As shown in the figure, a beam or electrons is emitted from a very small opening
ID: 1530437 • Letter: A
Question
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= 160 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_: up The potential of the upper plate should therefore be _ than the lower plate. lower 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 "cut-off voltage." What is the strength of the electric field between the two plates?Explanation / Answer
work done by potential difference on electrons = change in KE
dV*e = KE
dV = KE/e
KE = 160*1.6*10^-19 J
dV = 160*1.6*10^-19/(1.6*10^-19)
dV = 160 V
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electric field E = dV/D = 160/0.4 = 400 V/m
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