012-03498C
Analyzing the Experiment
In
performing the experiment (described in the previous
section for the various setups), you will have collected
values of
Va
and corresponding values of
Ic.
To analyze this
data:
1.
Subtract
2.0
volts from each
of your recorded values for
V and record these value
as
in
your
data
table.
Va(adjusted)
is
proportional
to
the energy supplied
to the electrons by the field
between the cathode and the
grid.
Explanation:
The electrons emitted by the cathode lose
a
certain
amount of energy in escaping and gain some
energy when they are absorbed into the plate. The net
energy lost or gained in this transition is just the
difference between the work functions of the
two
metals-in
this
case
the electrons lose approximately
2.0
eV. This energy can be accounted for by subtracting
2.0 volts from the measured values of Va.
at which
I,
is
a minimum
Voltage difference between adjacent minima
2.
3.
4.
If
you did not use the oscilloscope to make your
measurements, plot a graph
of
plate current versus
plate current minima and record the corresponding
You should
that the current
are spaced at
intervals
of
4.9
volts, showing that the excitation energy
of the mercury atom
is
4.9
The
spectral
frequency
corresponding
to
this energy is
:
on the x-axis. Locate the
with
Of
in Table
2.
=
1.18
x
4.9
eV
4.133
x
eV
The corresponding wavelength is:
=
253.7
nm.
In their original experiments, Franck and Hertz verified
the presence of this ultraviolet radiation with the aid of a
quartz spectrometer.
Table
1
Acceleration Voltage and Anode Current
Table
2
Resonance Data