11th Class Physics Chapter 7 | Beats (7.9) | 11th Class Physics New Book 2025

Indexed: 2026-05-16

[00:00:06]Assalam Walekum, my student, this is Kashi Madid, your Physics teacher. Student, we are discussing Chapter Seven and today we have to discuss an important topic which is important for you in terms of long term and is also an important topic in terms of short and objective.

[00:00:19]Our topic today is beads. But when we started the chapter, we defined this process as when two waves whose frequencies are slightly different, what if there is a slight difference in two frequencies and both are moving in the same direction, then when they are superposed, they will produce beads for you. Now what students wait for is that if you have different musical instruments like you have a guitar, you have any musical instrument, then you see that when sound is produced in it, if you look at drums, then what is done after two different times, that you press it and what you do with it is that you produce sound. Now the sound is being produced near you, so you will see that somewhere you can hear the maximum sound. Somewhere near you, you hear that faint sound. Basically, you hear loud and faint sounds and they are mixed to create rhythm. Ok? So this is basically the phenomenon of beads. And what we can say in this is that two waves are moving towards you like this. Is it okay like this? And the second wave is also moving in the same direction. But what is its frequency? There is a difference between the two. The number of waves will be different.

[00:01:33]You can simply say that one has a frequency of 3 Hz and the other has a frequency of 4 Hz. Ok? Suppose, just as an example, what will happen to the frequency of both? If the difference and both are traveling in the same direction, then when they are superposed they will produce weights.

[00:01:54]Ok? So we can define it as when two waves of slightly different frequencies travel in the same direction. But what should be the direction? There must be beans.

[00:02:03]Overlap each other. When they overlap, then there is a periodic variation of sound between maximum and minimum loudness which is called beats. So maximum and minimum loudness will be produced.

[00:02:17]Ok? So, just now I have given you examples of different musical instruments. So what happens in that is that when beats are being produced, at some places you hear maximum sound.

[00:02:27]Somewhere you hear a faint sound.

[00:02:29]So the loudness is increasing and decreasing at some places.

[00:02:31]And this happens because of beats.

[00:02:33]Now we understand what is beats? Basically. Now suppose we understand this as an example and this is what the book has also discussed. So suppose you have two different tuning focuses. And you can take the tuning.

[00:02:52]Ok? And when you press A from Tunic Focus 1, then the number of bits, number of waves that are being produced, you can tell what is its frequency? It is 30 Hz. Meaning, when Source One or you can say the first tuning fog produced the sound, what were its frequencies? 30 Hz.

[00:03:11]Frequency is the number of waves in one second. How many waves is one producing in a second? Now its frequency is 30 Hz. Meaning it is producing 30 waves and it is producing, yes you have 32 Hz as its frequency and it is producing 32 waves in one second. Ok?

[00:03:28]Now when these two are superposed, we draw their resultant waves here. Now students, can you see what these two waves are doing at any point?

[00:03:41]Look here, it also has a crust. It also has a crust.

[00:03:44]When you get the maximum displacement of both, what output will you get? Maximum is coming.

[00:03:49]And at the point where you have the trough of one and the crust of the other, both are meeting, then what is being formed here? The node is being formed. Like in the stationary wave, you saw that if a wave is going like this, then it gets reflected from here and comes back like this. Ok?

[00:04:04]So in the same way, you can see that in this process also, nodes and anti nodes are being created. At some places there is maximum displacement, at some places there is minimum displacement. But what you have here is the phenomenon that both the waves are moving in the same direction.

[00:04:18]But in stationary waves, you saw that both the waves moved in opposite directions.

[00:04:22][sound of clearing throat] Okay? So in this you can see where you will have maximum displacement. When you see the maximum displacement at this point, maximum displacement, then the output resultant wave is also of maximum amplitude and as soon as you have this one minimum and this one maximum, you can see that this will be visible to you here and this is just one second, basically in one second, in 1/2 second, you have two picks being formed here and like this you also have this one 1/2 second, this one is also 1/4 second, this one is also 1/4, this one is also 1/4, so if you complete all this 1/1/4 then this one is being formed in 1 second.

[00:04:59]So in 1 second, if the difference in frequency of these two is the frequency of one source is 30 Hz and the frequency of the other source is 32 Hz.

[00:05:07]What is the difference between the two? Of 2 Hz. So the more the difference, the more beats will be produced here. So maximum and minimum.

[00:05:15]So what's the sound happening here twice? It is reaching maximum. And what sound do you have twice? It is becoming minimal.

[00:05:21]So what are you making here?

[00:05:24]Maximum displacement. And two minimums are being made here with you. So you can say that you are hearing loud and soft sound or faint sound. Now who has the louder sound? At a certain moment, let's say a point x. Ok?

[00:05:38]Above and below this we will see the displacement of both waves aligned in the same direction.

[00:05:43]Both are aligning in the same direction.

[00:05:45]Look, maximum here also, maximum here also. If they are aligned in the same direction, what will they do when they superpose? What will the resultant magnitude do? We will increase the displacement. So in this case you can see that both of them were in the same direction so the output you will get will be increased.

[00:06:01]But when it is in the opposite direction, here it is downward. Here it is upward. If both are in opposite directions then you can say that you will see minimum displacement above y.

[00:06:10]And because here the displacement is in the opposite direction. Both waves had its upwards. It has its downsides. So you can say that both are cancelling each other's vectors.

[00:06:18]So [nasal sound] you can say here that at a certain moment x the displacement of both waves align in the same direction. So here you will hear loud sound. When both are in the same direction and when they are in opposite direction, the displacement is opposite, then the minimum displacement will be at y. So what will you have?

[00:06:40]You will hear the faint sound at this Y position. Now if you discuss on G then again maximum here also, maximum here also, both will be aligned in the same direction, so the output you will get will also be maximum like this.

[00:06:51]Then after that, what will come to you again at this minimum point?

[00:06:57]Minimum displacement will occur. So you can see that here you have maximum production twice and minimum production twice. So whatever you have, it is maximum, then minimum, then maximum, then minimum. So this way you will have number of beats produced. So there's this beat and there's this beat you have. Ok? So you can say that there is a periodic variation of sound between maximum and minimum loudness which is called beats. What name would you give it? Will name the beats. Ok? Now next you can see in it.

[00:07:29]We talk about G bit frequency. Now what do we do for bit frequency?

[00:07:33]How will you find it? For which we should know the number of bits, that if there is one source whose frequency you have is 50 Hz and there is another source whose frequency you have is 55 Hz, now we have to see how many bits will be produced in both of them, in this case two beats have been produced, one here, one here, so two beats are being produced here, maximum is going to you twice, minimum is coming twice, so you will say that two beats have been produced here, but if the frequency of one is 55 and that of the other is 50, then how will we know for how much time beats will be produced here? So you can say that whatever will be the difference in the frequency of these two, then the difference in 1 second, you can say the number of beats per second, that is, how many number of beats are produced in one second, is equal to what will it be equal to? The Difference Between the Frequencies of the Tuning Fugue. The difference in frequency between the two tuning fugues will produce the same number of beats. Like I just gave you an example, if F1 supposes how much do you have? 55 Hz. Ok?

[00:08:36]And F2 you have G 50 Hz. Now you want to see how many number of beats will be produced? So you take the difference between these two. What will you do with the difference? 55 - 50 so how much will this cost? Five words.

[00:08:50]So here you will have five beads. The number of beats that will be formed is five. Ok? And its bead frequency is 5 words. Ok? So you can see this example in this.

[00:09:04]Similarly, we can also make a general formula for the number of beads, what should be FA - FB FB FA? There will always be a greater frequency. Like you subtracted 50 out of 55.

[00:09:15]You subtracted 30 from 32.

[00:09:17]So the maximum frequency will be called FA. And the minimum frequency will be what you call FB. So, how much difference did you feel between the two? 2 Hz had arrived. So this much you have, you can also call it bead frequency. So the difference between the maximum and the minimum frequency that you have is correct, in which you can say that one frequency was maximum and the other one was less than that. So what name will you give to the difference between the greater and minimum frequencies?

[00:09:44]You will name the bead frequency. Ok? So after that, we can use this phenomenon of beads to tune different musical instruments.

[00:09:53]Ok? And because of this, what can we do with the phenomenon of birds?

[00:10:00]Can also produce different rhythmic waves. Can create a nice effect.

[00:10:02]Ok? So you can say that its example book has used it. I have taken an example of guitar, an example of drums, an example of organ, an example of violin.

[00:10:10]This is the different musical instrument you have. So, in this you can see what happened?

[00:10:15]What do musicians do? Use the beads to create an intense, interesting rhythmic effect here. Ok? Different: You might have noticed that when you hear a sound, it seems like a pleasant sound. It sounds nice and pleasant. Whenever you listen to singing, what is done in it is that a different economic effect is created in it. Add Texture Texture is added. So you will say that wherever there will be a slight difference in frequency between two sounds.

[00:10:45]What will you produce if there is a slight difference in the frequency of two sounds? Beats. Now students, you must have noticed one thing. Here we are using the word slightly different to mean that the difference should be minimal. There should be a small difference.

[00:10:57]Now how small should it be and why are we using the word small.

[00:11:01]For this you can take the example that human is also an ability. Ok? That is the maximum number of beats we can hear in one second. So if we have this butt frequency of 10 Hz, how much butt frequency do you have? 10 Hz.

[00:11:18]If the difference is more than 10 Hz. That means if you suppose that you have a source of 60 Hz and another source of 70 Hz. Ok? There are two different source tunic fakes. One is producing a frequency of 70 Hz and one is doing what you have? Of 50 Hz. And what will be the difference between the two?

[00:11:35]How many hearts is 70 - 50? 20 Hz. So we will not be able to hear this 20 Hz at all.

[00:11:39]Why? Because the human mind also has a perception as to how much minimum or maximum we can hear? So the maximum frequency difference we can hear is up to 10 Hz.

[00:11:50]Ok? You have to see this for MCUS also that if the difference becomes more than 10 Hz then we will not be able to hear these frequencies. So you can say that that is why we are using this word slightly difference that there should be a slight change in the frequency.

[00:12:05]Like one is 30, one is 32, one is 20, one is 21 or 22 or 23, it could be 24. So, maximum you can observe a difference of up to 10 Hz. So that's why we are using this word slightly difference.

[00:12:19]So hopefully we have understood this topic well for you.

[00:12:23]You have to prepare it well for the long term. See you in the next lecture, Inshallah. Allah Hafiz.

[00:12:31]na [music] [music] [music] so