A Simple Mass Spectrum Device As shown in the figure, a beam or electrons is emi

Question

A Simple Mass Spectrum Device As shown in the figure, a beam or electrons is emitted from a very small opening called an "orifice" from an electron source. Above the source, a set of parallel planes is used to create an electric field E. The distance between the two planes is D= 40 cm. The electrons enter the electric field with the same kinetic energy K_E= 260 eV (electron-volt), through a small hole (or "pin hole") at the center of the lower parallel plane. The electrons are identical and do not interact with each other. The motion of the electrons is along the electric field lines. You can ignore gravity. If the electric field is used to prevent the electrons hitting the upper parallel plane, then the direction of the electric field should be : The potential of the upper plate should therefore be than the lower plate. What is the minimum voltage that should be applied between the two plates to stop the electrons from reaching the upper plate? This is called the

Explanation / Answer

(1) Since charge is negative that means electron therefore force will be in opposite to the direction of motion.
So to prevent this to touch the upper plate the direction of electric field should be upward.
(2) Upper plate potential should be higher to avoid motion of the electron to the upper plate.
(3) We know that when Electron stop at that moment its complete kinetic energy will get converted to the
potential energy therefore
  260 *1.6*10-19 J = 1.6*!0-19*potential
Potential = 260 V
(4) We know that electric field = Potential / distance = 260 / 40*10-2 = 650 N/C

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