Electric dipoles & dipole moments

two equal and opposite charges separated by some distance is called an electric dipole die means two so think of it as like two opposite poles of electricity but my question has always been what's so special about these dipoles and for every dipole there is something called a dipole moment what exactly is that and why is it a vector quantity why should we care about all these things if you two have these questions it's time we got answers for them so let's begin our story begins with two charges plus five and minus three coulombs let's say they're stuck let's imagine they're not allowed to move somehow then they're going to create electric fields everywhere right and the fields are going to interact in all of that my question is what would the field look like if you went very far away from this group so let's say i'm zooming out zooming out zooming out zooming out zooming out in fact you know what i zoom out so much that the two pretty much look like a single dot to me and if you can't see it that's that's the idea it's so far we've gone so far away that's the theme over here okay the theme of this video is what would the electric field look like if we go very very far away so let me show you that these charges are still there you can imagine there are millimeter apart let's say and i'm asking you the field electric field kilometers away i want you to pause the video and think about it based on what you've studied so far what would the electric field lines look like if you went very far away from this group okay so i have a simulation and we go look at the simulation together so right now we're looking at the field when we are close to these charges plus five coulomb minus three coulomb look at what the field and how the field lines look like the way i make sense of this is let's imagine that each coulomb of charge you know throws out 10 field lines then since this is five coulombs it's throwing out 50 field lines now i don't know if there are 50 of them but let's imagine 50 feet lines are thrown out now this negative three coulombs of charge sucks in 30 field lines and that's what's happening out of 50 30 is being sucked in because it's negative charge and so the remaining 20 are able to sort of like escape from this suction and eventually go towards infinity and so when i go far away it's these 20 that we'll be interested in and i want you to look at it now look at these uh these 20 which are going to escape look at how those field lines eventually transform into you ready let's go all right here it goes we're going farther away look at them look at them they're becoming more and more straight can you see that look at what we see we see that those 20 field lines are almost perfectly straight they're perfectly radial it looks like those field lines are coming from a point charge over here interesting isn't it so let's go back to our drawing board and so what we saw is that when we are very close to these charges the field lines are very complicated but when we go far away the 20 field lines which eventually escape become radial and so from far away because we're getting 20 field lines it looks like it's coming from a single point charge of plus two coulombs and that looks nice to me because when i take plus 5 and minus 3 keep them very close to each other it looks like a single charge of plus 2 coulomb that makes sense to me okay let's play with this more what if i change the charges i make this plus 10 and minus 8 making sure the total charge still becomes plus 2. now what would the field lines look like when i go far away would it still look the same or do you think something changes can you pause and think okay we can use the same logic as before this time the plus 10 is going to send out a hundred field lines but out of them 80 are gonna get sucked in so only still 20 gets you know 20 escape and eventually they will become radial that means from far away looks like nothing has changed so what's incredible is that the individual charges don't matter the strength of the group depends on the total charge not the individual charges when i'm looking at it from far away let's play even more the climax is coming okay what now if i what if now i uh increase the distance between them it was one millimeter before now let's say i make it two millimeters i double it or i make it three millimeters i triple it what do you think would happen to the field far away when i go kilometers far away do you think it'll matter what do you think well remember these two dots are so close to each other that whether you bring them two millimeters or three millimeters or five or ten millimeters from my when i look at it from kilometers far away it's still going to look like a single dot to me right so that's not going to change anything when i look at far away so as long as the distances are very small the value of the distance really doesn't matter which is also incredible and finally hopefully you'll agree even the orientation won't matter right if i had kept that minus 8 on the left side or on the top or the bottom or somewhere over here do you think it would have mattered no because the field is radial right so the whole thing will turn but nothing changes so if we summarize we could now say what is the electric field far away depend on it only depends on in fact it's proportional to the total charge that's all that matters the individual charge the distance between them or their orientation none of them matters only the total charge and what's interesting is that there's nothing special about two charges i could have had three charges or four charges oh this means it's such a general result this is a general result if you give me any group of charges it doesn't matter how many are there if i want to look at the field far away it only depends on the total charge nothing else beautiful isn't it but there are special groups of charges that completely break this any guesses which ones yup the dipoles now let's say instead of this minus three coulombs of charge we had a minus five coulomb of charge what do you think would the electric field look like when i go far away from this can you use this can you use whatever we learned so far about field lines and try to visualize this yourself first so here's our simulation what's different is this time all the 50 lines that are thrown out all of them get sucked in there is no escaping them so all of them will loop back so what will happen if i go far away well let's look at it can you see all the field lines are looping back and so the field far away does not look radial the field lines are no longer straight we get something very different and of course very beautiful this is the dipole field this is why dipoles are special but my question is what does the strength of the dipole field depend on because the total charge is zero so what does it depend on well let's do the same exercise as before first let's change the individual charges instead of plus 5 and minus 5 what if i make them plus 10 and minus 10 can you visualize what the new field will look like would be the same the pattern would be the same but would the whole thing look the same or would it change well if i make it plus 10 and minus 10 this time 100 field lines are coming out and all the 100 field lines are looping back which means there'll be twice the number of feed lines compared to before so this means the electric field will double everywhere individual charges matter let's look at that here it is this is what happens when i double them the beauty is the total charge is still zero but the individual charges matter they are doubled and so the field everywhere doubles so let's write that down so this time i find that the electric field far away due to dipole it does not depend on the total charge because total charge will always be zero but it depends on the individual charge and let me just draw the field lines all the field lines get sucked back in and so this is what we saw okay second question what if i were to increase the distance from one millimeter i make it two millimeters or three millimeters just like before when i look at it from far away it looks like nothing has changed they will still look like a dot to me but do you think the field lines would change your mind is gonna get blown now let me show you the simulation here's the simulation and i've zoomed in so you can see the charges going farther and coming closer all right so let's see look at the field lines far away see what happens as i move them all right what do you see you find that when i move the charges farther away the field lines actually come closer over here which means the field gets stronger if you think about it that's mind-blowing if i make that distance from one millimeter to two millimeters when i go to go kilometers far away it still looks like a dot but the electric field over there has doubled ah that's just i don't know it's hard for me to comprehend why is that happening well that's because when the charges are closer if you bring them closer then the the the free lines get sucked very quickly and therefore the field lines go far away very quickly and so the field becomes weaker everywhere so the mind-blowing thing about dipoles is that the field far away depends on the teeny tiny distance between the charges in fact it turns out to be proportional to it but here's my question what if i were to double the charges and bring them closer and make the distance half now what would happen to the field far away well now you will see since this got doubled and this got half the effect cancels out and the field far away stays the same oh that means what does the field far away due to a dipole depend on it doesn't just depend on the charge it just doesn't depend upon the distance it depends on the product the product now decides the strength of the dipole and therefore we give a name to this product this product is called the dipole moment dipole moment the unit the sorry the symbol for that is p and the dipole moment is the product of the charge on the dipole and the distance between the two charges and why should we care about that because that represents how strong the dipole is and so tomorrow if you have a dipole and you want to know what's the effect of it somewhere far away you don't need the individual charge you don't need the distance you don't care about them all you ask for is the dipole moment all you need is the product the product that decides the strength of the dipole finally what if i change the orientation of my dipole for example i bring that minus 5 coulomb up would the field everywhere stay the same well now because the field is highly directional the whole field is going to change and so again what you find is that if you make a tiny change in this very tiny thing you change it field kilometers away everywhere is going to change this time orientation matters so if you just tell me the value of dipole moment i can't tell what the field looks like everywhere you have to tell me the orientation as well and that's why to take care of the orientation we make dipole moment a vector quantity so our dipole has a direction so how do we choose the direction of the dipole though well look at the field well over here below you can see the field is being blown downwards and from here the field is being sucked and so we like to say the type hole has a direction that looks like this and if you think about it this was the negative charge the blue is a negative charge this is the positive charge and therefore the direction of the dipole moment is from negative to positive from minus q to plus q that's the direction of the dipole moment so long story short why are dipole special well because when you are far away from any group of charges which are not dipoles the field that they create does not depend upon internal details because it looks like a dot the only thing matters is what's the total charge but when you're dealing with a dipole when you go far away even they look like a dot but the field they create depends on all the internal details and guess what dipoles are found everywhere in nature a lot of molecules like water for example are dipoles and so knowing their strength or dipole moments help us understand how they interact with other molecules and eventually help us understand how these elements or compounds behave