Java程序辅导

(ii) Simulation of Photoelectric Effect
This simulation program is to study the relationship between the cathode material
and the backing (retarding) voltage. This work will also show the relationship
between spectral lines of varying wavelengths using an incandescent light source
(not shown in spplet) and the backing voltage.
A freshly polished, negatively charged zinc plate looses its charge if it is exposed
to ultraviolet light. This phenomenon is called the photoelectric effect. Careful
investigations toward the end of the nineteenth century proved that the photoelectric
effect occurs with other materials, too, but only if the wavelength is short enough.
The photoelectric effect is observed below some threshold wavelength which is
specific to the material. Especially the fact that light of large wavelengths has
no effect at all even if it is extremely intensive, appeared mysterious for the
scientists.
Albert Einstein finally gave the explanation in 1905: Light consists of particles
(photons), and the energy of such a particle is proportional to the frequency of the
light. There is a certain minimum amount of energy (dependent on the material)
which is necessary to remove an electron from the surface of a zinc plate or
another solid body (work function). If the energy of a photon is larger than this
value, the electron can be emitted. From this explanation the following equation
results:
KEmax = eVmax
= hν − φ
where: KE is the maximum kinetic energy of an emitted electron, h is Planck
constant (6.626×10−34 Js) ν is frequency of the incident photon and φis binding
energy or work function of the cathode material.
This Java applet simulates an experiment for the determination of the Planck
constant and the work function: A single spectral line is filtered out from the
light of a source (not shown in animation). This light strikes the cathode of a
photoelectric cell and causes the emission of electrons (or not). In order to find
the maximum kinetic energy of the ejected electrons it is necessary to increase
the retarding (backing) voltage (V) by means of a potentiometer connection so
much that no more electrons arrive at the anode. You can see from the voltmeter
whether electrons reach the anode. Increase the retarding voltage (V) slowly and
note the retarding voltage value required to stop all the electrons from reaching
the anode (when voltmeter reads zero). This value is known as the stopping
potential. The product of this stopping potential and charge of the elctron (eV)
will give the maximum kinetic energy (KEmax)of the electrons.
If a series of stopping potentials (maximum retarding voltage) and frequency are
obtained, and the KEmax is then plotted against frequency, the gradient of the
line is the Planck constatn, h and the work function φ of the respective cathode
material (in eV, i.e. electron volt) can be directly read from the intersection with
99
the vertical axis.
Simulation Tasks:
By changing the ”cathode material” (ie the metal which is being illuminated),
”backing/retarding voltage” and ”wavelength” choices, determine if the retarding
voltage and hence the maximum kinetic energy of an electron is dependent on
the cathode material for the same wavelength (spectral line).
• What happens to the work function as the cathode material is varied?
• The cathode materials Cs, K, Na and Li used in this simulation belong to
the same row in the periodic table. Why does the work function increases
as the atomic mass decreases (ie the stopping potential for these elements
decreases as the atomic mass decreases)? Explain in terms of atomic number
and atomic radius.
100