Important Summary JEE Main Physics - Electrons and Photons

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Important Summary JEE Main Physics - Electrons and Photons

  1. 1. 9011041155 / 9011031155 • Live Webinars (online lectures) with recordings. • Online Query Solving • Online MCQ tests with detailed solutions • Online Notes and Solved Exercises • Career Counseling 1
  2. 2. 9011041155 / 9011031155 Electrons and Photons Electrons Electron Experiments with a Crookes tube first demonstrated the particle nature of electrons. In this illustration, the profile of the Maltesecross-shaped target is projected against the tube face at right by a beam of electrons.[1] Elementary particle[2] Composition 2
  3. 3. 9011041155 / 9011031155 Generation First Interactions Gravity, Electromagnetic, Weak Symbol e−, β− Antiparticle Positron (also called antielectron) Theorized Richard Laming (1838–1851),[3] G. Johnstone Stoney (1874) and others.[4][5] Discovered J. J. Thomson (1897) Mass 9.10938291(40)×10−31 kg Electric charge −1.602176565(35)×10−19 C Magnetic moment −1.00115965218076(27) μB[7] 1 Spin 3 ⁄2
  4. 4. 9011041155 / 9011031155 Photon a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy. E  h 4
  5. 5. 9011041155 / 9011031155 Photo electric effect Electrons are present on the surface of conductor. These electrons are loosely bounded with the nucleus. In few metals these electrons are released from its surface when the metal is expose to light. This effect is known as photo electric effect. “Phenomenon of emission of electrons by metal surface under the action of light is called as photoelectric effect. The emitted electrons in this process are called as photo-electrons.” Alkali metals like sodium, potassium shows this effect. 5
  6. 6. 9011041155 / 9011031155 1. Frequency of incident radiation. 1. Initially P.D. & intensity of radiation are kept constant. It is observed that, at low frequency, photoelectric current cannot flow. This certain minimum frequency at and above which photoelectric current starts is called as threshold frequency ( ). 0 Threshold frequency is differs for different metals. Wavelength of light corresponding to threshold frequency is called as threshold wavelength (λ0) 6
  7. 7. 9011041155 / 9011031155 2. Now, frequency (greater than threshold frequency) & potential difference are kept constant. It is observed that photoelectric current increases as intensity of incident radiation increases. Photoelectric current is directly proportional to intensity of incident radiations. 3. Now frequency (greater than threshold frequency) & intensity incident radiation are kept constant 7
  8. 8. 9011041155 / 9011031155 It is observed the photo-electric current stops at certain -ve potential. This value of potential is called as stopping potential (Vs). It is also observed that value of stopping potential increases as the frequency of incident radiation increases. The relation between kinetic energy (K.E.) and stopping potential (Vs) is K.E.=Vs.e 8
  9. 9. 9011041155 / 9011031155 4. As soon as light radiation incident on photo sensitive material, electron emission starts and current flows. That means photoelectric effect is an instantaneous process. 9
  10. 10. 9011041155 / 9011031155 Characteristics of photo electric effect. 1. Photo-electric effect occurs only when the frequency of incidence radiation is greater than certain minimum frequency called as threshold frequency (v0). 2. The number of electrons emitted per second is directly proportional to intensity of incident radiation. 3. The maximum K.E. of photo-electrons depends upon frequency of incident radiation K.E. increases with increase in frequency. 4. Photo-electric effect process. 10 is as instantaneous
  11. 11. 9011041155 / 9011031155 Einstein’s photo-electric equation. E  Work Function  K.E. 1 h  W0  mv 2max 2 Definition: “Minimum amount of light energy required to release an electron from metal surface is called as photo electric work function.” If frequency of incident radiation is decreased upto threshold frequency then, kinetic energy becomes zero. i.e. when ν = ν0, K.E. = 0 ∴ hν0 = W0 Substitute this value in equation 11
  12. 12. 9011041155 / 9011031155 1 2 mv max 2 1 2 h  h  mv max 2 1 2  h     0   mv max 2 h  h 0  1. From this relation it is clear that for emission of electrons frequent n of incident radiation n must be greater than threshold frequency. 2. From above equation it is also clear that, as frequency of incident radiations increases K.E. of emitted electrons increases. 3. The intense beam of light contains maximum number of photons. Due to this, number of collisions are more & number of electrons emitted per second are more. That is emission of electrons is directly proportional to intensity of incident radiations. 12
  13. 13. 9011041155 / 9011031155 4. Photoelectric effect process. 13 is an instantaneous
  14. 14. 9011041155 / 9011031155 Photoelectric cell: Cathode is a concave shape electrode & coated with photo sensitive material like sodium cesium etc. Anode is a platinum rod. There are two pins from t electrodes to connect the cell into external circuit. There is a non conducting base to the photocell. 14
  15. 15. 9011041155 / 9011031155 Applications of photo cell Photocell is used in the following applications: 1. Exposure meter: 15
  16. 16. 9011041155 / 9011031155 2. Lux meter : 16
  17. 17. 9011041155 / 9011031155 3. Burglar alarm: 4. Production of sound recorded on cine film: Sound is recorded on the part of cine film. When light from this part incident on photocell from this part, due to change in intensity, magnitude of current changes. Using this current, a sound is produced. 17
  18. 18. 9011041155 / 9011031155 • Ask Your Doubts • For inquiry and registration, call 9011041155 / 9011031155. 18

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