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Finding values for Planck's constant and the work function of the metal

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1)Switch on the mercury lamp, and confirm that it is lit. If at any point in the experiment, the lamp is turned off, call a TA for assistance. The lamp must remain off until it cools or the service life of the lamp may be affected. The lamp takes approximately 15 minutes to either warm up or cool down. Write down the time you turn your lamp on, and the time you begin your experimental readings.


2) Once the lamp is on and warming, familiarize yourself with the equipment. Practice safely changing the lens aperture and the wavelength selection filters independently. This is necessary for you to answer questions in the discussion section.

3) Set the voltage range to (-2 ~ +2 V) with the voltage selector button. You will be working with various current ranges to conduct this experiment. Find the current range selector on the Planck Constant Testing Meter, and set it to 10-11 range as the initial setting.


4) CALIBRATION of Ammeter: Change wavelength setting to no light, and adjust the zero-set knob until the ammeter is – 0.00 A (start with a positive 0.00 A, and turn current zero knob until you see – 0.00 A).


Calibration must be done every time you change the current range.

Do not go to step 5 until the lamp is warmed up and stabilized (15 minutes after you turn it on).



5) Set the aperture to 8 mm, and the wavelength to 577 nm. You will now adjust only the voltage and see what voltage causes the current to again read – 0.00 A. Use coarse and fine voltage adjustment knobs to get – 0.00 A current at the current display. The voltage at which you get zero current is the stopping voltage. Record the magnitude of the stopping voltage up to 4 significant figures. The stopping potential is negative, but we are interested only in the magnitude of the voltage.

If your current display window shows: . It means that you have exceeded your current range and you need to change to a different one. Remember about the calibration procedure when you change the current range.

6) Repeat the procedure for the other four wavelengths



. 7) Add the following in your report(Note: bullet pointsÞ will be marked in report)

Þ Present your results in a table. The initial table must include at least three columns: wavelength, frequency, and stopping voltage (include units). Also note the current scale, voltage range near the bottom of the table. A possible title would be, "Table 1: Part A: Finding Stopping Voltage for various frequencies of light".


Þ Make a graph of the stopping voltage versus frequency. Regarding units, it is recommended to plot not the frequency itself, but f /1014 Hz. [So, your x-axis will be f, (x1014 Hz)].



Part B: Current-Voltage (I-V) curves and finding saturation current

On the photoelectric tube set the diameter to 8mm and the wavelength of filter to 365 nm. Vary the voltage from Vs to 30 V and record the resultant current.

It is recommended to start current scale at 10-11 A. During this process, you will need to change the current setting, and the setting of voltage ranges. Remember to re-calibrate whenever you change the current settings.

Note: The whole range from Vs to 30 V should be divided into three approximate intervals from Vs to 0.00 V, from 0.00 to 2.00 V, and from 2.00 to 30.0 V. When making the voltage sweep the increment size should depend on the interval as shown in the table:


# Voltage interval Suggested increment size Voltage regime Number of Readings
from Vs to 0.00 V 0.2 V -2 ~ +2 V
from 0.00 to 2.00 V 0.5 V -2 ~ +2 V
from 2.00 to 30.0 V 2.0 V +2 V ~ +30 V


Þ Present your data in a table, with 30 readings of voltage and the resultant current. Make the graph with a logarithmic (base 10) current scale on the dependent Y-axis (units - 10-11 A).

Þ Be sure to include both major and minor gridlines in your graph


Part C: Determination of the effects of varying the light intensity

Set the apparatus to 8 mm aperture opening, and the voltage reading to +15 V. Record the current for the largest and smallest wavelength. Change the aperture to 4 and 2 mm, and record the new currents.

Þ Present your results in a table.


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