11th Class Physics Chapter 9 | Numerical Problems 9.6 and 9.7 | 11th Class Physics New Book 2025

[00:00:06]G Next numerical problem we have 9.6 a disk of area 10 cm² What is the area of ​​the disk you have? 10 cm² is it okay? Now you know what kind of unit area is? Meter Square.

[00:00:20]So to move from centimeters² to meters squared you multiply 10 to the power -4. You can also see this way that centimeters are square.

[00:00:30]What is the value of 1 centimeter? 10 to the power is equal to -2. So if centimeter is square then if we take its square then it will come to 10 power -4. So simply what do you have to do? If we multiply by 10 power -4 then what will the value be converted from centimeter square to? It will be converted into square meters. Because what kind of unit is area? Meter is square.

[00:00:49]Next you place it placed in a vertical electric field.

[00:00:55]And what is the electric field there? 5 * 10 coulomb to the power 5 newtons.

[00:01:01]And next if the plane of the disc makes an angle of 30° with the horizontal. And if the area that you placed is a vector area, it is neither parallel nor perpendicular.

[00:01:13]Rather it is making some angle 30° with the axis. Ok? Here it is making an angle of 30° with the horizontal axis. Determine the electric flux through the disc. You have to find out the electric flux.

[00:01:24]We denote electric flux by phi.

[00:01:26]We have to find out the value of electric flux.

[00:01:27]Now student you know that for electric flux we had dealt that when any angle is being made here specific angle theta. Ok?

[00:01:35]Or you can say that the angle here is denoted by theta. So for that you would use ea cos theta. E here is the electric field. A here is the vector area. And here cos of angle theta between E and A. The angle between E and A will come.

[00:01:51]So this is the mathematical form you will use. How much value of a do you have? 5 * 10 to the power 5 and how much area is given? What is the value of 10 * 10 to the power -10 sorry -4 and into cos? If you press cos 30 cos 30, you will get 0.86. Similarly, 10 * 10 to the power of -4 is one and what will be the result of -4? -3 You should simplify these. If you multiply this with this, what will you get? What is the unit of 4.3 * 10 power 2 and electric flux?

[00:02:26]Newton meter² per coulomb. Ok? So in this way you have to apply the full electric flux. Ok? And the electric flux has to be found. The next wrong question you have is 9.7 which says that a circular copper rod is 50 cm. Long one you have a copper road what is its length? Yes, what is the length of Copper Road? You can also write this from LC.

[00:02:53]How much of this do you have? 50 cm So again if we convert this from centimeter to meter then divide it by 100 and you will get 0.5 meter. Next, the diameter of this rod is also given and its diameter is 1 cm. Now the value of diameter is 1 cm. I denoted it by DE.

[00:03:12]So to convert centimeters to meters, divide by 100 and you will get 0.01 meters. Now students, you have to find the radius from the diameter.

[00:03:22]Ok? Because this will be our next use. When we talk about area, we will use πr² for it.

[00:03:29]Ok? So what you have to do is divide the diameter by half to get its radius. If you halve the diameter, then divide this value by two, then you will get the value of radius i.e. 0.005 meters.

[00:03:42]Now next look into this next statement is find the resistance cross its end. You have to find out the resistance of the copper end wire you have used. So we are using C for copper. So RC you have to find out the resistance of copper wire. See next second part. What should be the side of square cross section of 50 m long tungsten wire? Now you had taken copper wire earlier. Now we will take tungsten wire.

[00:04:09]Ok? And its length is also the same.

[00:04:14]You have the same length of tungsten wire as the length of copper wire. So that's why we wrote it down from General L as well.

[00:04:19]Ok? If you want, you can also write it as LC and LT.

[00:04:22]Because T stands for tungsten and C stands for copper. Next, its resistivity is also given to you. In this you can see 50 Same Long Road If The Resistance Is The Same. Now he is saying that the resistance is the same. The resistance of the tension wire is the same as the resistance of the copper wire. The resistance of both should be same. Then you have to tell us what is the area of ​​cross section of the road you are using.

[00:04:51]Ok? Any. So in that area cross section, if you have a length or square side, then you can consider it as a square, that is, if you are getting a square form, then what will happen in it is that if one side is given then you have to find out its other side. Ok? So, what is the length of one side of this? You have to find that out. Ok?

[00:05:14]And you will use the first case.

[00:05:17]What is in the first case? What is the resistance of both copper and tungsten? It is beans.

[00:05:20]Using this concept, you have to first find out the resistance of copper. We will find the resistance of copper. That's what we can use in its place.

[00:05:28]So this is our first task.

[00:05:30]Secondly, you have to find out the length so that if you change or replace the wire, then the resistivity of that wire is also given. Ok? You are given the resistivity of tension and copper also.

[00:05:43]So what you have to do is use this relation and find out its length.

[00:05:48]Ok? So what should we say?

[00:05:50]Because given that both have the same resistance. So for the same resistance you will use the relation RC. What is RC R equal to you? R equal to is row LL over A okay? So, there is resistance here for this. L here is the length of the wire. Here A is the area of ​​cross section. Ok? So you can use it like this.

[00:06:11]So in this case if we are using it for copper then C and C will come. The length is the same for you in both cases. So we will use l only and the area of ​​cross section will be obtained for that. So what will we use for area? Is the formula πr² correct? And in this way we will put value in it.

[00:06:29]What is the value of row C? 1.69 * 10 to the power -8 and how much length do you have? 0.5 will come. And we found the value of pi for area as 3.14 and r as radius. We will take its square. You multiply these two and what do you do with it?

[00:06:45]Divide it by taking squares. So what will come to you? RC = 1.08 * 10 to the power -4 ohms you will get the resistance of the copper wire.

[00:06:55]Now who will have this much resistance? RT will also be same for this. So you can say that if this is RC then what will be RT? 1.08 * 10 to the power of -4 ohms. You will also get the same RT because what are both? It is beans. This is given to us in the statement. Ok? Now we have to find out the length and in the second case we have wire change. We are using tungsten wire.

[00:07:20]So you would use the formula RT = RotL over A, okay? Now this area is basically in the form of a square. Ok? So what will we do for this?

[00:07:34]What will happen to the area? length * length. So you can also call it L². You can put it in place of its area.

[00:07:39]So what will come to you? Because he has mentioned it.

[00:07:44]Here you will see what should be the side of a square cross section of 50 cm.

[00:07:49]Long length? So it has square sides. So what are we going to use here? Instead of area, we will use L².

[00:07:56]Ok? Now what you have to do is I replaced the RT. Replaced it with RC only.

[00:08:01]Because both were beans.

[00:08:03]So you can use it here like this also. If you want, you can also write RC. Now you replace both of these. Shift this value over here. It will come here. So what is l² equal to? RowL over rc Now what is the value of Student Row? There you have it: 5 * 10 to the power of 8, and the length is the same in both cases. So we will use only 0.5. And how much resistance have you received so far?

[00:08:32]Multiply these two by 1.08 * 10 power -4.

[00:08:36]Divide by this value. Later we will also have to take the square root. Because we have to find out the length. So I also placed the square root on both sides.

[00:08:44]Now student, you will get l and its value will be 1.52 * 10 power -2 meter. Now what you have to do here is that you can also write the prefix of this -2 as senti. So what will be the length? 1.52 cm Ok?