The Photoelectric Effect

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The Photoelectric Effect

This lab will investigate the photoelectric effect.  The photoelectric effect is the emission of electrons from a surface when illuminated with light of a certain frequency.  The first insight to understanding this phenomenon was presented in 1900 by Max Planck.  His formula, E = hf, related the energy of a photon to its frequency.  Albert Einstein extended this idea of quantized photonic energy to a stream of photons (electromagnetic radiation) and explained the photoelectric effect.

1. Go to http://phet.colorado.edu/simulations/sims.php?sim=Photoelectric_Effect and launch the simulation.
2. Make the following adjustments to the simulation once it has launched.
• Increase the intensity to 50%
• Check the box “electron energy vs. light frequency”.

Once these adjustments have been made you should notice the ejection of elections from the surface.

1. Increase the wavelength of the light until electrons are no longer ejected. Record the wavelength in the table below and complete the calculations.  1 electron-volt (eV) = 1.6 X 10-19 J
2. Repeat the above step for each of the metals under the pull down menu.
 Metal Wavelength (nm) Frequency(Hz) Energy (J) Energy (eV) Sodium 543 nm 5.52 x 1014 3.64  x 10-19 2.28 Zinc 293 nm 1.02 x 1015 6.73  x 10-19 4.21 Copper 270 nm 1.11 x 1015 7.33  x 10-19 4.58 Platinum 202 nm 1.49 x 1015 9.83  x 10-19 6.14 Calcium 430 nm 6.98 x 1014 4.61  x 10-19 2.88

 Metal Threshold Frequency (Hz) Sodium 5.69 x 1014 Zinc 1.06  x 1015 Copper 1.18  x 1015 Platinum 1.58  x 1015 Calcium 7.16  x 1014
1. The minimum frequency of a photon that can eject an electron from a surface is called the threshold frequency, ft. What is the threshold frequency, ft, for each of the metals?

 Metal Work Function(J) Work Function (eV) Sodium 3.77  x 10-19 2.36 Zinc 7.03  x 10-19 4.39 Copper 7.82  x 10-19 4.89 Platinum 0.11  x 10-19 0.07 Calcium 4.75  x 10-19 2.97
1. The minimum amount of energy required for an electron to escape from a metal is called the work function, W, and is given by the equation W = hft. Calculate the work function for each of the metals in joules and electron-volts using the threshold frequencies for each metal.
• h = 6.63 X 10-34 Js or h = 4.14 X 10-15 eVs

1. When an electron is ejected from the surface, what type of energy does the electron possess?
The ejected electron has kinetic energy.

• Check the box “current vs light intensity”.
• Select the metal platinum.
1. Adjust the frequency of the incident light slightly above the threshold frequency.
2. Vary the intensity of the light and observe any changes in the number of ejected electrons.
3. Increase the frequency of the incident light until it is well above the threshold frequency.
4. Vary the intensity of the light and observe any changes in the number of ejected electrons.

1. What’s the relationship between the frequency of the incident photon, threshold frequency and the ejection of electrons?
If the frequency of incident photons is higher than threshold frequency then electrons will be ejected. However, if the frequency of incident photon is lower than threshold frequency, no electrons will be emitted.

1. What’s the relationship between the energy of the incident photon, the work function and the ejection of electrons?
Electrons will be ejected as long as the incident photon energy is equal to or greater than the work function. In case of greater, excess energy goes to the knocked out electron as KE.

1. What’s the relationship between the kinetic energy of the ejected electrons, the energy of the incident photon and the work function?
Electrons will be ejected as long as the incident photon energy is equal to or greater than the work function. In case of greater, excess energy goes to the knocked out electron as KE.

1. What’s the relationship between the intensity of the incident light and the average kinetic energy of the ejected electrons?
The two are not related to each other.

1. What’s the relationship between the intensity of the incident light and the number of the ejected electrons?
As intensity increases, the number of ejected electrons also increases.