Ph102 Lab: Plancks Constant Worksheet (Page 3 of 3) in pdf

Ph102 Lab: Plancks Constant Worksheet Page 3

PH102 Lab: Planckʼs constant

g) In excel, plot this data (I on the y axis and V on the x axis), and make a linear trend-

line through your data. The x-intercept of your trendline gives the threshold voltage

LED I color = ____________

LED II color = ___________

Slope = _______________ A/V

y intercept = __________________ A

x intercept = __________________ V

h) Repeat the steps above for your second LED (which should have a different color).

Experiment II: Estimating the wavelength

Estimate the wavelength of light emitted by your LED with the color charts provided. If

there is time, you can measure the wavelength directly using the diffraction gratings as

we did before.

LED I: __________ nm

LED II: __________ nm

Analysis

= V

= hc

From the equations above, we can derive:

th,1

Calculate both sides of this equation and record your answer below:

Now estimate Planckʼs constant for each LED, using c = 3.0 x 10

m/s and average the

two results. Note that your wavelength

should be in meters, your voltage V

in volts, e

= 1.6x10

-19

J/V, and c=3x10

m/s. This gives h in units of J-sec.

h = eV

λ/c

= ________________ h

= ________________

avg

What is the percentage error, compared to the accepted value of Planckʼs constant?

What is a possible source of error? (Hint: where is the energy going?

When you are ﬁnished:

Clean up and turn in a hard copy of your report

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Ph102 Lab: Plancks Constant Worksheet Page 3