InterviewSolution
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InterviewSolution
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A small plate of a metal (work function `=1.7eV`) is placed at a distance of 2m from a monochromatic light source of wavelength `4.8xx10^(-7)m` and power 1.0watt. The light falls normally on the plate. Find the number of protons striking the metal plate per square metre per second. If a constant magnetic field of strength `10^(-4)T` is applied parallel to the metal surface, find the radius of the largest circular path followed by the emitted photoelectron. (use `h=6.63xx10^(-34)Js` , mass of electron=`9.1xx10^(-31)kg`, charge of electron=`1.6xx10^(-19)C` |
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Answer» Photon energy `=(hc)/lambda=((6.6xx10^(-34))xx(3xx10^(8)))/(4.8xx10^(-7))` `=4.125xx10^(-19)J` The rate of emission of photon from the source `=(1.0Js^(-1))/(4.125xx10^(-19)J)=2.425xx10^(18)s^(-1)` No. of photon striking per square meter per second on the metal plate `=(2.425xx10^(18))/(4pir^(2))=(2.425xx10^(18))/(4xx(3.14)xx(2)^(2))=4.82xx10^(16)` The maximum K.E. (K) of the photoelectrons emitted from the plate having work function `phi_(0)(=1.17eV)` is given by `K_(max)=(hc)/lambda-phi_(0)` `=(4.125xx10^(-19)J)-(1.17xx1.6xx10^(-19)J)` `=2.253xx10^(-19)J` The maximum velocity of photoelectrons `v_(max)=sqrt((2K_(max))/m)=sqrt((2xx2.253xx10^(-19))/(9.1xx10^(-31)))` `=7.036xx10^(5)ms^(-1)` The radius of the largest circle traversed by photoelectron in magnetic field B in given by `r_(max)=(mv_(max))/(eB)=((9.1xx10^(-31))xx(7.036xx10^(5)))/((1.6xx10^(-19))xx(10^(-4)))` `=0.04m=4.0cm` |
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