1 / 17

Physics 2

Physics 2. Chapter 27 Sections 1-3. Blackbody Radiation Curve. All objects emit EM radiation Usually consists of a continuous distribution of λ As temp increases the more the max intensity shifts to shorter λ

farhani
Download Presentation

Physics 2

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. Physics 2 Chapter 27 Sections 1-3

  2. Blackbody Radiation Curve • All objects emit EM radiation • Usually consists of a continuous distribution of λ • As temp increases the more the max intensity shifts to shorter λ • As temp increases the total energy given off by the body (area under curve) increases

  3. Classical physics predicts that as λ approaches 0 the amount of energy radiated should become infinite Instead data shows that as λ approaches 0 the amount of energy radiated also approaches 0 Blackbody Radiation Curve

  4. Blackbody is a hollow object with a small opening through which light can enter Energy gets absorbed every time light hits the wall inside If box is insulated the energy absorbed causes the temp inside to rise Energy will be radiated inside the box and some will escape Blackbody Radiation

  5. Planck’s Theory • Planck said a blackbody is made up of a large number of atomic oscillators, called resonators, which emit EM radiation of various frequencies • Resonators can only absorb and give off energy in discrete amounts given by E = nhf where n=quantum # (0,1,2…), f=frequency of resonator, h=Planck’s constant = 6.63 x 10-34 Js

  6. Planck’s Theory • Energy is quantized • Discrete units of light energy are called quanta • Resonator will only radiate/absorb energy when it changes quantum states The idea was so radical that even Planck didn’t think it was realistic. It was just a mathematical model.

  7. Photoelectric Effect • Phenomenon where electrons are ejected from a metal plate when light of certain frequencies shines on it

  8. Photoelectric Effect Classical physics says: • When light waves of any f strike the metal they should eject electrons if their intensity is high enough • At low intensity, electrons can be ejected if wait long enough for electrons to absorb the incoming energy • Increasing the intensity increases the KE of the ejected electrons

  9. Photoelectric Effect Observations: • No electrons are emitted if frequency is below a certain level, regardless of intensity • If frequency exceeds this threshold frequency the number of electrons emitted is proportional to the intensity • KE of emitted electrons is independent of intensity • KE increases as frequency increases • Electrons are emitted almost instantly, even at low intensity

  10. Einstein’s Theory • Photon – discrete unit of light energy EM waves are composed of photons Each photon has an energy, E, given by E = nhf that can be absorbed by an electron

  11. Einstein’s Theory Einstein says that if light hits a metal it can do 1 of 2 things: • Photon can give its energy to an electron in the metal • Photon can do work of removing the electron from the metal

  12. Einstein’s Theory • Work function – minimum amount of energy required for an electron to escape from a metal, hft, where ft is threshold frequency If photon has more energy than hft the rest goes into KE of ejected electron

  13. Einstein’s Theory photon E = energy to remove electron (work fn) + max KE of ejected electron hf = hft + KEmax

  14. KEmax ft f If f < ft then no ejected electrons KE only depends on f not intensity (increase in intensity means more ejected electrons but at same KE) Slope is h (Planck’s constant) Einstein’s Theory

  15. Sample Problem The work function for a silver surface is 4.73 eV. Find the minimum frequency that light must have in order to eject electrons from its surface.

  16. Compton struck an electron at rest with an xray photon Noticed scattered photon has a frequency that is less than the incident frequency Difference between the 2 frequencies depended on the angle at which the scattered photon left Compton Effect or Shift

  17. Compton Effect or Shift • Phenomenon in which a photon is scattered with a smaller frequency or larger wavelength than the incident photon This phenomenon became the basis for the idea of wave-particle duality.

More Related