Photoelectric Effect and Planck's Constant
Obj: Determine Planck's Constant Using Stopping Potentials

Materials: two multimeters, several LEDs of known wavelength, variable power source, several wires
1.  Record the wavelength of each LED in Table 1.   Determine the positive and negative sides of the LED.  Arrange the power source, LED, ammeter, and voltmeter as shown in the schematic diagram.  Be sure the power source is set at zero volts before energizing.  The ammeter should be set to read in the mA range and set to the 200μ amp setting (or 200 m).
2.  Slowly increase the voltage from zero until the LED conducts current.  Adjust the voltage until the ammeter vacillates between 0.0 and 0.1 μA.  Record this voltage as the stopping potential Vo (and K.E. = qVo).
3.  Repeat the procedure for several other colors of LEDs.

1.  Complete the table below.
                                                                         Table 1






Frequency (Hz )


Stopping Vo


Kmax Energy (J)


Energy (eV)









































































  2.  Plot the graph of Energy (Joules) vs. Frequency (Hz).  Calculate the slope.  What is its significance?  Calculate a percent error using the accepted value on your reference table.
3.  Which LED has the highest work function (Wo)?  Explain what this means.
4.  Choose one of your LEDs, then calculate the final velocity of an electron as it travels across the LED point gap.  How would this velocity change with color?  (Hint:  K.E.max = qVo = ½mv2)
5.  If the point gap approximates 1.0 mm, calculate the acceleration (v2 = 2ad) of the electron and the time t to cross the gap.  What vector field supplies the accelerating force?

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