11th Class Physics Chapter 9 | Use of Galvanometer | 11th Class Physics New Book 2025

[00:00:06]Assalam Walekum Dear Student This is Kash Majeed Your Physics Teacher Student Today our topic is Use of Galvanometer Well in the previous topic which we have discussed till now in which wet stone bridge and second potentiometer we had used galvanometer in both of these and by using galvanometer we had achieved balancing condition and on this condition we had said that what is the voltage across the galvanometer at both the ends? It becomes beans. And in this case current does not flow and galvanometer does not show deflection.

[00:00:37]And what was the measurement you had in this case? You could have done accurate or precise measurement. Ok?

[00:00:44]So basically these points have been discussed in this topic that if you used galvanometer in Wheat Stone Bridge then what was its function? Then you used a galvanometer in a potentiometer, what was its function? So first if you look at the function of galvanometer, it is an instrument used to detect a current. Ok?

[00:01:04]We mostly use it in circuits to detect current. And it is often used in a null method.

[00:01:10]What is Null Methods? In which you basically achieved the balancing condition. In which we make the current zero, by making some arrangement we have to make the current zero.

[00:01:20]To achieve a precise measurement in an electrical circuit.

[00:01:25]In any electrical circuit, we mostly use it for bridge circuit or we use it in complex circuit.

[00:01:31]And in this case our aim is to achieve the null point. And what will be the deflection of the galvanometer at that point with the current you have? It will become zero.

[00:01:41]And it becomes easy for you to do the measurement.

[00:01:43]So this is how the use of null method is null method is vitally used in bridge circuits and in bridge circuits there is basically a weight stone bridge and potentiometer. Ok? You have discussed both of these so far.

[00:01:55]And then the book described both of them here. It's the same wording you used earlier. But this is written in the sense of the use of galvanometer.

[00:02:04]So you can say that null method is used to measure unknown resistance. So here's the null method that you used in the Wheat Stone Bridge. The purpose of this was that you had to measure the unknown resistance.

[00:02:17]Meaning you had four resistors and in this way they formed a bridge and between both the ends you connected the goniometer. Then we would change the variable resistor and what would be the potential difference at both ends after changing the variable resistor? It would have been equal.

[00:02:34]So in this case the current goes to you to zero. So this is what is said here that the variable resistance that you have, you used to adjust it in such a way until the galvanometer shows no deflection. And then what you do after that is we derived a formula R1 R2 = R3 over R4 and the value of this R4 which was the unknown resistance we find it in this way because R1 and R2 were fixed non resistance and R3 you had was still non but you had variable resistance. We used to vary this R3 and achieve balancing condition and in that case what you had to do is to make the unknown resistance mean. So this is basically what has been explained here.

[00:03:17]Now in potentiometer you also used null method and it meant that unknown voltage has to be measured or compared. The first is to measure the voltage of the cell and the second is to compare the EMF of the two sources of cells that you have, which were working as cell sources.

[00:03:36]This is your work, it was here.

[00:03:39]What did you do in this case also? Basically, you used to move ah context C that whatever register you had like this.

[00:03:48]Ok? We have just discussed it in the previous topic.

[00:03:50]So you had this point like this and you could have put any unknown source here.

[00:03:56]Ok? And you will install the galvanometer here like this. And with this you would have a sliding contact C. You have point A and point B and accordingly you used to connect A and B like this.

[00:04:14]Basically, you would connect the EMF of non-source.

[00:04:18]Ok? So this is the kind of setup we just discussed earlier. So you can also explain these things here. So in this case also, what you had to do was move this sliding contact. Then for this, if the potential on this side of the galvanometer became equal to the potential on the other side, then you would say that here the current of the galvanometer would become zero and the deflection would not show and in this way we could measure the unknown EMF of the cell. The EMF of this cell, which was named ex, was basically equal to the EM V voltage along A to C which dropped. Ok? So here also you have to use the same wording and explain it. And next we have to discuss its advantages.

[00:05:06]Advantage of using galvanometer.

[00:05:09]In null methods you are using galvanometer and what advantage do you get from it? So first you have that with this you can eliminate the effect of internal resistance.

[00:05:19]Internal resistance means the resistance of the galvanometer, due to which there is no change in your value.

[00:05:26]Ok? So you can say that eliminate the effect of internal resistance on the measurement. Whatever measurement you are doing, there should be no effect of internal resistance.

[00:05:35]Resulting in a more accurate reading. And you get a more accurate reading if we use a galvanometer and because the current is zero, you can say that its resistance does not change your value. So this way you get accurate readings. Second is Galvanometer is highly sensitive instrument that can detect the order of 10 power-6 ampere. This means that it can detect even current in the range of micro amperes.

[00:06:02]So, therefore, you can say that this is also a very helpful thing that even if there is a minor current, it can detect that too, which is mostly in the range of micro amperes.

[00:06:12]Then there is the third point you have that no reflection indicates a direct and clear condition of balance making it easier to identify the null point. So in this way it becomes easy for you to find the null point. So simply this question has the same points which you used in Wheat Stone Bridge and the concept which you used in potentiometer. Whatever explanation you had added for balancing condition or to achieve null point, you have to simply use the same here, what is the function of galvanometer in this? So hopefully you must have understood this concept.

[00:06:52]In the next lecture we will discuss the registers that you have and similarly we will discuss different concepts and after that we will discuss numerical problems.