 If a lot of people speak together, we can't make out anything. But a radio can. Even though it continuously receives hundreds of different radio signals, we can make it listen to any particular channel. How does it do that? Well, radios are something that we don't. LCR circuit. So let's see how it uses resonance to tune into specific channels. We've already seen before that even if you keep the voltage or the values of inductance or capacitance or the resistance, all of them are constant and just change the frequency, the current will change dramatically. If the frequency is very low or very high, we saw that the current is very tiny. The pink arrows over here are the current. But if the supply frequency equals a magic number given by 1 by 2 pi square root of LC, we saw resonance. This is when the impedance in the circuit becomes minimum and the current goes haywire. Now if you need a refresher on this, then we have a dedicated video called LCR resonance and resonant frequency. Feel free to go back and check that out. But what I want to do here is to understand this relationship between the frequency and the current a little bit better. And one of the ways to do that is to draw a graph. So what I want to do is I want to go ahead and plot a graph of the current I0 versus the frequency. Okay, what will happen to I0? I basically want to plot the same thing that you're seeing over here, but on a graph so that I don't have to keep watching this animation. Now, before I draw it, I want you to take a shot at this. One clue is that we know that at resonant frequency, the current is maximum. So let's assume that that resonant frequency is somewhere over here. So given that, can you make a guess as to what this graph would look like? Go ahead. Give it a shot. All right, if you're given it a shot, let's see. Here's how I'm thinking. I know that at this point, my current has to be maximum, which means if I go away from here on either sides, the current should decrease. So with that, if we plot the graph, here's the maximum and on either sides, the current decreases, decreases, and that's what the graph would look like. Again, notice it's telling us the same story at resonant frequency. The current is maximum. But if you go away from it with too low or too high frequency, then notice the current becomes minimum. So we can get rid of this animation. We can now just focus on this graph. Okay. Now with this graph, let's see if we can decipher the mysteries of radio. The important thing to understand is that different, different radio stations get broadcasted at different frequencies. For example, just name a couple of famous radio stations that I am aware of. I know one radio station called Radio Mirchi and all the messages over here gets broadcasted at 98.3 and that's why you may have heard this number a lot. 98.3 megahertz of frequency. So whenever you're listening to Radio Mirchi or if you want to, then that is that frequency of 98.3 megahertz. Similarly, let me just, let me just show one more. Another one, which I remember is big FM. And again, they have that punchline, 92.7 big FM. And that basically means they are broadcasting all their messages at 92.7 megahertz. Now let's imagine that in our radio currently, the LC values are such that if you calculate the resonant frequency, it happens to be exactly 98.3 megahertz. Let's assume that. What's going to happen? Well, here's our radio and here are both the radio waves I've shown. There are multiple radio waves due to lots of channels, but let's assume just two, they're both hitting our antenna. And as a result, antenna vibrates with both the frequencies. What an antenna does is basically converts the radio signals into electrical signals. So because of these two, the antenna is trying to drive our LCR circuit at both these frequencies. But look at what look from the from the graph, we can see that whatever messages are coming at 98.3, it's able to generate a much larger current. Let me show you that. It's able to generate a much larger current. The current that that is generating is over here due to the radio mirchi signals. And the current that is generated due to the big FM, look at that. That current generated is very tiny. And as a result, this very high current goes through the speaker. And this current, well, it can hardly drive the speaker. And as a result, you will only hear the messages from this particular channel. Now imagine you want to switch to big FM. What should you do? I think you can now guess. Well, now we need to reduce the resonant frequency of our LCR circuit. How do we do that? Well, you have to either, now since you want to reduce this, we need to increase this. So you either increase the inductance or you increase the capacitance. And there are ways in which it can be done. Different radios do different ways. For example, in some radios, when you turn the knob, the distance between the plates of the capacitor changes. And you might know as that distance changes, the capacitance changes. Interesting, isn't it? And so just by turning the knob, one of them will change. And in this particular case, all we have to do is increase the value of capacitor. And as you increase the value of capacitance, the resonant frequency will start shifting towards the left. So the graph will start shifting towards the left. And so your new graph, as you increase the value of that capacitance, your new graph will eventually come to this. Now the opposite happens. Now it's the big FM. It's these messages that start exciting your radio because your radio's resonant frequency has changed. And as a result, your radio will start picking up those signals. And this is how by using resonant frequency concept, resonant concept, you can tune into any station that you want. Beautiful, isn't it? Alright, now let's make things a little bit more interesting. Imagine your friend also has her own radio. And let's assume for the sake of simplicity that in her radio, the value of inductor and the capacitor as of now is exactly the same. So it's resonating at 98.3 MHz. But there's one difference. Let's say that the resistor, the resistance of her LCR circuit is higher than your LCR circuit's resistance. So the value of resistance is higher. I want you to predict what that new graph of this new radio is going to look like. Same resonant frequency but higher resistance. Again, can you pause the video and give this a shot? Alright, because it has the same resonating frequency, it'll peak at the same frequency. However, at the peak value, the recurrent would be smaller because the resistance is higher. And so the graph that we could expect is somewhat like this. You immediately see the graph is much flatter than before and that has consequences. For example, if you just concentrate on this graph, so let me just get rid of the other one, and now look at what's going to happen to the radio. Just like before, this frequency is going to generate the maximum current. But notice, because the graph is so flat, the current generated by the big FM frequency is considerable. It is smaller but it's considerable. Which means in this particular case, your radio might be giving some amount of message from this as well. There will be some mixed messages that you'll be getting. You'll be able to hear this as well as a little bit of this as well. Compare that with what we got before. Before, the difference was so huge, it was able to select this frequency very nicely and reject everything else. But now notice, it's not all that great. So you can pretty much agree, let me get both the graphs. Okay, we agree that this curve has a higher quality of selecting the frequency and rejecting everything else, which is of resonance, higher quality. But this one has lower quality. So not all LCR circuits have equal quality. If you want a very high quality ability to select very specific frequency and reject everything else, you need the graph to be very, very sharp. And so such sharp graphs are what we call high quality LCR circuit. Now, of course, the engineer inside you might actually be more curious and wondering, okay, can we give a number to it? How do I calculate this quality? And that's something we'll talk about in a future video. There's a number called quality factor, but let's not worry about that right now. But I'm pretty sure one thing you can immediately get is that quality is inversely related to the resistance. Notice if you have lower resistance, you end up with a higher quality circuit. With a higher resistance, you end up with a lower quality circuit. And so although we don't use radios as much today, it's a perfect reminder of how LCR resonance circuits can be used to tune into very specific frequencies. Sometimes I wish I too had this ability to listen to just what I want and filter everything out.