question_answer 37)

Answer the following questions : (a) Quarks inside protons and neutrons are thought to carry fractional charges [ e;(-1/3)e]. Why do they not show up in *Millikan's oil-drop experiment ? (b) What is so special about the combination el m ? Why do we not simply talk of e and m separately ? (c) Why should gases be insulators at ordinary pressures and start conducting at very low pressures ? (d) Every metal has a definite work function. Why do all photoelectrons not come out with the same energy if incident radiation is monochromatic ? Why is there an energy distribution of photoelectrons ? (e) The energy and momentum of an electron are related to the frequency and wavelength of the associated matter wave by the relations : But while the value of  is physically significant, the value of v (and therefore, the value of the phase speed ) has no physical significance. Why?  

Answer:

(a) In case of the Millikan oil drop experiment, the charge on the electron is measured. The electron revolve outside the nucleus and each has a charge e. Thus, we do not observe the fractional charges (b) We know that the energy acquired by the electron when accelerated through V volt is Similarly, the force on the electron in an electric field isand force on electron in a magnetic field orClearly, the dynamics of electrons is determined by  in all the above relations instead of e alone. (c) At ordinary pressure, molecules of gas keep on colliding with each other and the ions formed do not have a chance to reach the respective electrodes to constitute a current because of their recombination. At low pressure, however, ions donot collide frequently and are able to reach the respective electrodes to constitute a current. (d) Work function in fact is the energy required to knock out the electron from highest filled level of conduction band of an emitter. In the conduction band, there are different energy levels which collectively form a continuous band of levels. Therefore, different amounts of energy are required, to bring the electrons out of the different levels. Electrons once omitted have different kinetic energies according to the energy supplied to the emitter. (e) Since frequency for a given matter wave remains constant for different layers of the matter but wavelength  changes so 'k is more significant than v. Similarly  energy  is also constant for a given matter wave so phase  is also not physically significant.