Photoelectric Effect - A-level Physics

the photoelectric effect you might have guessed involves two things light and electrons it's actually the main piece of evidence for the quantum theory of light that means that it proves that light acts as a particle incidentally Isaac Newton was the first person to come up with a quantum theory of light when he thought that light travelled in little packets called core puzzles but he thought that they had mass but then a guy called Huygens or if you want to pronounce it properly it's a ho Hunt's he came along instead actually it acts like a wave and we see that way of diffraction but this is the main piece of evidence that light acts as a particle that means that light comes in discrete packets of energy we call these photons now the energy of one photon we find out is equals to H F F being the frequency of the light an HB in Planck's constant a six point six three times 10 to the minus 34 joule-seconds our eyes I was the one to revive Newton's particle theory of light but with a few changes Einstein and people after him did experiments with light and electrons to prove that this is the case what we find is that if we have a piece of metal we know that metals are lattices of ions surrounded by delocalized electrons what we find is that if we shine light at a metal we actually see electrons being popped off that's an electron and that's an electron now this would make sense with a wave theory of light but the problem is if the wave theory of light was correct or it was the only correct model for light then we should see electrons liberated but there's jumping off the surface of the metal with any frequency of light so long as it's bright enough it's light just acted like a way that we'd have a brighter light and more waves coming in and they'd be absorbed by the electrons the electrons get the energy from the light and they would be liberated but we do not see that being the case now and these electrons are liberated from the surface of a metal they have kinetic energy so they're able to move away from the metal because they have this kinetic energy the question is how do we measure this kinetic energy here's the setup that we have now this is an evacuated tube because you want electrons to jump from one plate to the other without bashing into anything else so no gas is allowed in there what we do is shine a light on this plate here what happens the light comes in and it's absorbed by the electrons on the plate and they jump off there what do we have we have a current being set up in the circuit forget about this battery for now so when we shine light electrons are liberated off the surface of this metal they jump to the other side and we have a current being set up here we measure a current with our ammeter but how can we measure how much energy these electrons have incorporate a battery into our circuit if we have a battery set up this way around then it's actually going to try and stop the electrons from reaching this plate here and that's what we do to measure kinetic energy of electrons what we do is turn the voltage up on the EMF on our battery until no electrons reach the other side we see on the plate what we've hit is our stopping potential call that V s now we know that any voltage is energy divided by charge this energy therefore that's applied to the electrons that should match the kinetic energy that they have coming off this plate here so let's just reject this a little bit vs the stopping potential is potential enough to come throughout the kinetic energy half MV squared divided by the charge now the charge of an electron is e so that's EK has kinetic energy divided by e therefore the kinetic energy of an individual electron can be calculated by doing the charge of electron times the stopping potential one point six times ten to the minus nineteen times whatever potential we need to stop the electrons jumping this gap here after they've absorbed a photon of light now let a little space here because I need to put in here became X now we put the max in there because we really are only concerned about electrons liberated from the surface electrons can be liberated from maybe one layer down two layers into the metal we only care about electrons that are coming right off the surface of the metal if that's the case then they're going to have the maximum possible kinetic energy to jump this gap so that's how we can measure the kinetic energy of these electrons so let's see what happens when we now change the frequency of light that's coming on to this plate here and we measure the electrons EK max now what we see happening is when we have a very low frequency we get no electrons liberated at all so we have a flat line and that will carry on being flat if we increase the frequency until we hit a certain frequency and then all of a sudden electrons will start being liberated and higher the frequency the more kinetic energy they get we call this frequency here the threshold frequency now why is that why do we have to reach a certain threshold frequency before we start getting electrons being liberated that's because we extrapolate this line down here we end up with a negative energy and we're going to call that minus Phi this energy here is the energy needed to liberate an electron in other words if we have this frequency of light here the photons do not have a energy to give the electrons to liberate them from the surface of the metal so only after we hit this threshold frequency that we start getting electrons being liberated this energy here is always the energy that an electron needs to be liberated and then any energy it nests has left over it's going to have as kinetic energy so now I have a straight line graphed any straight line graph is y equals MX plus C where m is our gradient C is our y-intercept but in this case we don't have white we have ekm x equals now it turns out that the gradient of this line is Planck's constant itself times X which is our frequency in this case and what y-intercept is minus Phi so people find this equation fairly confusing but it really isn't this here is the energy left over for an electron after it's been liberated this is the energy given to it by the photon and this here is the energy needed to liberate it energy equals energy take away energy we calculate the energy left over as kinetic energy by taking the energy given to it to begin with by the photon take away the energy needed to liberate it that's what we have left over so this is our photo electric effect equation now we know that when we hit the threshold frequency then the photons just giving the electron enough energy to be liberated but it's going to have no energy left over so we can say that in this case 0 there's no kinetic energy equals HF minus Phi but F is our threshold frequency so that means that our threshold frequency is equals to 5 the work function divided by Planck's constant Phi this energy needed to liberate something is what we call the work function of a metal it's going to be different for every type of metal so what did the photoelectric effect proof first thing that it proved is that higher intensity that's more photons did not increase the kinetic energy of the electrons that must mean one photon is absorbed by one electrons in other words you can't have two photons being absorbed by the same electron and therefore it having twice the energy that doesn't happen as the light we said earlier as well that means that light exists as quanta in other words it exists in tiny little bits of energy not just one big messy wave it is specific individual bits of light known as photons I hope you found this useful if you did please leave a like and if you think I've missed anything or have any questions please leave a comment down below and I'll see you next time

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Keywords: questions, paper, answers, shorts, science, 11th, school, diploma, sat, practical, british, aqa, mark scheme, investigation, solutions, ocr, experiment, 10th, crash course, explained, tutorial, ap, past, grade, college, stopping potential, spoken, edexcel, technique, 9th, exam, 12th, university, electrons, equations, advanced, sats, placement, unintended, lesson, mcat, lecture, kinetic energy, homeschooling, revision, entrance, naked, high school, soft, fun, english

Id: I_7ZfMlIKhk

Length: 9min 39sec (579 seconds)

Published: Mon Feb 20 2017