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

y = 24.723x - 42.855 y = 18.383x - 48.329y = 26.839x - 50.129

y = 14.881x - 39.413

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

1.0 1.3 1.5 1.8 2.0 2.3 2.5 2.8 3.0 3.3

Input Voltage /v

Cur

rent

/ m

A

V Blue led / v

V Red led / v

V Green led / v

V Yellow led / v

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