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Questions on the Photoelectric Effect

Q7.

The stopping potential VS for a photoelectric cell is measured for light of two frequencies. The following results are obtained:

stopping potential VS/V
0.93
1.41
frequency/1014 Hz
6.10
7.30

Use this data to determine a value for the Planck constant.

For photoelectrons:

EK = eVS

Now

hf = φ + EK(MAX)

so

hf = φ + eVS

and

φ = hf - eVS

We can fill in the data for this equation for each potential and frequency given to us. This will give us a set of simultaneous equations. Now, φ is characteristic of the metal so it is the same for both sets of data and we can eliminate it from the equations. This gives us the following:

hf1 - eVS1 = hf2 - eVS2

h (f1 - f2) = e (VS1- VS2)

h (7.30 - 6.10) x 1014= e (1.41 - 0.93)

h = 1.60 x 10-19 x 0.48 /(1.20 x 1014)

h = 6.40 x 10-34 Js

 

(Total 4 marks)