012-04049J
h/e Apparatus and h/e Apparatus Accessory Kit
13
®
Theory of Operation
In experiments with the h/e Apparatus, monochromatic
light falls on the cathode plate of a vacuum photodiode
tube that has a low work function,
W
0
.
Photoelectrons
ejected from the cathode collect on the anode.
The photodiode tube and its associated electronics have a
small capacitance which becomes charged by the photo-
electric current. When the potential on this capacitance
reaches the stopping potential of the photoelectrons, the
current decreases to zero, and the anode-to-cathode volt-
age stabilizes. This final voltage between the anode
and cathode is therefore the stopping potential of the
photoelectrons.
To let you measure the stopping potential, the anode is
connected to a built-in amplifier with an ultrahigh input
impedance (> 10
13
Ω
), and the output from this amplifier
is connected to the output jacks on the front panel of the
apparatus. This high impedance, unity gain (Vout/Vin = 1)
amplifier lets you measure the stopping potential with a digi-
tal voltmeter.
Due to the ultra high input impedance, once the capacitor
has been charged from the photodiode current it takes a
long time to discharge this potential through some leak-
age. Therefore a shorting switch labeled “PUSH TO
Zero” enables the user to quickly bleed off the charge.
However, the op-amp output will not stay at 0 volts after
the switch is released since the op-amp input is floating.
Due to variances in the assembly process, each appara-
tus has a slightly different capacitance. When the zero
switch is released, the internal capacitance along with
the user's body capacitance coupled through the switch
is enough to make the output volatge jump and/or os-
cillate. Once photoelectrons charge the anode the input
voltage will stabilize.
Technical Information
Содержание h/e Apparatus
Страница 2: ......
Страница 14: ...h e Apparatus and h e Apparatus Accessory Kit 012 04049J 10 Notes...
Страница 22: ......