Bibliography - School of Physics - University of Melbourne

7.4. EXPERIMENT 83
Figure 7.4: The apparatus for the observation of the photoelectric effect.
• Roll the light shield of the photocell (black tube) back and rotate the cell until the
line is centred on the aperture of the photodiode. Tighten the locking screws on the
photocell and grating.
• Place the transmission filter on the front of the photocell to allow 100% of the light
to enter the photodiode. If you have chosen the yellow or the green spectral lines
remember to use the appropriate filters. Connect the voltmeter to the output of the
photocell and switch the photocell on. Zero the photocell by pushing the red discharge
button.
Question 7.1 Record the stopping potential after a minute or so. Is it necessary to measure
the stopping potential for all transmission filters? If not, why not?
There are two effects that make the measurement of stopping time t s difficult. The first is the
swiftness of the electron buildup - it is simply not viable to accurately measure time-spans
of less than about a second by hand with a stopwatch. The second is the inaccuracy of the
end-point. Note how the stopping potential is not constant, but instead fluctuates constantly.
Certainly, in the act of measuring the stopping potential, it is difficult to tell when the final
point is reached - and your choice of end-point will markedly affect the results. Instead, if
we assume that the build-up is linear, we can instead choose some arbitrary factor κ 3 with
which to multiply the stopping voltage, and then measure the time taken to reach κV 0 , which
will be κt s . The factor κ needs to be small enough that the end-point is now well defined, yet
3 0 < κ ≤ 1.