Presentation is loading. Please wait. # The Photoelectric effect Before we begin:  voltage = work done per coulomb  ie W = QV  if an electron passes through a potential difference and slows.

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The Photoelectric effect Before we begin:  voltage = work done per coulomb  ie W = QV  if an electron passes through a potential difference and slows down, its Ek is converted to electric potential so Ek = 1 / 2 mv 2 = QV = eV  small amounts of energy could be measured in eV  1 eV = 1.6x10 -19 J

The Photoelectric effect

Ultraviolet Radiation Quartz window Electrometer e-e- e-e- e-e- e-e- Vacuum This has been further investigated using this apparatus:

Ultraviolet Radiation Quartz window Electrometer e-e- e-e- e-e- e-e- Vacuum UV passes through the quartz window and ejects electrons (called Photoelectrons) from the negative cathode. They travel to the positive anode and constitute a tiny current detected by the electrometer. The results are inexplicable with the wave theory of light.

Photo current Intensity 1The intensity was increased / decreased. 2The anode voltage was decreased and then taken negative until the photocurrent reduced to zero (this voltage is called the stopping potential V s ). Ultraviolet Radiation Quartz window Electrometer e-e- e-e- e-e- e-e- Vacuum 1 Results and Explanations

Ultraviolet Radiation Quartz window Electrometer e-e- e-e- e-e- e-e- Vacuum 1The intensity was increased / decreased. 2The anode voltage was decreased and then taken negative until the photocurrent reduced to zero (this voltage is called the stopping potential V s ).

Stopping Potential V s Frequency Ultraviolet Radiation Quartz window Electrometer e-e- e-e- e-e- e-e- Vacuum 3Different wavelengths of UV were used. 4Different metal surfaces were used.

1 Increasing the intensity increases the current 2 Decreasing the voltage until the anode is negative and still current flows. Vs is the same. 3 Decrease the frequency / increase the wavelength 4 Using different metal surfaces Wave explanation Particle explanation More waves liberate more electrons with more energy BUT they do actually have the same energy! Would not expect to get a Vs ie a stopping potential Would expect less electrons but should always get some ie would not expect to get a Vs anyway. Might produce lower or higher energies but there still shouldn’t be a cut off Unsatisfactory explanations More photons liberate more electrons with the same energy. One photon ejects one electron so there is a max energy that an electron can have. This corresponds to Vs. Enegry of a photon is less so energy of an electron should be less. Some energy used to liberate electron (work function W or  ). The work function is different so the E k of the electron will be different and hence the Vs will be different. SATISFACTORY EXPLANATIONS

Einstein explained the phenomena perfectly with a few simple assunptions as follows:  The light exists in small packets (photons) of energy E = hf (h= planck’s constant = 6.6x10 -34 Js).  The photons interact with electrons on a one to one basis.  Energy is required to remove an electron from the surface of the metal (the work function) and the amount is a function of the metal.  The remaining energy is liberated as the E k of the photoelectron.

so This is Einstein’s Photoelectric Equation: hf = W +E k or hf = W + eV s Energy of incident Photon Removes electron from metal Balance will be Ek of electron

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