Transcript 7_photo
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 mv2 = QV = eV small amounts of energy could be measured in eV 1 eV = 1.6x10-19J The Photoelectric effect BEGIN Consider a gold leaf electroscope which is charged. If it is exposed to ultraviolet radiation: If it is negatively charged If it is positively charged the leaf falls the leaf does not rise Why can’t this be explained by the UV ionising the air above the GLE? The Photoelectric effect The explanation is that the UV is able to dislodge the electrons from the surface of the metal when it is negatively charged but not when it is positively charged. So the UV is interacting directly with the electrons in the surface of the cap of the GLE. This has been further investigated using this apparatus: Quartz window Ultraviolet Radiation Electrometer e- eeVacuum e- 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. Quartz window Ultraviolet Radiation Electrometer eeeVacuum e- 1. The intensity was increased / decreased. 2. The anode voltage was decreased and then taken negative until the photocurrent reduced to zero (this voltage is called the stopping potential Vs). 3. Different wavelengths of UV were used. 4. Different metal surfaces were used. 1 The intensity was increased / decreased. 2 The anode voltage was decreased and then taken negative until the photocurrent reduced to zero (this voltage is called the stopping potential Vs). 1 Photo current Quartz window Ultraviolet Radiation Electrometer eeeVacuum Results and Explanations Intensity e- 1 The intensity was increased / decreased. 2 The anode voltage was decreased and then taken negative until the photocurrent reduced to zero (this voltage is called the stopping potential Vs). Quartz window Ultraviolet Radiation Electrometer eeeVacuum e- Results and Explanations 1 and 2 Photocurrent Increased illumination Vs Anode Voltage Quartz window Ultraviolet Radiation Electrometer 3 Different wavelengths of UV were used. e- 4 Different metal surfaces were used. eeVacuum Stopping Potential Vs Frequency e- Wave explanation 1 Increasing the intensity increases the current More waves liberate more More photons liberate more electrons with more energy electrons with the same energy. BUT they do actually have the same energy! Would not expect to get a Vs 2 Decreasing the ie a stopping potential 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 Particle explanation 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 Would expect less electrons but energy of an electron should be should always get some ie would less. Some energy used to not expect to get a Vs anyway. liberate electron (work function W or ). Might produce lower or higher The work function is different so energies but there still shouldn’t the Ek of the electron will be be a cut off different and hence the Vs will be different. Unsatisfactory explanations 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 = 6.6x10-34Js). 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 Ek of the photoelectron. so Energy of incident Photon Removes electron from metal Balance will be Ek of electron hf = W +Ek or hf = W + eVs This is Einstein’s Photoelectric Equation: At the minimum frequency to liberate photoelectrons (f=f0 = cutoff frequency) this becomes hf0 = W so hf = hf0 +eVs We can rearrange the original equation to get h W Vs f e e If we vary f, Vs will vary . This is of the form y=mx+c such that if we plot Vs on the y axis and f on the x axis, we will obtain a straight line of gradient h/e and intercept on the y axis of W/e.ie if you know e you can evaluate W and h.