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NAMASTE

NAMASTE. Dr. More S.S. Dept. of Physics Yashwantrao Chavan Mahavidyalaya , Tuljapur. Chap.Ist PHOTOELECTRIC EFFECT. Lenard’s Method to determine e/m for Photoelectrons – * The value of e/m for photoelectrons was determined by Lenard using magnetic field to deflect the photoelectrons.

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NAMASTE

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  1. NAMASTE Dr. More S.S. Dept. of Physics YashwantraoChavanMahavidyalaya, Tuljapur

  2. Chap.IstPHOTOELECTRIC EFFECT • Lenard’s Method to determine e/m for Photoelectrons – * The value of e/m for photoelectrons was determined by Lenard using magnetic field to deflect the photoelectrons. * Apparatus Consist of evacuated glass tube, side tube T, cathode C, window W, anode A with small pin hole, two metal electrodes p1 and p2 connected to the electrometers E1 and E2.

  3. *If cathode C is given – ve potential and illuminated . – vely charged particles are emitted which are accelerated towards anode A. *These charged particles passing through pin hole in A they move towards p1 with uniform velocity ,then electrometer E1 shows deflection . * A uniform magnetic field B is applied perpendicular to the plane of figure . So that photoelectrons are deflected towards P2. due to this electrometer E2 shows deflection.

  4. *Lenard studied relation between current and potential to cathode. * High positive potential V is applied to cathode , the current is zero. Then decreases +ve potential up to +2 V there is small current shows that few electrons passes enough velocity to overcome the retarding potential of +2 V. *If the potential is further decreased ,the current increases and reaches a maximum value for -20 V.

  5. *K.E. of the electrons is ½ mv2 = eV------------(1) e-charge, m- mass of photoelectrons. * R is radius circular path in magnetic field of strength B, then centripetal force required for circular motion is equal to magnetic force. mv2/R= Bev mv/R= Be v = BeR/m -------------(2)

  6. Using this value in eq. 1 1/2m . B2e2R2/m2 =eV ½ B2eR2/m =V e/m = 2V/ B2 R2 -------------(3) * Knowing V, B and R value of e/m is calculated.

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