Download Planck Constant

Planck Constant
Albert Hong
St. Mark’s School
Why LED?
• Syllabus
– HKALE
– HKDSE – Atomic World
• Let student familiar with the connection of
the circuit – a taste of electronics
• Cheap
– $0.5/LED
– With $200 for setting up 10 set for assessment
Structure
Formation of Depletion Layer
Forward Bias
• Current starts to flow when
the potential difference is
large enough to overcome
the barrier created by the
electric field.
• When electrons cross the
boundary and move into
holes, they move from
higher-energy states to
lower-energy ones.
 Photons emit.
Experiment (1)
Experiment (2)
• By changing the voltage supply, different
currents can be obtained
• y-intercept the graph equals to hc/q
• By giving students , h can be obtained
Experiment (3)
• The experiment can continue by giving
students LED of unknown wavelengths
• Using h found to find unknown 
• Blue, green or even IR
Apparatus
• Breadboard
• LED
Worksheet
Students’ work
Students’ work
Assessment
• Graph plotting technique – suitable scale
• Extract information from unfamiliar
situation – no such experiment in most of
the AL textbooks.
• 10 groups can be easily setup – TAS can
be done in the same session
Extension
• Can integrate with the experiment of
finding wavelength with diffraction grating
• Use the  found to obtain h
• Investigation – different ways to find 
emitted by LED? Diffraction grating, circuit,
etc.
Points to note
• Red LED’s spectrum is narrower – Better if
you are using the LED in diffraction grating
experiment
• Start measurement from Vin = 8-10V
LED Current /mA Vs Voltage /v
7.0
y = 26.839x - 50.129y = 18.383x - 48.329
y = 24.723x - 42.855
V Blue led / v
V Red led / v
6.0
V Green led / v
V Yellow led / v
y = 14.881x - 39.413
Current / mA
5.0
4.0
3.0
2.0
1.0
0.0
1.0
1.3
1.5
1.8
2.0
2.3
Input Voltage /v
2.5
2.8
3.0
3.3
Acknowledgment
• Perimeter Institute
Other resources
• About dark matter
• Simple enough for students with basic
knowledge of gravitation and circular
motion
• Video with worksheets for students
• http://www.perimeterinstitute.ca/index.php
?option=com_content&task=view&id=691
&Itemid=657
More
• EinsteinPlus
• ISSYP