Measuring Planck’s Constant Using Light Emitting Diodes (LED’s) Department of Physics and Astronomy Youngstown State University Dr. Michael Crescimanno Snowflake Kicovic
Purpose • To find an essentially simple, straightforward method for deriving Planck’s constant using a device that we can build. • This device has to be build easily. It should be durable and feasible. • The results yielded should give an accurate value for Planck’s constant. • This method, depending on the results, can then be used in an entry level physics lab, such as that of a high school physics lab.
Planck’s Constant • 1900, Max Planck proposed discrete behavior for an object of subatomic dimensions - Planck’s constant h - the natural unit of action 6.626 x 10-34 J-s, or kgm2/s • It also represents angular momentum. • 1905, Einstein stated that electromagnetic radiation is localized in photons with frequency f and energy: E = hf • 1913, Niels Bohr extended idea to electron existing between states of discrete energy. Transitions are accompanied by absorption or emission of photons with f = E/h.
The Photoelectric Effect • 1902 it was proven that the KEmax of an electron is independent of intensity of light ray and dependent on the frequency f. • 1905, Einstein formed a fundamental theory where light is composed of photons = energy quanta. • Electrons are ejected (with great velocity from the atom) by the E of the photon. • Each light quantum consists of an amount of E = hf
Light Emitting Diodes • Light Emitting Diodes have p-n junctions where voltage yields a flow of current. The carriers (electrons and holes) are injected across the junction producing light.
Procedure • We first build the device, approximately taking 15 minutes. The device consists of 5 different colored LED’s, a 6 volt battery pack, a potentiometer, an on/off switch, a 330W resistor, a loose set of black and red wire, and a wire with an alligator clip. • The apparatus is turned on. • The alligator clip is attached to a LED lead.
Procedure (cont’d) • The loose wires (black and red) are connected to a Multimeter (which reads the voltage across the LED). • Turning the room lights off, we vary the voltage (with the potentiometer) to see the max voltage before shutoff of the LED. • We record the value. • After, we turn the potentiometer back to maximum, and we measure the wavelength of each diode with a spectrometer.
Data DiodeVoltage (V)Wavelenghts (l) Blue 2.196 640 Green 1.536 695 Orange 1.507 695 Large Red 1.530 700 Small Red 1.287 680
Experimental Results • From before E = hf, therefore, we used the formula h = (e V l) /c • <h> = 5.84 x 10-34 • h = (5.84 +s) x 10-34 • s = [ (1/n)Sin (hi – h)2]-1 • s = 1.05
Conclusion • Being that the s was 1.05, it is evident that the errors in the experiment were random rather than systematic. • This goes to show that this experiment is very effective and efficient, while at the same time being very simplistic. • These conclusions therefore exhibit the perfect characteristics for an entry level physics course while making it an interesting and EASY method for obtaining one of nature’s constants.