Which of the following statements on the photoelectric
effect is correct?
(A) Kinetic energy and number of emitted photoelectrons depend
on the light intensity
(B) Kinetic energy and number of emitted photoelectrons depend
on the wavelength
(C) The kinetic energy of the emitted electrons depends on the
wavelength, their number depends on the light intensity.
(D) The kinetic energy of the emitted electrons depends on the
light intensity, their number depends on the wavelength.
(E) (B) and (C) are correct.
Which of the following statements on the photoelectric
effect is correct?
(A) Kinetic energy and number of emitted photoelectrons depend
on the light intensity
(B) Kinetic energy and number of emitted photoelectrons depend
on the wavelength
(C) The kinetic energy of the emitted electrons depends on the
wavelength, their number depends on the light intensity.
(D) The kinetic energy of the emitted electrons depends on the
light intensity, their number depends on the wavelength.
(E) (B) and (C) are correct. (see next page)
In class, I focused on the most simple observations in the photoelectric effect (answer C). After class,
one of your classmates correctly pointed out that the number of electrons also depends on the wavelength.
So here is the full story:
Primary effect:
The photon energy determines the kinetic energy of the ejected electrons by the energy
balance
Ekin = hn – F.
The intensity determines the number of ejected electrons (the more intensity, the more photons, the more
ejected electrons). That would be answer C in the clicker question.
Secondary effects:
As the photon energy increases (shorter wavelength), electrons can be ejected that are more tightly bound
(i.e. lower in the energy diagram), rather than just the most weakly bound electrons. Overall, this results in
an increase of the number of electrons as there are more possibilities for any one photon to eject an electron.
Now imagine that the photon energy increases further, but you keep the intensity fixed. Since each photon
carries more and more energy as you keep decreasing the wavelength, fewer photons are hitting the surface
per unit time, even though the intensity (power per area) remains the same. Fewer photons mean fewer
ejected electrons. In that sense, answer B is also correct, even though it is a secondary effect.
Which of the following energy diagrams correctly
describes the photoelectric effect?
(A)
(B)
E
(C)
E
Ekin
E
kin
FF
E
EFkin
Ekin
hn
hn
hn
FEkin
hn
F
• Ekin is the kinetic energy of the emitted electron
• F is the work function of the surface
• hn is the photon energy
Fhn
hn
E
kin
Which of the following energy diagrams correctly
describes the photoelectric effect?
(A)
(B)
E
(C)
E
Ekin
E
kin
FF
E
EFkin
Ekin
hn
hn
hn
FEkin
hn
F
• Ekin is the kinetic energy of the emitted electron
• F is the work function of the surface
• hn is the photon energy
Fhn
hn
E
kin
Which of the following ideas for the H atom makes most
sense to you?
+
-
(A) Electrons are discrete particles embedded in a
smeared out positive background charge.
-
- - - - - -
(B) Electrons orbit the nucleus just like the planets
orbit the sun. In principle, their orbit can have
any radius.
(C) Electrons move on circular orbits around the
nucleus, but they are only allowed to have certain
radii.
(D) Electrons don’t actually orbit the nucleus.
Their positions and momenta cannot be predicted exactly.
Which of the following ideas for the H atom makes most
sense to you?
+
-
(A) Electrons are discrete particles embedded in a
smeared out positive background charge.
 Thompson’s “Plum Pudding”
(B) Electrons orbit the nucleus just like the planets
orbit the sun. In principle, their orbit can have
any radius.
 Rutherford’s Planetary Model
-
- - - - - -
(C) Electrons move on circular orbits around the
nucleus, but they are only allowed to have certain
radii.
 Bohr’s Model
(D) Electrons don’t actually orbit the nucleus.
Their positions and momenta cannot be predicted exactly.
 Schrödinger Equation
What would you expect for the trajectories of the
-particles according to Thompson’s Plum Pudding Model?
(A) ... They all fly straight through the Au foil
with very little deviation, if at all.
(B) ... They are all bounced back, because the Au foil is
too thick.
(C) ... Most of them fly straight through, but some of them are
scattered back at large angles
J. J. Thomson (1856-1940)
What would you expect for the trajectories of the
-particles according to Thompson’s Plum Pudding Model?
(A) ... They all fly straight through the Au foil
with very little deviation, if at all.
(B) ... They are all bounced back, because the Au foil is
too thick.
(C) ... Most of them fly straight through, but some of them are
scattered back at large angles
J. J. Thomson (1856-1940)
The Rutherford Model of Atoms
Based on the idea that electrons circle the nucleus on classical orbits,
formulate a relationship between the motion of the electron and the
interaction between electron and nucleus.
(A) The gravitational interaction between
electron and nucleus must balance the
centrifugal force.
LL
(B) The kinetic energy of the orbiting
electron must be the same as the potential
energy coming from Coulomb attraction between
nucleus and electron.
(C) The force keeping the electron on the orbit
must equal the Coulomb force.
FF
rr
v,v p
(D) Linear momentum must be conserved.
(E) … (B) and (C) mean the same thing, anyway, and they are both
correct.
The Rutherford Model of Atoms
Based on the idea that electrons circle the nucleus on classical orbits,
formulate a relationship between the motion of the electron and the
interaction between electron and nucleus.
(A) The gravitational interaction between
electron and nucleus must balance the
centrifugal force. Gravitation plays no role here
LL
(B) The kinetic energy of the orbiting
electron must be the same as the potential
energy coming from Coulomb attraction between
nucleus and electron. No. In fact, Ekin = -1/2 Epot
(C) The force keeping the electron on the
orbit must equal the Coulomb force.
(D) Linear momentum must be conserved.
Only in absence of external forces!
(E) … (B) and (C) mean the same thing,
anyway, and they are both correct.
FF
rr
v,v p
Rutherford’s model has at least one problem that could
never be resolved:
(A) The model does not explain the stability of atoms.
(B) The model does not explain line spectra.
(C) The model does not explain larger atoms, only H.
(D) ...(A) and (B)
(E) ... (B) and (C)
Rutherford’s model has at least one problem that could
never be resolved:
(A) The model does not explain the stability of atoms.
(B) The model does not explain line spectra.
(C) The model does not explain larger atoms, only H.
(D) ...(A) and (B)
(E) ... (B) and (C)