AP Physics
Atomic Physics FRQ Review
1. In an experiment on the photoelectric effect it is found that photoelectrons are
emitted from a surface only for radiation of wavelength 5.2 x 10-7 m or less. For
a certain shorter wavelength, to be determined, a stopping potential of 2.5 volts is
required to reduce the photocurrent to zero.
(a) What is the frequency of the 4.8 x 10-7 m radiation?
(b) Determine the work function of the surface
(c) Determine the energy of the photons associated with the 2.5 volt stopping
potential
(d) Determine the unknown shorter wavelength
(e) Make a graph showing maximum kinetic energy of photoelectrons as a
function of frequency. Label the axes and the threshold frequency
2. In a dental x-ray unit electrons are accelerated through a potential of 100,000
volts and hit an anode where x-rays are produced.
(a) determine the maximum frequency of the x-rays produced
An x-ray of maximum energy produced by the unit collides elastically with an
electron at rest in the patient’s tooth. If the frequency of the scattered x-ray is 2.0
x 1019 Hz and relativistic effects can be neglected
(b) What will be the kinetic energy of the recoil electron
(c) What will be the momentum of the electron?
3. An atom has a ground state of -15.0 eV. When a gas composed only of these
atoms in the ground state is irradiated by an ultraviolet source with a continuous
spectrum, only the wavelengths of 200 and 150 nm are absorbed.
(a) Calculate the photon energies(in eV) of the two absorbed wavelengths
(b) Make an energy level diagram showing the ground state, the excited
states, and the energy above which the spectrum becomes continuous
(electrons free) Show by arrows on the diagram all possible transitions
that would produce emission lines
(c) How much energy would be required to ionize this atom in the ground
state
(d) What wavelength, other than the ones absorbed, would be expected in the
emission spectrum of the gas?
Answers 1. (a) 5.8 x 1014 Hz (b) 2.4 eV (c) 4.9 eV (d) 254 nm (e) graph
2. (a) 2.41x1019 Hz
(b) 1.326 x 10-14 J (c) 7.0x10-23 Kg m/s
3. (a) 6.22 and 8.29 eV
(b) diagram (c) 15.0 eV
(d) 600 nm