Electric Charges And Fields Part 1 | NCERT Line by Line | NCERT NEET 2025 Physics | Shreyas sir

0:03 good evening my dear Lions welcome welcome welcome to the vant need English Channel hello Apna hello madhumati hello Tisha good evening good evening uh UD prakash yes your captain is here uh so we're starting off with the 12th standard syllabus and the first chapter of your ncrt electric charges and Fields uh we will try to complete as much as possible and if it takes too much of time then we'll probably divide into two parts okay so we'll see how the lecture goes uh welcome aboard uh it's going to be around 2 to 3 hours that's the expected timeline and this is your captain STS on the vantu need English Channel teaching and nurturing kids for more than 13 years and uh this is now your opportunity your time to get

0:49 yourself thorough through all the concepts to get yourself Guided by my particular teachings and experiences and make sure that you crack need exam and any other comp exam that you going to give the following year okay so make sure you're liking the video if you have not done that yet give your Guru Dak by Smashing that like button and stay subscribed so that you'll not miss any other class or else you might just lose track of where did I watch sir where did I see this series yes hello shuti hello satri good evening Krishna Warrior pandus are here hello goam hello n pratiba welcome welcome madhumati yes thank you for all the lovely hearts and lovely welcome let's begin with the class okay now this is a big chapter and

1:36 Carries High weightage lot of Concepts obviously a lot of things are there and it's not just important from your board's point of view but also obviously need point of you and this chapter of electric charges and Fields like you can see over here it starts by the understanding of what charges are and what forces do charges apply and then we slowly move to the concept of electric field and electric flux and towards the end we deal with something called as dipoles and also gos law with its applications that's what the broad outcome of this chapter is okay there is a second chapter following this which is

2:21 electrical energy electrical potentials capacitance and all of that that is the chapter number two this is chapter one of 12th standard okay so let's begin hello hello hello shti hello Trisha hello Manoj good evening my dear lovely Warriors see uh the concept of charges so that's what is there in the introduction the concept of electric forces was known much before and people knew it by experience and it was slowly formalized later on like we all have experienced some kind of a spark or heating of a crackle when we take off of your clothes or sweater particularly in dry weather and sometimes you might have tried to rub a scale and touch bits and pieces of

3:08 paper they get attracted when you remove your shirt suddenly you feel some spark or when you uh Touch Somebody you might have felt a shock all these are related to electrostatic charges the charges which are there around us even lightning thunder storms they involve electrical charges and forces obviously in them so you might have also heard this is due to generation of static electricity this is the precise topic which we're going to discuss in this chapter static means anything that does not move so this is a very important word over here static means anything which does not move or change with time so that is why the chapter's name is sometimes referred to as electrostatics it is also referred to as electrostatics okay so uh here we'll be

3:56 dealing with you know the forces the field the potentials Etc due to stationary charges only that is why static electrostatics cool that is basic introduction part now this is where the real chapter actually begins now see if something has to apply Force something has to cause some phenomena it has to be due to something very preliminary something very Elementary something which happens to be found in nature and that is basically your charge that charge which is there in all the bodies around us in all the atoms and the molecules that is the thing which causes that is the thing

4:44 which causes the forces the crackling the thunderstorms the lightning the Sparks the static Behavior electrical discharge that is that thing so historically the credit was given to you know u a long long time back to Greece around BC but and that's how the name came okay from there many such pairs of materials were known which on rubing could attract light objects Like Straw pit balls and bits of papers so just imagine this this is a lot of time back thousands of years back people knew that when you rub some cloth on some object or if you rub two objects together they were known to attract or repel few things so people knew that there is something and they attributed it to something called as electron meaning

5:31 Amber and there were many such pairs which were known which were known on rubbing they would they would attract you know objects like you know plastic paper or pit balls and stuff like that so that is what you know the history of charges all right and we all know that when you take a glass rod and you rub it with you know uh rub it with different cloth and if you touch it to another plastic rod or another glass rod either they will you know start repelling or attracting each other depending on the charges which are present on the other body this is something which you have already seen as either in 9th or 10th standard or you would have experienced

6:19 this in your school when you take a plastic scale rub it on somebody's hair rub it on somebody's hair and you attract bits and pieces of paper 100% would have done it yes Trisha it will be around 3 hours you can expect it always go with new ncrt yes always go with new ncrt do not use the old ncrt so this is interesting to see that when a glass rod is rubbed with silk the rod will acquire some kind of charge and the silk acquires some another kind of charge and this is true for any pair of objects that are rubbed to be electrified you rub pandu against Champa pandu will become one type of charge

7:05 Champa will become other type of charge pandu becomes positive Champa becomes negative you rub Champa on ladu ladu will become negative Champa might become positive so they become electrified they become charged bodies when they are rubbed so what happens is when you rub you know two objects onto each other the charges of one kind go onto the other side so the charges of one one kind go on to the other side so one becomes maybe positive the other becomes negative so it depends on the pairs of the bodies this depends on the pairs of the bodies which are being rubbed okay it's not like every time the first object which you rub only will become positive it depends on the material okay so you will see that if

7:52 the electrified glass rod is bought in contact with the silk with which it was rubbed they will no longer attract each other they also not attract or repel other light objects as they did not being electrified so you will see that if you just bring them close without rubbing they will not attract or repel it's only after rubbing that they were getting electrified that means that rubbing action was involved in electrification just by taking that glass rod near that cloth it will not do anything rub it then that electrification process happen so why do they acquire charge is something which we get to know from this particular thing all material is made of atoms molecules although most of the materials

8:39 are neutral they do contain charges just think about it even in an atom there is a positively charged nucleus there is a positively charged nucleus and the negative charges go around it the negative charges will go around it as many number of positive charges are there exactly that many number of negative charges are there so as such as such we know that the atoms are electrically neutral but they do contain charges they do contain some positive and the negative charges which are exactly balanced now what happens is now what happens is that when you rub things when you rub things you know you will see

9:24 that okay all these things are Basics there is nothing great over here they just mentioned that you know the atoms and these molecules hold these charges together Etc now this electrical forces play a very important role when you rub atoms together when you rub molecules together when you rub objects together that is explained in the next slide you will or maybe over here only what happens is these electrons which are orbiting they might get knocked out they might get knocked out and move on to the other at they might get knocked out and move on to the other atom so what happens imagine this atom loses few electrons it will become less negative which means more positive these electrons will jump onto something else

10:10 that will become more negative so that is how that electrification process happens because these electrons jump from one to the other when you rub them when you rub them and there are certain kind of atoms which love those electrons they will pull them certain atoms also love those electrons but maybe they are not able to hold them so properly so there is no good adition adition is when things stick when things attract things stick together that is addition so maybe that addition or the liking towards that electron is not that great so it is not able to control that electron so much but some other atom which has come close to it by rubbing action you will see it is able to capture that electron and take it away so it becomes more negative

10:57 this becomes more positive is that point clear how this electrification process happens everybody yep awesome awesome for more physics classes three you should be a part of the DNA batch I'm taking almost four classes per week two to hours it's crazy the amount of content that I am giving yeah yes it can't believe but it is true now once a body gets charged once a body basically gets charged with this you know um you can detect whether a body is charged or more charged or less charged or not charged at all using this device

11:44 called as electroscope so what is an electroscope electroscope is a simple operatus is a simple operatus to detect a charge on the body which uses the gold leaf as an as an setup what it consists is there is a metal rod in a box you can see there is a metal that orange thing is a metal rod in a box with two Thin leaves attached at the bottom end imagine this is a rod and there are two leaves like the plant leaves are there imagine two leaves in the bottom okay and all of this setup is inside a box it is filled with insulation and there is nothing which can uh conduct or out or inside that Rod now what happens is

12:29 there is a small point of contact on the end of the rod at the top which is also called as the metal knob through which the charges can enter or exit because everything else is insulating nothing else can enter or exit through that Rod or from anywhere except that top point so what happens is if I have a Charged body imagine my head dis charged so I bring my head close to that rod and I touch that Rod's top knob what will happen the charges will conduct through the rod and they will go till the very end out of the leaves those two leaves will become charged the moment they become charged they will repel they will repel the moment these leaves go like this I'll be like yes there is a charge on my head then I call Champa hey Champa

13:16 come here let's see whether there is a charge on your head so Champa takes her head and puts her head on this knob and touches it like this Champa puts her head on the knob the charges through Champa flow through that rod and enter the leaves till the very end and the leaves go even more then cha is more charged if the Divergence is more there is more charge on it so the degree of the Divergence is an indication of the amount of charge the degree of Divergence is an indication of how much charge is there then ladu comes he ladu come bring your head ladu touches that particular metal knob over here the charges if at all flow and I see that the leaves are not going only then I'll say ladu listen there is no charge on

14:03 you the leaves did not move at all so no charge no Divergence more charge more Divergence got it understood all right perfect perfect guys I need more attendance harboran that's all yeah all right now now conductors and insulators now that we understood what a body will be charged not charged how to detect that charge all right so we have till now focused on how the charges get transferred from one body to other body uh how different pairs were identified even thousands of years back and then we also saw atoms are neutral there is equal amount of positiveness

14:48 negativeness and electroscope is a device used to measure how much charge is there more or less or no charge at all now there are basically two major objects classified classified as conductors as and inductor sorry conductors and insulators depending on their electrical Behavior conductors are the ones conduct electricity insulate that means you separate out that means they do not conduct electricity so some substance readily allow passage of electricity through them some substances readily allow the passage of electricity to them others do not others do not others do not those

15:37 which allow electricity to pass through them easily are basically called as conductors they are called as conductors and which are conductors basically most likely Metals humans and even animal bodies and even Earth are very good conductors they are very good conductors Earth human body most of the animals basically if you touch a lose wire you will get shock you will die also even an animal by mistake if it happens to touch these wires they will die because they can conduct electricity most of the metals aluminium gold silver copper all these also can conduct electricity whereas you will see the most of the

16:24 non-metals most of the not all like glass porceline plast itic nylon wood they offer very high resistance to the passage of the electricity to them they are called as insulators they are called insulators because they offer High Resistance most substances fall into one of these class classifications anything that you take around you either it will be an insulator or it will be a conductor later on you will see there are certain devices which are like conductors like insulators somewhere in between they are called as semiconductors there is a separate chapter for that that is Ward the end that is towards the end okay don't worry about it right now so basically the reason why certain objects are like conductors is only because they have

17:10 free electrons in them they have free electrons in them you can see they have electrical charges electrons comparatively to to the non conductors or insulators so what happens these free electrons can easily flow they are ready to move they're ready to conduct that is is why they behave like a conductor that is why they behave like a conductor now we have laid the basic Foundation now let's talk about charges all right so whatever charges whatever things which create that crackling sound thunderstorms which attract bits and pieces of paper when you rub glass rod on wool that they can attract some other objects there is some

17:58 kind of attra R active and repulsive forces between them inside an atom also there is some positive and negative charges these charges obey certain basic rules some basic guidelines have some properties what are those properties let's have a look first of all you should knowe that there are two kinds of charges whenever you talk about charges either it can be positive or it can be basically negative there are certain objects which do not have only any charge and that is basically neutral objects neutral objects example Neutron Neutron does not have positive or negative it has no charge at all the example of positive charge is basically a proton proton has

18:44 a positive charge the example of a negative charge nothing but an electron which has a negative charge so what are the basic properties there are three major properties addition conservation and quantization we'll talk about it but remember most of the we treat our charges like point charges because the size of a proton the size of an electron the size of a neutron for our day-to-day activities is so small you can treat it like a point so most of the times we treat them as point charges the size of the charges are very very small as compared to the distances between them hence we treat them like point charges usually you will see we'll show the charges like a point

19:30 although the size of a charge might be there but for our day-to-day activities it is very very small so what are the basic properties of charge guys first one addition of charges meaning this is completely based on this is completely based on the property that charges are basically scalar quantities like mass time right volume all these are scalar quantities so you can just add the charges if there are two or more charges I can say that the total net charge is just q1 + Q2 plus Q3 like that till q1 I can just add them that is the meaning of addition of charge now it might happen

20:17 one of the charge is plus two another charge is -3 another charge is +4 another charge is - 7 you have to do algebraic sum so 2 - 3 -1 - 1 + 4 + 3 + 3 - 7 is -4 so you know the total charge will be negative so that way you have to consider the signs also while adding just don't take the magnitudes which is common sense the total charge is basically conserved this also kind of makes sense the total charge is conserved because no matter what happens you cannot destroy that charge or even if it omit a charge just imagine uh say for example there is a neutral body no charges there you

21:02 destroyed somehow you will see an equal amount of positive and equal amount of negative will come so earlier charge was zero so you'll get plus two positive plus two negative still the net charge will be zero same way no matter what you do let's say you rub body and you see one body becomes positively charged like sir this become positively charged sir so how is charge conserned remember the other body has got negatively charged the other body has got negatively charged so the total charge in this universe will remain conserved so the total charge initially the sum is equal to the total charge finally in this universe or basically in an isolated system in an isolated system

21:50 so if we take an isolated system you'll see whatever was there before will be there afterwards best examples like this imagine one is positive two another is -5 this is there before let's say this object becomes let's say this object becomes minus one charge so how much charge will be there on the other one just calculate the total just calculate the total 2 - 5 is basically minus three so the total should still be minus three which means the charge on this object will be minus 2 because -1 -2 will become minus 3 so that net charge remains conserved for an isolated system yep is that clear very very important isolated means like imagine you're

22:37 inside a classroom and teacher locks the door isolated nobody will go out nobody will come in that's it that's an isolated system so no charge can go out no charge can come in last one quantization of charge we'll talk about it I think in the slide after this yes quantization of charge which says that every charge is a multiple integral multiple of some basic charge and that basic charge is the smallest charge possible you cannot go beyond it just imagine for example I tell you hey in this world St sir says sh sir says you can only have 200 rupe notes you can only have 200 rup

23:23 notes so a person can have 400 rupees because he can have two 200 rupe notes can have 1,000 rupees can have basically 5 200 rupe notes but can a person have 150 rupees no 250 no 377 no 1,688 no 1,600 yes basically all multiples of 200 are allowed that's the smallest denomination allowed this is the quanta this is a Quant of money and you can have only 200 rupe notes in this new world so same way way it is found that in our dat in our universe the smallest charge which God gave us is 1.6 into 10^

24:10 -9 kums Kum is the unit of charge which is also the charge of an electron or a proton you can't get a charge lower than that any charge around you will be a multiple of this will be a multiple of this that's what it means so you will see any charge is n * of e th the charge Q on a body is a multiple of e that e is uh the basic unit of charge and N is basically any integer which could be positive or negative this is also this basic unit of charge like I said is also a charge on an electron or a proton by convention

24:57 the charge on an electron is taken to be negative therefore the charge on electron is written as negative e and that on a proton is positive which kind of makes sense okay so the charge on a electron is negative e whereas the charge on a proton is positive e because proton is positive this is just convention you can ask sir why electron is taken negative sir I feel bad for electron why not electron is positive fine you take electron as positive then proton will be negative then the whole history will change then then each person will come up with their own reason and Convention so we need as a standard so take an electron as negative proton as positive let's not debate about that this principle because electric

25:44 charge is always multiple of a basic charge this is called as quantization of charge when it is an integral multiple that is called as quantization of charge now usually what happens is usually what happens is the number of charges which you are dealing with let's say there is a lump of stone it is charged or my hand got some static or there is a car which is moving the metallic body got some charge on it there is a thunder lightning happening there is some charge in it okay or some electrical circuit is there some charges are there in it so in usual day-to-day activities the charge that you deal with although it is not very large it is small only because if it is too large then it will give you a

26:29 big shock and even if you know you are taking a decent amount of charge there are so many electrons or protons in it there are so many electrons and protons in it that you virtually don't see much of difference in this quantization principle just imagine if I talk Amani's wealth Amani's wealth okay has he has sir uh something you know some crazy amounts are two uh this something 5 six uh 7 6 8 0 0 something like that you will not think whether it is an integral multiple of 200 rupees or

27:14 not you'll be like okay fine even if there is something wrong let's say by mistake I wrote you know 827 I know this will not be an integral multiple of my basic 200 rupe node but I'll will be like okay fine even if it is supposed to be 800 that 27 rupees will not make so much of a difference to his wealth getting my point so when we deal with too many electrons okay it's okay to not worry about this principle so much because a small difference here and there in writing the value in the last decimal will not change our result or calculation so much is that okay okay

27:59 everyone with me so uh that is something which you should know and here they have just talked about who discovered it and who demonstrated it so maybe in math the following you might get it sometimes so just for that so uh the quantization of charge was first suggested experiment uh you know uh was suggested by the experimental laws of electrolysis discovered by farad Michael farad suggested this but it was experimentally demonstrated by milikan in 1912 so it was suggested by Faraday but it was proven by milikan milikan oil

28:47 drop experiment cool everyone with me yes e is negative and for proton it is plus e yes so we are going to take the magnitude most of the times so what's the value of e that is also given right over here like you can see e is nothing but 1.6 into 10^ minus 19 Kum so if you're wondering how Kum is defined it is defined as the amount of charge which flows when you take one ampere of current for one second that definition has been mentioned here so the SI unit the SI unit of a charge is basically called Kum and is denoted by the symbol C in terms of the current it is is defined so 1 Kum so 1 Kum is the charge

29:32 flowing through a wire in 1 second for a current of 1 amp current is a fundamental quantity charge is a derived quantity please remember charge is basically a derived physical quantity derived physical quantity keep this in mind okay everyone till this point now if you notice the amount of charge of a proton or an elect Elon it is so small 10 the^ minus 19 means so many decimal places you have to go to get one nonzero number so it is very very small so if you take one Kum charge imagine the number of protons or electrons in it positive 1 Kum or negative 1 Kum there

30:18 will be around 10 to the^ 18 electrons now 10 to the^ 18 when you write it is okay but when you think or imagine it it's so huge remember 10 the power 10 the^ 6 is a million 10 the power 9 is a billion 10 the^ 12 is a trillion so there is 10 ^ 6 more to 10^ 12 so it's a million trillion electrons it's a million trillion electrons in just one Kum just take a moment to you know grasp this number to grasp this particular number it's so huge which means which means that you

31:05 know one Kum is a very huge amount of charge usually which the charges which we use in most of our day-to-day activities is in mum micrum nanum pum which are smaller units of basically the same thing so 10 to the^ 9 electrons this is ncrt exercise 1.1 come on every every Warrior should start putting up the answers in the uh chat box glad it is useful Nan joo's but I want everybody to attend these classes or else there will be no motive for us to conduct this classes continuously remember that so if 10 the^ 9 electrons move out of a body to another body every second how much time is required to get a total charge of one Kum on the other body that is the

31:51 question come on think about this think about this how we'll do this particular question as mentioned 10^ 9 electrons move out of a body every second so to get a total charge of 1 Kum now one Kum has how many charges well think about it one Kum has how many charges so each charge is 1.6 into 10^ -9 Kum into the number of charges will give me basically one Kum of charge so the number of charges in 1 Kum is 1 / 1.6 into 10 ^ -9 this is the number in basically 1

32:38 Kum now if 10 the^ 9 electrons move every second so to get one Kum how much how much you know time is required so think of it this way guys the number in 1 Kum is 1 / 1.6 into 10^ -9 okay whatever that number be if you divide this number with you know 10 the^ 9 whatever electrons or basically charges charges per unit time basically per one second if you divide this if you

33:24 divide this what will you get these many charges you need if you divide it with these many charges per second I will basically get the total time which is required to make these many charges flow to get one Kum of charge think of it this way if you're not getting it imagine total number of charges which I need is basically 1,000 and I know that per second per second example exle 10 charges come per second per second so how much time will I take to bring in those th charges if I divide th with 10 I will get basically 100 so 100 seconds

34:11 will be needed for me because every second 10 charges come so every second if 10 charges are coming if I wait for 100 seconds 100 into 10 will give me 1,000 same way this is a number if I divide it with the number of charges per second whatever I get will be basically the time that's what they would have done they would have divided this with this so I will get basically the time and it comes out to be 198 years guys that's a long long time so just look at it even though 10 the^ 9 electrons seems like a big number but to get one Kum of charge it is taking me 198 years that means I need few generations for them to conduct this particular experiment so this just tells you how big one colum of charges how big one Kum of charges it's

34:58 not so easy it's a very big number so usually we deal with micro mums or nanocs is that clear everyone till this point all right moving on to the next one how much positive and negative charges is there in a cup of water now this example which is there in ncrt they have assumed that the cup of water is 250 g obviously you can say why 250 I will take it as 220 so there is no uh you know Clarity on that but just assume a small cup of water is 250 gam then or 250 ml basically so we know that the molecular weight of water is 18 G per mole so 250 g will have how much moles

35:47 250 g will have how much moles basically you divide 250 divided with 18 you will get the number of moles of water number of moles of basically water if you multiply by the by the avagadro number which is 6.02 into 10 to the^ 23 what you will get is the number of molecules what you will get is the number of molecules in it is that right yes you'll get the number of molecules in it what do you do with that well each each molecule of the water H2O will have 10 electrons think about it by oxygen oxygen guys atomic number eight

36:33 right and hydrogen atomic number one so H2O so there are two electrons in those two hydrogens and eight electrons in that oxygen so I would just say that two electrons from here and two protons from here and from oxygen you will get eight electrons here and eight protons over here so if you see totally there are 10 positive charges and over here there are basically 10 negative charges so each molecule gives you 10 positive and 10 negative charges so these many molecules will give you into 10 you multiplied by 10 amount of positive or negative

37:20 charges that many number of positive or basically negative charges so you just have to multiply this whole number by 10 is this point clear very good last step how much charge is there now guys every charge is 1.6 10^ -9 Kum 1.6 10^ - 19 Kum so whatever number of charges you got 8.3 into 10 ^ 25 if you multiply this with 1.6 10^ minus 19 what you will get is the charge of of positive or the charge of negative magnitude wise as a whole in that entire 250 ml of water if

38:06 you notice this charge is very very huge look at this 10 to the power positive 7 so many zeros it's like 13 million Kum of charge if you calculate it it is actually 13.38 million Kum of charge in just a cup of water but the fun fact is there is equal amount of positive and negativeness so it is electrically neutral they cancel out so you don't feel anything but even a small cup of water has this much amount of charge that is what this example signifies what these two examples tell you although even small amounts have very huge

38:52 amounts of charge but in equal amounts it's very difficult to separate them out and to just get you know an unbalance of more positive or less positive so it this example shows it takes a lot of effort lot of time lot of energy to you know create that unbalance to get a huge kolum of chge that is the significance of these two examples did you understand the meaning of these two examples hello Tani Tamil did you understand the significance it's not just for calculation see nobody tells you while Sol this examples what is the significance of it the significance is exactly that even though there is huge amount of positive and negative kums to create even a Kum of unbalance takes a

39:40 lot of time and effort so that is a meaning of it all right so that was about charges shall we go ahead my dear Warriors and let's talk about the next concept which is basically the kum's law yep kums law okay so now we know that there are some main properties of charges and we saw that right over here the charges can be added or subtracted right where did it go yeah charges can be added or subtracted total amount of charges conserved and yes the total charge is always basically Co quantied it is

40:25 always a multiple of the basic charge charge which is this particular value now let's talk about the forces between the charges this is basically given by our kums law so kum's law is a quantitative statement quantitative means where you can calculate it gives you the quantity the amount of whatever things you are calculating qualitative means I just tell you listen two positive charges will repel opposite charges positive and negative will attract it's a qualitative it's just telling the quality of that object or the quality of that particular thing it's not telling me the exact amount I'm not telling you how much will I repel how much will I attract schum law gives me a qu quantitative not qualitative uh

41:14 uh statement about the force between two point charges see very very important point charges you cannot find the force between a cube and a solid sphere using kum's law and unless you divide the cube and the sphere into small small small small points and find the force between every single point pair and then add them up to get the total Force as a whole you cannot apply unless they are point charges everybody agrees with this point charges yep very good so when the linear size of the body is much smaller than the distance then the size may be ignored and the charge bodies are treated like point charges so just imagine imagine if there is a cube and a

42:01 sphere I will not use I cannot use kums law I cannot use kum's law 100% because they are very very close right now they are very very close right now but imagine this Cube goes very very far like 5 six kilometers away the sphere is 5 six kilometers away from this Cube so because right now they are very very far away very very far away now I am allowed to use kum's law I can treat this like a point and treat this like a point and then I can uh you know apply kum's law so this is like point charges where the size may be ignored and the charge bodies are treated like point charges when the distance is very very large

42:47 when the linear size is small as compared to the distance size of the sphere size of the cube is small as compared to the distance Okay so so this says this particular law says that you know the force which is there the force between two charges varies inversely as the square of the distance between the charges and is directly proportional to the product of the magnitude of the two charges and it is acted along the line joining the two charges what it means is imagine there is a charge here and there is a charge here this this I call it q1 this I call it Q2 this charge and this charge will have some force of attraction or

43:33 repulsion depending on their signs that force is inversely proportional to the square of the distance between them this is the distance between them and it is also proportional to the product of the charges so proportional to q1 Q2 and that Force which I'm showing this particular Force always acts along the line joining the two charges meaning the force won't be like this the force won't be like this it will be always along the line joining it will be either like this or like this depending on attractive or repulsive it will always be on that line joining the two charges it can't be in some random direction is that clear so when you combine both of them when you combine both of them it is directly

44:20 proportional to q and Q2 inversely proportional to r² you will see that the force is some constant into q1 Q2 ided r² this particular constant that you are seeing we'll be talking about it so this is our constant of [Music] proportionality proportionality which is basically called as our kums constant which is called as our kums constant because this law is given by kulum this law is given by Kum okay so that is the formula that is something which you are obviously supposed to remember so let me just block this off let me just block it

45:07 off yes sum constant into q1 Q2 by r² now what is the value of that particular kums constant we will uh see that the value of the kums constant K is 9 into 10 ^ 9 in SI unit this is another value which you should remember 9 into 10^ 9 very very important uh while the original exp okay find kums law will magnitude impact the other way around kums lock and now be unlo to furnish okay don't have to worry about these things these are not important lines uh they are just telling the history that this law can be used to define the unit of charge not the other way around not important so much okay ignore it s unit of the value of Kum okay unit of charge that results from

45:54 this so this particular paragraph tells you how you can define a Kum Etc when imagine you take two charges one Kum each or let's say unknown charges and they are separated by exactly 1 M and I see the force of attraction or repulsion between them is Imagine 9 into 10^ 9 9 into 10^ 9 Newton 9 into 10^ 9 Newton then each of these charge must be 1um then each of this charge must be one Kum because it kind of makes sense look over here this is one this is one this is also one 1 into 1 divid 1 square is 1 so K will become equal to F so the force is

46:40 9 10 ^ 9 K is also 9 10^ 9 this can only happen when q1 Q2 is 1 one each assuming R is also one so that is how you can define a using this particular law is that right okay I hope this is absolutely clear everyone till this point yep okay so here the main takeaway is the choice of K determines the size of the unit of the charge determines the size of the unit of the charge because if you change the value of K then q1 and Q2 will be defined differently because Newton is already defined what is 1

47:27 Newton when you take 1 kg Mass accelerate by 1 m/ second Square the force acting is 1 Newton distance already defined this much length is 1 M so if somebody chooses a different set of units K will be different Q will be also different in that new unit of system some pandu system of unit that is the main takeaway okay but it's okay we are mainly focusing only on SI unit otherwise that statement will make sense okay now this particular thing I have already explained over here that is already done so what is one of charge alternative definition when the distance is 1 M and the force which is experienced is of 9 into 10^ 9

48:15 Newton now this particular Kum constant which you saw over here this Kum constant that is also written down as that is also written down as 1X 4 Pi Epsilon the Kum constant K is also alternatively written down as 1 by 4 Pi Epsilon so you can remodify the same law which you saw over here and write it down as instead of K Force is 1 by 4 Pi Epsilon q1 Q2 by r² what is this modification for you will understand it later on when you study dialectric material but when you write it like this this Epsilon not which you see over here

49:01 that Epsilon not which you see over here this Epsilon not is basically called as the permitivity of free space or permitivity of vacuum and the value is basically 8.85 into 10 ^ minus 12 this permitivity of free space is basically medium dependent medium decides is that it tells me how much how much how much electric electric force how much electric force is permitted is permitted in that particular medium how much electric force is permitted in that

49:48 Medium right now most of the times we use air or vacuum just like you might have studied refractive index of air is roughly one same like vacuum same way you know if you conduct the same experiment of two charges in some water you get very irritated you drown those charges in water the force will change because the medium has changed so this value will change epile or not will change it will no longer be this it will be something else it will be permitivity of the medium not of free space because free space means vacuum or basically almost like air almost like air is that clear okay so we have dsha maam over here in the physics class

50:34 welcome DHA ma'am yes I hope you guys have been attending all the classes of all the ncrt lines by all the teachers guys already most of us have done like or almost done two chapters in ncrt lines very good nice to see all the Jo good evening good good evening great so till this point I hope it is clear what is the meaning of permittivity how much field or how much force is permitted in that medium electric force is permitted in that Medium that is what this decides if the medium where the charges are placed is different this

51:19 value will definitely change now there is this Vector form which is pretty useless I feel uh but still we'll just write about it or we'll just talk about it how do you write the kums law in Vector form I'll teach you a simple way uh I mean the final formula is here so I'll just tell you a simple way of noting this down see imagine there is a charge one here imagine there is another charge over here let's say there is some distance between them and charge one on charge two applies a force so this is the force on to due to one same way charge number two on one will apply some Force so I will write it as force on one divided uh one because

52:06 of two this Force either this or that the magnitude the magnitude of the force we have already seen it is K * it is basically K * q1 Q2 divided by the distance Square we have seen that but when you write it in Vector form remember any vector quantity any vector quantity is nothing but the magnitude and then you have the unit Vector always a force of 5 Newton I a force of 7 Newton jcap if it is in some other direction that whatever magnitude

52:54 78 you know n hat n hat is the directional Vector of that particular vector quantity it is having a magnitude of one it is just used to signify the direction so same way think about it if I tell you I want to find the force on two because of one how about how about showing a vector called r21 which is nothing but the position position of two with respect to 1 remember in 11th standard vectors 1D 2D motion you have studied this position of two with respect to 1 is R2 minus R1 velocity of two with respect to 1 vs2 minus V1

53:42 acceleration of two with respect to one A2 minus A1 Cha's velocity with respect to pandu Cha's velocity minus pandu's velocity this is the pure definition which we have seen even in relative motion vectors everywhere else so if I Define a small unit Vector over here something like this which is R2 1 Hat R2 1 Hat it is just an unit Vector it's the same as the direction of this force is the same as the direction of this Force hence can I not say F21 bar this is the vector is magnitude magnitude is K q1 Q2 divided by distance distance is this which is r21 which is

54:30 basically r21 square unit Vector what is that unit Vector it is this one so it is r21 hat this is the vector form of kum's law this is basically the vector form of kums law alternatively if somebody asks me what is F12 also F12 will be the same thing it will be K q1 Q2 this is distance now whether you write r21 or R12 it's one and the same thing r21 R12 square and here it will be nothing but R12 hat R12 hat just like r21 and R12 the order will

55:15 change the order will change think about it this thing over here this thing over here technically is r21 hat everything else is the same the magnitude is the same it has the same magnitude it has the same magnitude but the direction will be opposite of 21 which means F12 bar will be technically negative of F21 bar these two forces are magnitude wise same but Direction wise they are opposite of each other it has to be because Newton's third law if a body applies a force on another body another

56:01 body also applies the force on the first body these forces are opposite equal but act on different objects if one is called action the other is called reaction so this is Newton's third law make sense make sense everyone with me on this perfect so this is exactly what has been explained over here also if you notice F21 is 1x 4 Pi Epson q1 Q2 by r² R2 1 bar so same formula has been written down right over here has been written down right over here and if you're wondering how do I find r21 Cap basically any unit Vector can be found using this definition any Vector cap is

56:47 the vector itself is the vector itself divided by its magnitude divided by its magnitude a hat is a bar by a magnitude V hat is V bar by V magnitude that's how you find any unit Vector definition that is what has been mentioned right over here r21 cap is r21 Vector divided by the magnitude so this is basically the vector this is basically the magnitude this is basically the magnitude give this in mind okay rest everything else whatever I had written before also r21 R2 - R1 same thing is written over here as well same thing is written over here as well clear is this

57:33 Vector form absolutely clear yep awesome and because F12 is negative F21 what does this tell you F21 is basically equal to F21 this just tells that it is consistent with Newton's third law action if one is basically action the other is basically reaction equal and opposite I have also mentioned that now we are not worrying about the medium so much we are just considering them in vacuum if they are placed in some another medium then we'll be studying that later on uh remember I had just mentioned about permitivity that

58:19 permitivity value will change where is that permitivity yes this permitivity value will change depending on on the medium being present right now we are not worried about it so this is not an important line let's talk about this question let's talk about this question yes s we'll be doing Laws of Motion after next week we'll be doing kinematics in 2D and then we'll be starting off with Laws of Motion in our RNA series okay so kum's law between two charges and Newton's law of gravitation both are having similar looking format because for electrostatics it is K q1 Q2 ided R square for gravitation it is G M1 M2 / r² R square is there below product is there on the top constant is there

59:06 outside so since their laws are very similar question is to compare their forces for uh for I think electron and a proton um estimate the acceleration of electron and proton due to the Force Bend they are one amstrong apart okay masses of proton electron are given okay compare the strength of the forces by determining the ratio of their magnitudes okay so basically over here the first first part is for two protons and uh second part is for one electron and one proton okay see if you divide force of electrostatics divided by force of gravitation it will look something like this K charge on the electron or charge on the proton is the

59:52 same thing which is e only I'm just interested in the magnitude divided by r² similarly gravitational force will be G mass of the proton or mass of the electron so 1 M1 M2 divided R sare so R square R square will cancel so it will be k e Square by G M1 M2 when it is basically scenario number one where is it scenario number one for an electron and a proton this masses will be these ones one will be 1.67 into 10^ minus 27 and this mass will be 9.11 into 10^ - 31 these

1:00:39 electrons or proton charges will be the same thing 1.6 into 10 ^ -9 and this will be whole Square in the first case in the second case for two protons it will still be the same it will still be the same this will be the same thing for whatever I have written down over here this will be different because there are two protons first mass will be this one the second mass will be the second mass will be exactly this one 1.67 into 10 ^ minus 27 so we'll get two different answers so basically you will get two different answers the ratio of the electric force by the gravitational force in the first case versus the ratio of electric force upon the gravitational

1:01:27 force in the second case these ratios after calculations comes out to be I think this one 2.3 10^ 39 2.3 2.3 into 10^ 39 and for the other one it comes out as U 1.3 10 ^ 36 13 into 10^ 36 what does this example tell you see it's not about calculation it is telling you something it is telling me electric force is so so stronger whether you take electron electron electron proton proton proton as compared to the gravitational force it is much much stronger much much

1:02:15 stronger as compared to you know gravitational forces this is the significance of this particular example that is why when you draw the free body diagram in most of the electrostatic problems you never show the gravitational force or you are not bothered about gravitational force between electron electron electron proton isn't it amazing yep that is the significance of this there is a second part also find the acceleration of electron or proton so if you know the force divided by mass you will get the acceleration I think that is what is done later on I know the force the force interestingly look at it the force the force okay between electron

1:03:02 proton or proton proton comes out to be very very small very very small if you substitute the values K q1 Q2 by R square charges are 1.6 10 ^ minus 90 and the distance they have taken it as 10 the^ minus 10 m which is 1 amstrong it is very very small so then you'll be like sir find you said electric force is very strong as compared to gravitational force but the electric force only sir look at this it is so so small then should we even bother about electric force do you get my point electric force is strong as compared to gravity so fine we neglected gravity but electric force itself is so so small so why should we

1:03:48 even worry about electric force that reason comes in the next step if you divide the force by Mass you will get acceleration mass is so small that the acceleration comes out to be very very large so even though the force is small the mass being so so small the acceleration produce becomes so huge that you need to consider the electric force got it understood the significance of this particular example my dear Warriors my dear lions shall we go ahead yep all right there is another example which is there let's have a look

1:04:34 at that yes yes Krishna acceleration is very very large now talking about the next example there is a charge metallic sphere which is suspended by a nylon thread another charged metallic sphere held by an insulating handle brought close so there there is a sphere here there is a sphere here you are holding it using an insulator so that the charge does not go out or go in and you're are placing it 10 cm away from each other so there will be some electric force between them so there is some deflection so the repulsion is noted okay uh by you know noting the deflection in the shadow so they are using light so because light falls on

1:05:20 that sphere there is a shadow the shadow will move some amount so noting how much the shadow moved they are able to find how much the deflection is and thereby estimating the force is so much now what you do is these spheres are touched by uncharged spheres these both are touched to two identical uncharge sphere so what will happen immediately imagine yeah imagine if this had a charge Q this will become Q by2 This will be become Q by2 if this had a charge of capital Q this will become now capital Q by2 capital Q by2 the charges will get divided now and now you take

1:06:07 away those C and D separately away from each other now you bring back those A and B spheres because the charges have reduced the force will reduce so you reduce the distance and make it 5 cm now what will be the new Force between them you have to compare you have to basically compare the old Force versus new Force If You observe carefully originally whatever the force would be there between them F not okay F not that F will be k q q / r² later on what happens when you touch uncharged sphere

1:06:54 the charges become High and you bring them back over here the new force will be let's say F1 the new force will be let's say F1 now remember the charges are also half the charges are also half so what is that new Force k q by 2 Q by2 divided by distance instead of R it is now R by2 so I'll put R by 2 whole Square interesting Thing 2 two two two cancels you'll still get it as K QQ ided r² which means our new Force just happens to be equal to the old force it just happens to exactly equal the old Force so the force hasn't changed so the electrostatic force

1:07:41 remains unaltered it is the same it is just coming out to be as F that's what this example is all about I hope before this if you never understood what is happening in the example now you have understood so there are two spheres they are individually touched to some uncharged bodies so the charges will get distributed equally because they are similar now those spheres will repel less so you bring them closer so it so happens that even if the charge reduce the distance compensated for it and they ended up attracting by the same amount that is the significance of this particular example cool let's move to the next question

1:08:29 now we studied how to find out the force between two point charges what if there are more than two charges 3 4 5 six that's when you use this particular principle okay when there are more charges like q1 Q2 Q3 Q4 Q5 Etc and there are many charges which will keep on repelling or attracting each other to find the net force what you do is basically you just add them vectorially that's what it says this is exactly the principle of superposition so what they have done in order to find the force on one due to two as well as due to three you first find the force by

1:09:15 two on one which will be here you find the force by 3 on one which will be here these are two vectors using parallelogram triangle polygon you find the resultant Vector that is what they have done so they found Force One Two Force 13 and then add them vectorially to get the net force they have written it down in Vector form but when you actually do it it is much more simpler so this is exactly the principle of superposition so the principle of superposition says that in a system of charges in a system of charges the force on one due to Q2 is the same as given by kum's law and it is unaffected by the presence of other charges the total

1:10:00 force on any charge is due to all the charges and is just given by fub1 plus F2 plus F3 like that that's it so you just do vector addition vector addition you should not think that sir this charge came sir so will this charge affect this charges force on this no this is cha this is pandu this is ladu pandu loves Champa ladu loves Champa just because pandu loves Champa ladu would't say I will love you less they are not affected by it pandu loves Champa irrespec of whether ladu loves Champa or not so they are unaffected by the presence of other

1:10:47 charges everyone together will love Champa so Champa will experience net force Champa will experience the love from every so that is what has been mentioned here so in a system of charges the force on one due to two is the same whether other charges are there or not the love of Champa the love towards Champa will be there whether other person is loving or not I don't care and in fact Champa will experience the love from everyone the force by all the charges together will be applied using vector addition as simple as that okay all right cool shall we do some questions okay so here are the questions consider three charges q1 Q2 Q3 uh okay uh each

1:11:33 equal to Q on the white axis of an equilateral triangle what is the force on charge Q which is placed at the centroid of the triangle now in this particular question uh if you look at the solution you will break your head what the hell have they done really I mean this is so stupid what they have done the solution it was unnecessary isn't it like obvious if this charge this charge this charge are equal this charge will give a force here this charge will give a force down it will be basically they will cancel the net force will be zero because this will be 120 this will be 120 this will be 120 this is F this is F this is f net force is

1:12:20 zero so the net force on this will be zero that is what they have actually found out it is zero by doing all this nonsense they have written it down in Vector form so guys just because you learn something does not mean you have to apply it this is a very classic example of that chuma say they wanted to show off that okay listen I taught you Vector form for a reason remember this Vector form where did it go somewhere here only some crazy looking Vector form this is useless I mean most of the times you don't need it most of the times you don't need it you can just do it by parallelogram law in fact they have said that also parallelogram or triangle law using triangle parallelogram law addition of vectors and kums law you can solve most of the problems you will be

1:13:05 able to all electrostatics is basically end of the exactly that so you know using Vector form is a bit of a showof so this is basically a showoff remember that cool otherwise you don't need it even for the next exam answer is zero for for this even for the next example I won't use Vector form I won't use Vector form why because what is there instead of QQ and Q There is q q and minus Q okay no issues think about it for this particular charge what will you do versus this particular

1:13:51 charge this is a negative this is positive this is negative this is positive negative positive negative positive will attract so 100% there will be some attractive Force let's say I call it fub1 uh this will or let's say I will call it as X this also X Force okay these two together will produce a resultant these two together will produce a resultant over here on the charge at basically C on the charge at basically C and remember the angle between them is basically 60° this is an equilateral triangle so I just can say FC is nothing but root of root of what a s + b² + 2 into a into B into COS

1:14:42 of angle between the two vectors so XXX will come outside I have one remaining one remaining here and over here 2 cos 60 is basically half 2 into half is 1 so 1 + 1 + 1 is root3 where X is nothing but the force between this and this charge which will be k q q divided by this is just the magnitude divided by the distance whatever is the distance let's say it is L so l squ so it is magnitude wise k q ² by l s multiplied root3 you will get the force on the charge kept at the vertex C isn't it simple isn't it simple my dear Warriors similarly think if I want to find the

1:15:27 force on this point on this charge over here I will look at it like this this positive and this negative will attract definitely each other with an amount X positive and positive will repel so the repulsive force will be here amount will be still X magnitude will still be X only because charges magnitude are same distances are the same so X will be nothing but k q q divided l s which is k q ² by l s still but the resultant now will be over here the resultant and this angle will be 120 so fine so the resultant Force at Point a will be root of x² which is a square + B sare + 2

1:16:14 into a into B into COS of 120° so X is common Take It Outside 1 + 1 - 2 into uh 1 by 2 why minus because cos 120 is negative half 2 2 becomes 1 1 - 1 is 0 so it will become xun 1 + 1 - 1 which is 0 so X < tk1 is basically X so X is nothing but this whole thing so you can find the force on a very simply by using your original old formulas which you have learned in 11th standard for vectors if you notice what they have done over here you will break your head again they have tried to find the answers using vector addition and all of that that is not needed at all is that

1:17:01 okay everyone that is not needed cool awesome they have also mentioned one more thing if you add this Force this force and this Force which will give you the total force on all the three charges together that answer will come out to be zero it kind of makes sense it kind of makes sense because this is the net force on the system the net force on the system that will be zero because all these forces will be actually internal forces internal for the particular triangle these three charges attracting repelling each other is an internal Force internal forces will always cancel

1:17:47 so you will see that the net force will be zero okay uh they will not produce any resultant as kind of makes sense yeah for FB also it will be the same thing very good somebody is asking sir can you solve for FB just think this this positive and this positive will repel it will be here this positive and this negative will Attract it will be here so the resultant of both of them will be here isn't this diagram the same as this diagram just Mirror Image this is same as this and obviously the angle between them is 120 reason is just like this particular internal angle was 60 over here this internal angle is also 60 so this external angle is 120 so the

1:18:33 same logic applies and you'll still get the answer as you know uh X which is KQ s by L sare got it Kisha absolutely clear very good awesome let's move ahead let's move ahead let's talk about electric field now let's talk about electric field so we have talked about electric force till now we have talked about charges till now we have talked about forces formula in kums format and also superp position how and more than one charges are there how you find the net force by adding them like simple vectors nothing great

1:19:19 now let's talk about electric field let's talk about electric field just imagine just imagine pandu and Champa love each other pandu and Champa you know fall in love with each other pandu attracts Champa Champa attracts pandu they're very attractive hot people okay so when pandu saw Champa passing the other day on the road pandu fell in love for Champa pandu started moving towards Champa when

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