Celebrating 100 years of Frank
James Frank and Gustav Hertz in 1914 demonstrated experimentally
the existence of discrete
or quantized energy levels in atoms, as proposed by Niels Bohr’s
atomic model. This is the
100th
year of their experimental demonstration.
The experiment demonstrates that electron can occupy only
discrete or quantized energy
states as per quantum theory. The experimental
pressure mercury gas filled in i
electron emitter), anode (collector) and grid (for acceleration
and reducing space charge
effect). The electrons are thermionically emitted by a heated
cathode, and then accelerated
toward the grid which is at a positive potential, relative to
the cathode. The collecting plate is at
a lower potential and is negative with respect to mesh grid. A
few electrons have sufficient
energy on reaching the grid, will pass through the grid, and
reach colle
measured as current. Electrons which do not have sufficient
energy will be reflected backward
at the grid.
As long as the electron collision is elastic, the electrons will
not lose energy on colliding with
gas molecules in tube. As the accelerating potential increases,
the current also increases.
A plot derived from measurements of accelerating voltage versus
anode current shows a
sharp drop of current at regular intervals of applied
accelerating voltage
19V for neon). Typically, the
mercury atom is 4.9 eV.
At accelerating voltage of 4.9 V, free electron possesses 4.9 eV
of kinetic energy. As a result
the inelastic collision between free electron and mercury atom,
free electron’s kinetic energy
converted into potential energy by raising bound electron of
mercury atom to higher energy
state. Due to the loss of kinetic energy of free electron, it
can
required to reach at the ground electrode, which in turn leads
to sharp drop in the current. If
voltage is increased above 4.9 V, electron will participate in
one inelastic collision, lose their 4.9
eV, but then continue to be ac
Theme: RMC 2014
Celebrating 100 years of Frank – Hertz Experiment.
James Frank and Gustav Hertz in 1914 demonstrated experimentally
the existence of discrete
or quantized energy levels in atoms, as proposed by Niels Bohr’s
atomic model. This is the
year of their experimental demonstration.
The experiment demonstrates that electron can occupy only
discrete or quantized energy
states as per quantum theory. The experimental setup comprises
an evacuated
in it. The tube has a triode arrangement with
electron emitter), anode (collector) and grid (for acceleration
and reducing space charge
The electrons are thermionically emitted by a heated cathode,
and then accelerated
rid which is at a positive potential, relative to the cathode.
The collecting plate is at
a lower potential and is negative with respect to mesh grid. A
few electrons have sufficient
energy on reaching the grid, will pass through the grid, and
reach collecting plate, and will be
measured as current. Electrons which do not have sufficient
energy will be reflected backward
As long as the electron collision is elastic, the electrons will
not lose energy on colliding with
As the accelerating potential increases, the current also
increases.
A plot derived from measurements of accelerating voltage versus
anode current shows a
sharp drop of current at regular intervals of applied
accelerating voltage
the threshold potential required for the electronic excitation
of
accelerating voltage of 4.9 V, free electron possesses 4.9 eV of
kinetic energy. As a result
the inelastic collision between free electron and mercury atom,
free electron’s kinetic energy
into potential energy by raising bound electron of mercury atom
to higher energy
state. Due to the loss of kinetic energy of free electron, it
can not overcome the potential
required to reach at the ground electrode, which in turn leads
to sharp drop in the current. If
voltage is increased above 4.9 V, electron will participate in
one inelastic collision, lose their 4.9
eV, but then continue to be accelerated with the remaining
kinetic energy
James Frank and Gustav Hertz in 1914 demonstrated experimentally
the existence of discrete
or quantized energy levels in atoms, as proposed by Niels Bohr’s
atomic model. This is the
The experiment demonstrates that electron can occupy only
discrete or quantized energy
n evacuated tube with low-
with cathode (thermionic
electron emitter), anode (collector) and grid (for acceleration
and reducing space charge
The electrons are thermionically emitted by a heated cathode,
and then accelerated
rid which is at a positive potential, relative to the cathode.
The collecting plate is at
a lower potential and is negative with respect to mesh grid. A
few electrons have sufficient
cting plate, and will be
measured as current. Electrons which do not have sufficient
energy will be reflected backward
As long as the electron collision is elastic, the electrons will
not lose energy on colliding with
As the accelerating potential increases, the current also
increases.
A plot derived from measurements of accelerating voltage versus
anode current shows a
sharp drop of current at regular intervals of applied
accelerating voltage (4.9V for mercury,
required for the electronic excitation of
accelerating voltage of 4.9 V, free electron possesses 4.9 eV of
kinetic energy. As a result of
the inelastic collision between free electron and mercury atom,
free electron’s kinetic energy is
into potential energy by raising bound electron of mercury atom
to higher energy
not overcome the potential
required to reach at the ground electrode, which in turn leads
to sharp drop in the current. If
voltage is increased above 4.9 V, electron will participate in
one inelastic collision, lose their 4.9
with the remaining kinetic energy. In this manner, the
current rises again after the accelerating potential exceeds 4.9
V. At 9.8 V, the situation changes
again. There, each electron now has just enough energy to
participate in two inelastic coll
excite two mercury atoms, and then be left with no kinetic
energy. Once again,
is observed at this accelerating voltage
process repeats, each time the electrons will undergo
This experimental observations are in well agreement with
quantum mechanical predication
that atom can absorb energy if and only if it exceeds than
required to lift an electron from
lower into higher energy state. Thus,
James Frank and Gustav Hertz were awarded Nobel Prize in
Physics, in 1925 for their work.
current rises again after the accelerating potential exceeds 4.9
V. At 9.8 V, the situation changes
again. There, each electron now has just enough energy to
participate in two inelastic coll
excite two mercury atoms, and then be left with no kinetic
energy. Once again,
at this accelerating voltage. Thus it is observed that, at
intervals of 4.9 volts the
each time the electrons will undergo one additional inelastic
collision.
This experimental observations are in well agreement with
quantum mechanical predication
that atom can absorb energy if and only if it exceeds than
required to lift an electron from
higher energy state. Thus, energy levels of electron in atom are
discrete.
James Frank and Gustav Hertz were awarded Nobel Prize in
Physics, in 1925 for their work.
current rises again after the accelerating potential exceeds 4.9
V. At 9.8 V, the situation changes
again. There, each electron now has just enough energy to
participate in two inelastic collisions,
excite two mercury atoms, and then be left with no kinetic
energy. Once again, the current drop
Thus it is observed that, at intervals of 4.9 volts the
one additional inelastic collision.
This experimental observations are in well agreement with
quantum mechanical predication
that atom can absorb energy if and only if it exceeds than
required to lift an electron from
energy levels of electron in atom are discrete.
James Frank and Gustav Hertz were awarded Nobel Prize in
Physics, in 1925 for their work.