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Experimental setup & saturation current: photoelectric effect

Let's explore how to measure the no. of photoelectrons emitted per second experimentally, in photoelectric effect. We will also draw a graph of the photocurrent vs anode potential. Created by Mahesh Shenoy.

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Video transcript

while performing experiments on photoelectric effect people were interested in measuring two things one is how many electrons are coming out per second these electrons are called photoelectrons because they're coming out due to light but how many of them are coming out per second and secondly with what energies are these electrons coming out in fact we wanted to know with what maximum kinetic energy would the electrons have and as you might know the story as you've seen in the previous videos when we saw how the intensity and the frequency of the light affected these two things it blew our mind it changed the way we thought about light and it led to the whole quantum revolution but the question i want to explore in this video and i think we should ask these questions more often in science is how do we measure these things what kind of experiments did we do that allowed us to count how many electrons are coming out per second and with what energies they're coming out and and remember we're talking about the 1900s and so in this video we'll focus on building an experiment that will allow us to count how many electrons are coming out per second and in in the future video we'll look at how to measure with what energies these electrons are coming out so where do we start well for photoelectric effect we at least need a metal so we know we can begin over there some metal that shows photoelectric effect so back then we knew zinc showed photoelectric effect that's how we discovered it it was the first metal i think so we knew zinc shows photoelectric effect and we discovered that when you shine ultraviolet light on it zinc was becoming positive so from this we guessed that electrons are coming out from zinc but now the question is how do we count how many electrons are coming out well what we can do is we can put another metal over here which can sort of act as the collector the goal of this metal would be to collect the electrons so this would be the collector and since the zinc is emitting the electrons that zinc would become or the metal which we are choosing over here would that would become the emitter this would be the emitter but how does that help how does collecting electrons help us count how many electrons are coming out well imagine the collector is collecting 100 electrons per second that means there is a current of 100 electrons per second if we can find a way to measure that current then that current itself will be an indicator of how many electrons are being collected how do we measure the current well we can use an ammeter but we can't use it over here so what we can do is we can make a path for these collected electrons to go back so we can make a path like this right because think about it as this collector collects electrons it becomes negative this becomes more positive the electrons want to go back so as this is collecting 100 electrons per second those hundred electrons will go here and so there will be 100 electrons going here per second and i can measure the current here by using an ammeter back in the 1900s we had invented ammeters so ammeters were there so if i put an ammeter over here maybe a milliampere because the current might be very small the current here is a direct indicator of how many electrons are being collected per second boom i found a way to measure or count electrons so let me write that so this current here is a direct indicator of number of electrons collected collected per second but wait we can't party it because we're not done yet there's still a problem with this setup see this only measures how many electrons are being collected per second and that's not necessarily the same as the electrons emitted per second because not all the emitted electrons might reach the collector some of the electrons may just not reach the collector and you can't i mean you can't practically build a collector to collect all the electrons just like that right so some of the electrons may not so this number is not a true indicator in fact should be smaller than actually how many electrons are emitted because a lot of electrons might be missing the collector so what do we do so now the question is well now the problem is not all electrons emitted are being collected so the question is how do we do that how can we somehow can we somehow find a way to ensure that all the emitted electrons are being collected how do we do that well for that we need to find a way for the collector to somehow attract electrons towards it or maybe for emitter to somehow repel or push the electrons away from it how can we do that i want you to pause the video and think about what can you do how can you modify this setup for the collector to start sucking in these electrons well electrons are charged particles they are negatively charged particles right so just make the collector positive it will start attracting it make the emitter negative it'll start repelling it that's all we have to do but how do we do that hey we can use a battery and put a voltage so we can put a battery here let's put a battery here and if i make let's see i want to make the collector positive so if i put the positive of the battery here and the negative of the battery here now my collector starts becoming positive and starts sucking more electrons into it and i'm getting what i wanted i am now able to suck in the emitted electrons and because we want to ensure that all the electrons or at least almost all the electrons even the ones that are ejected at you know different angles even they need to get sucked in what we can do is we can put a very high voltage right if the voltage is very high then there's a stronger pulling force and more you know good chance that all the electrons get sucked in but my question to you is as you're running this experiment let's say you put a high voltage how will you be sure that even at the high voltage all the electrons have gotten sucked in i want you to pause and think about this for a while this is you we have everything but think experimentally how will you confirm that all the electrons have been sucked in all the emitted electrons are definitely being collected what can you do which to pause and think here's what we can do put some voltage and measure the current now increase that voltage if the current increases that means more electrons are getting collected that means we are not there yet further increase the voltage again if the current increases that means still more electrons are being collected we're not there yet as you increase the voltage finally there comes a point where the current will no longer increase what does that mean ah that means no more electrons are being collected that means we must have hit a saturation almost all the emitted electrons must now be getting collected and so when the current reaches a maximum value and it doesn't increase after that now we know for sure that all the electrons that are being emitted almost we can say are definitely getting collected so the model of the story is in this experiment wait for the current to reach maximum so let me write that over here so max current or we can say saturation current that is equivalent to the number of photoelectrons oops photoelectrons ejected per second or collected per second pretty much the same at this point so the last thing i want you to do because practically you know how people from this experiment they draw graphs and i know graphs can seem intimidating but they are really insightful so what i want you to think about we now have the whole story i want you to predict this graph let's draw a graph of voltage on the x-axis the voltage that you're going to put a battery over the voltage of the battery versus the current that you detect over here and that current is often called the photocurrent because it's the current due to photoelectric effect so over here we'll measure we'll plot the photocurrent i want you to pause and think about as you change the voltage from zero from when the battery was not there all the way to increasing the voltage how would the graph look like can you pause this is the last time i'm asking you to post can you pause and try all right let's see so let's start with when the voltage is zero when we don't have a battery do we get zero current no because even without the battery we saw there will be some electrons ejected in the direction over here they will get collected and there will be a current so there might be some current to begin with even when the voltage is zero now as i increase the voltage more and more electrons get start getting sucked in and so we would expect the graph to increase we go up and then eventually we know we hit a maximum and after that the graph won't increase anymore so you can kind of guess the graph would go somewhat like this and this point at which we reach saturation that represents our you know that represents our photoelectrons number of photoelectrons coming out per second so this means if i repeat the experiment let's say i change something about my light i repeat the experiment and in the new experiment let's say i get a graph in which the saturation current level is somewhere over here i now know for sure in this experiment less electrons are emitted per second in fact i can by measuring this i can say okay half the amount of electrons are being emitted per second or if i change something of my something about my light and now in the new experiment let's say i plot a graph and i get a graph in which the the new saturation current is here i again know for sure now that more photoelectrons are being emitted per second so you see using this clever experiment we can just by looking at the maximum value of the photocurrent we can know how many photoelectrons are coming out per second the last thing to think about is that if there are air molecules in between then that will mess up our experiment so this experiment should ideally be conducted in vacuum so the electrons are not lost due to you know by colliding with the air molecules and that's why this experiment is often conducted in some kind of a transparent chamber transparent because we want the light to pass through and so we often choose glass or quartz or some other transparent chamber and we suck out all the air and create vacuum and there we go this is how pretty clever right we can now count how many electrons are coming out per second and in a future video we'll see how we can tweak the same experiment and measure with what energies they're coming out but until then i want you to think about it maybe discuss with your friends and teachers of how can you use the same setup tweak it a little bit and be able to find what's the maximum energy with which the electrons are coming out