Here we are, a day away from the Physics mock (on Monday 6th December 2021, at 8:30). This is the last topic I have yet to review so far, so I thought to revise it could be a good thing to go over what we've done. Overall what we need to learn seems a bit overwhelming, but splitting it into smaller parts makes it more manageable. The exams have creeped up on me though, so I guess I just need to make sure I know all of what's in these reviews for tomorrow.
First of all is the Photoelectric effect. This effect can be shown well with an electroscope, which is shown below:
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In the experiment, the electroscope is first charged. This causes the gold leaf to rise (as the leaf and metal stem are both negatively charged with electrons, so they repel). When UV light is directed at the zinc plate, the gold leaf lowers. This is because the photon's energy is absorbed by the electrons. These photoelectrons then travel to the surface, and escape. This means the leaf and stem get less charged, so repel less.
There are 2 equations which are necessary to this theory. The first is E=hf. This is used in this case to find the energy of the UV photons. h is Planck's constant, and f is the frequency. If you don't have the frequency, but have wavelength and wave speed, then you'll need c=λf (rearranged to c/λ=f). The other equation is E=Φ+Eₖ. This is showing that the overall energy= the work function (the energy needed to produce photoelectrons)+the kinetic energy, which will just be any left over energy. The good thing is that these equations are on the data sheet, so we just need to know how to apply them. Although I wrote them down from heart anyway, but I guess it's safe just to check.
With the equations, it is used to show that the energy of the photon equals the energy after. You can also substitute hf into E in the equation to directly show that. An important part is that the energy of the wave has to be at least the work function. The threshold frequency is defined as the minimum frequency needed to provide the work function, or produce photoelectrons. If this isn't met, so if the frequency isn't high enough, photoelectrons won't be produced, so the leaf won't lower. This is why UV is used for the experiment, as visible light or anything lower on the EM spectrum has too low of a frequency. If the freq of the wave is exactly the threshold freq, then hf=Φ, as there is no leftover energy. If the freq is higher than the minimum needed, then there will be kinetic energy.
Note: for this I think you might need to know how plot a graph, and get further results from that. Given the energy and freq, plotting shouldn't be too hard, but you need to take into account the hf=Φ+Eₖ equation, if you need to find out the other values. Putting energy on the y-axis and freq on the x, you could rearrange the equation to make it like y=mx+c, so Eₖ=hf-Φ. This would mean Planck's constant will be the gradient, and the work function will be the y-intercept*-1 (as it is -Φ)
Ok, onto energy levels now. Before we go onto all the learning content, we need to know how to convert J to eV. I explained it in my review on Matter and radiation, but I'll go over it again. To get from J to eV, you divide it by 1.6*10^-19 (the energy of an electron). The opposite is just the opposite (/).
