Part B :
The intensity of the incident light (from the mercury vapor lamp) can be altered by putting an
aperture in front of the photodiode.
Apertures available : diameter d = 2, 4, 8 mm
Different aperture means different light intensity (different number of photons of the same energy).
The number of photons should be proportional to the area of the aperture (not the diameter).
When the anode is positive relative to the cathode, electrons from the cathode (emitted because of the
photons shined on the cathode) will collect on the anode (collector plate).
When the stopping potential is zero (U = 0V) then the anode is positive relative to the cathode (because
the cathode is negative).
In this part of the experiment you are to investigate if the following statement is true:
Higher intensity of the light means MORE photons (not photons with more energy).
The theory says :
When higher intensity of the light means more photons then higher intensity of the light must also mean
that more electrons will collect on the anode.
The photocurrent measured is directly proportional to the number of electrons hitting the anode. Current
is defined as charge per time :
I=
q Z |e|
=
t
t
;
1A = 1
C
s
(7)
Z = number of electrons ; |e| = 1.602176. 10-19 C Note: I > 0 so you need to use |e| > 0
Using two different lines of the mercury spectrum (435.8 nm and 546.1 nm) and three different
apertures (2 mm, 4 mm, 8 mm) the photocurrent will be measured for the anode potential U = 0V. For
each of the 6 experiments the number of electrons collected on the anode per second will be calculated.
The area of each aperture will be calculated.
PROCEDURE :
Part A :
Refer to page 13 of the Pasco Instruction manual ‘PHOTOELECTRIC EFFECT APPARATUS’ which is
available in the Laboratory .
Experiment 1 - Planck's Constant, h
Measuring and Calculating Planck's Constant, h
Preparation before measurement
Do 1 - 10.
Go to page 14
Measurement
Do 1
Go to page 15
Do 2, 4
Make a table in your Lab Notebook :
Experiment
1
2
3
4
5
4mm Aperture
Wavelendgth,
365.0
404.7
435.8
546.1
577.0
l / nm
Frequency,
v /1014 Hz
Stopping
Potential, |V|/V
Record the stopping potential
Do 6 and 7 and record the stopping potential
Do 9
B3