1 x 10-9m) If lis in nm, the energy in eV is found from: Verify the answer in Example 1 . . .

  6. Incident light Cathode Anode A C Ammeter A + - The Photo-Electric Effect When light shines on the cathode C of a photocell, electrons are ejected from A and attracted by the positive potential due to battery. There is a certain threshold energy, called the work function W, that must be overcome beforeany electrons can be emitted.

  7. Incident light Cathode Anode A C Ammeter Threshold wavelength lo A + - Photo-Electric Equation The conservation of energy demands that the energy of the incoming light hc/l be equal to the work function Wof the surface plus the kinetic energy ½mv2of the emitted electrons.

  8. l = 600 nm A K = 1.10 x 10-19 J Or Example 2:The threshold wavelength of light for a given surface is 600 nm. What is the kinetic energy of emitted electrons if light of wavelength 450 nm shines on the metal? ; K = 2.76 eV – 2.07 eV K = 0.690 eV

  9. Incident light Cathode Anode The stopping potential is that voltage Vo that just stops the emission of electrons, and thus equals their original K.E. V A + - Potentiometer Photoelectric equation: Stopping Potential A potentiometer is used to vary to the voltage V between the electrodes. Kmax = eVo

  10. The slope of a line: y Slope xo x y x Slope of a Straight Line (Review) The general equation for a straight line is: y = mx + b The x-intercept xo occurs when line crosses x axis or when y = 0. The slope of the line is the rise over the run:

  11. Finding h constant Stopping potential V Slope y x fo Frequency Finding Planck’s Constant, h Using the apparatus on the previous slide, we determine the stopping potential for a number of incident light frequencies, then plot a graph. Note that the x-intercept fo is the threshold frequency.

  12. Stopping potential V Slope y fo x Frequency Example 3:In an experiment to determine Planck’s constant, a plot of stopping potential versus frequency is made. The slope of the curve is 4.13 x 10-15 V/Hz. What is Planck’s constant? h = e(slope) = (1.6 x 10-19C)(4.13 x 10-15 V/Hz) Experimental Planck’s h = 6.61 x 10-34 J/Hz

  13. Incident light Cathode Anode A V + - Stopping potential: Vo= 0.800 V Example 4:The threshold frequency for a given surface is 1.09 x 1015 Hz. What is the stopping potential for incident light whose photon energy is 8.48 x 10-19 J? Photoelectric Equation: W = (6.63 x 10-34 Js)(1.09 x 1015 Hz) =7.20 x 10-19 J

More Related