Idiomas disponibles
Idiomas disponibles
3. Description
The Planck's constant apparatus is for determining
the magnitude of Planck's constant h and the work
W done in emitting electrons from a caesium cath-
ode in a photocell using the back-EMF method.
It contains a vacuum photocell, a voltmeter for
measuring back EMF, a nanoammeter for measuring
the photocell current and a power supply for the
LEDs. Five different light-emitting diodes (LEDs) are
provided, which emit light at differing known aver-
age frequencies. The intensity of the emitted light
can be varied between 0 and 100% in each case. The
photocell itself consists of a cathode with caesium
condensed onto its surface and a ring-shaped anode.
When the apparatus is switched on, a voltage is ap-
plied between the two electrodes and this can be
adjusted by two knobs for coarse and fine adjust-
ment.
Power is supplied to the apparatus via the plug-in
transformer provided. The Planck's constant appara-
tus with order number 1000536 / U10700-115 is
designed for a mains voltage of 115 V (±10%) while
the version with order number 1000537 / U10700-
230 is for 230 V (±10%).
4. Technical data
Photocell:
Voltmeter:
Precision:
Nanoammeter:
Precision:
LEDs:
Dimensions:
Weight:
5. Theoretical principles
At the end of the 19th century and the beginning of
the 20th, it almost seemed as though Physics had
explained all there was to know, but the so-called
photo-electric effect was one of the last riddles. Clas-
sical theories were unable to account for this effect.
In 1905, though, Albert Einstein devised a brilliantly
simple theoretical description of the phenomenon
using the quantum theory which had been intro-
duced by Max Planck. His assumption was that light
consisted of particles, so-called photons (quanta of
light), with an energy E which was directly propor-
tional to their frequency f and a momentum p indi-
rectly proportional to the wavelength λ:
=
⋅
⋅
=
λ
E
h
f
p
h
/
The constant of proportionality here h was Planck's
"quantum of action". What it meant was that energy
in the form of electro-magnetic radiation could only
Type 1P39, caesium (Cs)
3½-digit LCD
0.5% (typically)
3½-digit LCD
1% (typically)
472 nm, 505 nm, 525 nm,
588 nm, 611 nm
280x150x130 mm
1.3 kg approx.
be emitted in small, discrete packets called quanta.
This minimum energy was dependent on the fre-
quency. Planck's constant is one of the fundamental
constants of nature and has a value to a high accu-
racy of h = 6.62606896*10
In this experiment light from the light emitting di-
ode connected in the circuit passes through a ring-
shaped anode before striking the cathode. If an
electron is struck by a photon, the photon can give
up all of its energy ( E
that energy may then propel the electron out of the
metal surface (the so-called work function W). The
rest of it is converted into kinetic energy for the
electron:
= ⋅ −
E
h f W
kin
The work done in emitting electrons from the cath-
ode is dependent on the material as well as on the
temperature. For caesium it is 2.14 eV at 0 K and
about 2 eV at room temperature.
Depending on the adjustment of the back EMF be-
tween the cathode and anode, a current of electrons
should flow from the former to the latter. This can
be measured using the nanoammeter. If the back-
EMF corresponds to the critical voltage U
⋅
=
= ⋅ −
e U
E
h f W
0
kin
then this current should have a magnitude of 0 nA.
e U ⋅ against f for the critical voltages
Plot a graph of
0
U
, measured for various frequencies of light f, to
0
obtain a line of gradient h crossing the y axis at W.
The point where the line crosses the y-axis is differ-
ent for all cathode materials, so that the correspond-
ing straight lines are all different too. The gradient of
the line depends on the cathode material.
6. Operation
6.1 Measurement of critical voltage at a light
intensity of 75%.
•
Plug in the transformer to supply power.
•
Set the intensity of the light source to 75%.
•
Insert the plug for the first light source into the
LED connector socket.
•
Push together the jaws of the clip for the sleeve
over the collector tube of the photocell and re-
move the sleeve.
•
Push the LED unit fully onto the collector tube of
the photocell until the jaws of the clip snap into
place.
•
Set the fine adjustment knob for the back-EMF
to a central position.
Note: it is worth waiting a few minutes to set the
critical voltage before starting the first measure-
ment.
2
-34
Js.
= ⋅ ) to the electron. Part of
h f
where
0,
-19
and e=1.6021x10
C,
