PHY 4822: photoelectric effect documentation
- What you have to do:
- align Hg lamp with filter wheel and photocell
- connect photocell to amplifier and to potentiometer providing retarding
voltage
- for every one of the six filters, measure photocurrent vs retarding
voltage; make sure you get enough data points in the region where
photocurrent is
small
- graph photocurrent vs retarding voltage (six plots)
- figure out a consistent way to determine the "stopping voltage":
- methods should be defined by a clear prescription (an algorithm)
in such a way that it is reproducible, i.e. repeated
application of the same algorithm with the same parameters to the same
data set should yield the same result;
(e.g. "voltage at which current begins to rise" is not
a prescription meeting the reproducibility criterion)
- try at least three different methods
(the third may be a variation of one of the two methods, but with
different parameter)
- for every method of determining stopping voltgage,
plot stopping voltage vs frequency; determine h from the slope
(give result using eV as energy unit)
- determine uncertainty on h;
use difference between results for different methods to estimate systematic
uncertainty on h.
- can you make any statement about the work function(s)?
- photoelectric effect
(chapter 4 from book on expts. in
mod. physics)
(for viewing only, not for printing)
- photoelectric effect (Chapter 4 of Melissinos'
book on expts. in modern physics, pdf;
note that this book was written in 1966)
- photoelectric effect (Princeton adv. lab)
- manufacturer information:
Another measurement of h:
from I/V characteristic of LEDs
What you have to do:
- connect a powersupply so as to apply a voltage across the
(LED + 100Ω resistor)
- use a DMM to measure the voltage across the LED, and another one to measure
the voltage drop across the 100Ω resistor (to measure the current through
the LED)
- For every one of the 6 LEDs,
- vary the voltage, starting from -0.5V to not
more than 4V, but with the restriction that the LED current should not exceed
50mA (i.e. stop raising the voltage when the current reaches 50mA)
- for every one of the six LEDs, measure LED current vs applied
voltage; make sure you get enough data points in the region where
the LED current is small (close to 0).
- graph LED current vs applied voltage (six plots)
- figure out a consistent way to determine the "threshold voltage" (the
voltage at which the LED becomes conducting):
- methods should be defined by a clear prescription (an algorithm)
in such a way that it is reproducible, i.e. repeated
application of the same algorithm with the same parameters to the same
data set should yield the same result;
(e.g. "voltage at which current begins to rise" is not
a prescription meeting the reproducibility criterion)
- try at least three different methods
(the third may be a variation of one of the two methods, but with
different parameter)
- for every method of determining threshold voltgage,
plot threshold voltage vs frequency; determine h from the slope
(give result using eV as energy unit)
- determine uncertainty on h;
use difference between results for different methods to estimate systematic
uncertainty on h.
- In your write-up, explain what happens in this experiment, and how
and why
the threshold voltage is related to the frequency of the emitted light
(consult appropriate references on photonics, e.g. those quoted in the
manual, or one of the websites linked below).
About LEDs: