Laboration Fotoelektrisk effekt

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					Laboration: Fotoelektrisk effekt
Uppgift:                 Att undersöka hur fotoelektronernas rörelseenergi beror av det infallande
                         ljusets frekvens, samt att bestämma utträdelsearbetet för anodmaterialet
                         samt Plancks konstant.

Materiel:                Fotocell med förstärkarenhet (UNILAB), glödlampa med filter och

Teori:                   Se UNILAB-bruksanvisningen nedan.

Utförande:               Belys fotocellen med monokromatiskt ljus av olika våglängder. Mät de
                         olika cut-off-spänningarna.

Resultat:                Rita ett diagram för cut-off-spänningen som funktion av ljusets frekvens.
                         Grafen skall vara en rät linje. Bestäm linjens lutning och beräkna h. Bestäm
                         även utträdesarbetet Wu ur grafen.

Kommentar:               Jämför ditt experimentellt bestämda värde med litteraturvärdet. Förklara en
                         ev avvikelse.


UNILAB®                  Notes for use                                     No.                     33


This device was developed in the University of Newcastle Tyne and Rutherford College of
Technology; primarily as a simple method of obtaining an estimate of Planck’s Constant h. It shows
very clearly that more energy is associated with light of shorter wavelengths; and the only
accessories needed are an oscilloscope (or AC millivoltmeter) and a filament bulb running from a
50Hz AC supply. A 12V car headlamp bulb, having a greater output of shorter wavelengths, is
better than a ’mains’ bulb,

Typical arrangement
Place lamp so that photocell is well illuminated. The
photocell current will be modulated at 100Hz and the 100Hz
signal is amplified and fed to the oscilloscope). Press the
’on’ button S1, and set the dial VR1 anticlockwise. This
“zeros” the electron-repelling voltage on the photocell
anode). Now slowly adjust oscilloscope gain (volts/div)
until a trace of amplitude about 4 divisions is seen.

To show that violet Light emits electrons having more
energy than those from red lights, place a No. 600 (violet) filter onto the Photoelectric Unit. Slowly
increase the repelling voltage using VR1 until the oscilloscope trace reduces to its minimum
amplitude. A ’cut-off’ voltage (VR1) of about 0.7V will be required. Repeat with filter No. 608
(red). The cut-off voltage will now be around 0.15V.

Explanation. Light ejects electrons from the photocell cathode, and they will reach the anode,
producing a current of a few nanoamps, unless the anode is sufficiently negative.

To stop electrons emitted by violet light, a higher repelling voltage is required. Therefore violet
photons give the electrons more energy than do red photons.

Estimating Planck's Constant. Einstein’s Photoelectric Equation (see Mckensie: A Second Course of
Electricity, 3rd edition p226) leads to the following equation for the voltage needed to prevent an
electron reaching the anode:
                                                                     h     W
                          W k  hf  W u  eU  hf  W u  U  f  u
                                                                     e      e
Where h is Planck’s Constant and f is the frequency of the photon causing the emission.
                         U                                                 c
The slope of the graph        multiplied by e should give h. (Note that f  .)
                          f                                                

Tabulate values of U and f (for the shortest wavelength (i.e. the highest frequency) shown on each
slide), and plot the graph.

Typical results
Slide No.     U/V          f max           (s–1)
608          0.14         4.8 x 1014              This yields a ’probable’ slope of 1.65 x 10–15Vs,
607          0.30         5.2                     giving:
605          0.36         5.7                     h = 1.65 x 10–15Vs x 1.6 x 10–19 Q = 2.6 x 10–34Js
603          0.52         6.3
602          0.56         6.9
600          0.65         7.8
This is somewhat less than half the accepted value. For causes of error, bear in mind that each slide
passes some wavelengths shorter than those specified, and refer to School Science Review March
1967 pages 371–374.

Use as amplifier. When the 3.5mm jack is plugged into the input socket, any input signal between
50 and 200Hz will be amplified by about x200; maximum input approx 3mV. Input impedance
about 500kΩ.

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