AP Free Response Question
2008 B7
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In an electron microscope, a tungsten cathode with work function 4.5 eV is heated to release electrons that are then initially at rest just outside the cathode. The electrons are accelerated by a potential difference to create a beam of electrons with a deBroglie wavelength of 0.038 nm. Assume nonrelativistic equations apply to the motion of the electrons.
(a) Calculate the momentum of an electron in the beam, in kg m/s.
(b) Calculate the kinetic energy of an electron in the beam, in joules.
(c) Calculate the accelerating voltage.
(d) Suppose that light, instead of heat, is used to release the electrons from the cathode. What minimum frequency of light is needed to accomplish this?
Topic Formulas
Description
Published Formula
elastic potential energy
impulse
kinetic energy
linear momentum
mass-energy equivalence
photoelectric equation
photon energy
photon momentum
potential energy
power
power
work
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