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Powerpoint - Determination of Planck's Constant
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Transcript of Powerpoint - Determination of Planck's Constant
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Determination of Plancks Constant By Multiple light emitting diodesCHAN, NERI, PAGDATO, TOLENTINOPHYSICS 73.1 X4INTRODUCTIONWHAT IS A LIGHT-EMITTING DIODE (LED)?A semiconductor device that emits light when voltage is applied across it.A p-n junction diodeConsist of p-doped and n-doped regions in its simplest formConductor material is typically aluminum-gallium-arsenide (AlGaAs)
PARTS OF A LIGHT-EMITTING DIODE
OPERATION OF AN LEDA. No applied voltageThe p- and n-doped regions are misaligned creating a potential barrier that blocks the flow of charge carriersResults to no current flow
B. Applied voltageConduction and valence bands align and the potential barrier height decreasesResults to little current flow
OPERATION OF AN LEDC. Applied voltage exceeds turn-on voltage VoThe conduction and valence bands align and the potential barrier becomes low enough to allow charge carriers to move across the pn-junctionResults to a current flow
MAX PLANCKProposed that atoms absorb and emit radiation in discrete quantities.
Mathematically expressed as E = nhf
In this experiment, the Vo of the diode is related to Eband gap and photon frequency by eVo = Eband gap = hf
OBJECTIVESThis experiment aims to determine Plancks constant by plotting the turn-on voltage vs frequency of the LEDs wired in a parallel circuit; and
To investigate the mechanism of LEDs and relate it to the concepts encountered in the course.
METHODOLOGY
24V POWER SUPPLYDirect Currentconnected to a printed circuit board
1k RESISTORin series
LIGHT-EMITTING DIODES (LEDs)in parallel with each other+-
VOLTMETERalso in parallel
AMMETERin series
RECORD VOLTAGE (V) and CURRENT (mA) READINGSat most 15 different readings*Voltage was not allowed to exceed 4VPLOT CURRENT vs. VOLTAGEestimate the turn-on voltage from the best-fit lineCURRENT (mA)VOLTAGE (V)Vo*just an illustration
6 POSSIBLE COMBINATIONS3!PLOT TURN-ON VOLTAGE vs. FREQUENCYfrequency of the LED with higher VohPLANCKS CONSTANTRESULTS AND DISCUSSIONVOLTAGE DROP*amount of voltage lost in the LED when operated at the certain reference current
RED, ORANGE, YELLOW AND YELLOW-GREEN (1.8V)PURE GREEN, BLUE, WHITE, UV (3.3V)
PARALLEL CIRCUITSSame voltage throughoutDifferent current
Connecting several LEDs in parallel with one resistor is generally not a good ideaCurrent will take the path of the least resistanceHence, LED with less Vo steals the current from the higher VoCOLOR COMBINATIONSWHAT WORKED:Red and OrangeOrange and BlueOrange and VioletBlue and Violet
WHAT DIDNT:Red and BlueRed and Violet
WHY DIDNT IT WORK?DIFFERENT ELECTRIC PROPERTIESDifferent colors Different sizes and manufacturersManufactured in different batches
COMPONENTS CHANGEChange in characteristics because of aging
HEATINGDifferent characteristicsHeat increases, Vo decreases.
2829COLORFREQUENCY (X1014 HZ)SLOPEY-INTERCEPTTurn-on voltage (V), experimentalTurn-on voltage (V), theoreticalRED-ORANGE4.900.03020.05351.7715232.026150ORANGE-BLUE6.530.00860.02172.5232562.700155ORANGE-VIOLET7.430.04360.12032.7591743.072305BLUE-VIOLET7.430.05440.1532.81253.072305
Slope (eV.s)3.9994x10-15y-intercept (eV)-1.6197x10-1TheoreticalPlancks Constant (eV.s)4.135x10-15PERCENT DEVIATION (%)3.2793%SOLUTIONSeparate resistors in parallelCalculate resistance
If goal is to light all the LEDs, connect in seriesNeeds higher working voltageNot the scope of the study
VCONCLUSIONCONCLUSIONDetermination of Plancks constant is still feasible with multiple LEDs in parallel
Only a certain combination of LEDs will light
The LED with the higher theoretical turn-on voltage should be usedRECOMMENDATIONSHigher voltage of power supply
More LEDs in parallel
More accurate measuring devices (more decimal places)
Account for differences (better experimental procedure)
Explore how the Plancks constant will be determined by connecting in seriesQUESTIONS?