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Equipotential surfaces (& why they are perpendicular to field)

Equipotential surfaces have equal potentials everywhere on them. For stronger fields, equipotential surfaces are closer to each other! These equipotential surfaces are always perpendicular to the electric field direction, at every point. Created by Mahesh Shenoy.

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Video transcript

we've learned how to visualize electric field by drawing field lines in this video let's explore how to visualize electric potentials and the way to do that or at least one way of doing that is by drawing something called equipotential surfaces so what exactly are these well as the name suggests these are surfaces and these are three dimensional surfaces over which the potential at every point is equal equipotential surfaces let me give an example so if we come over here let's say from this charge i go about two centimeters far away over here there will be some potential at that point let's call that as 10 volt let's imagine that to be 10 volt now if i went 2 centimeters over here from the charge what would the potential there it should also be 10 volts what about 2 centimeters from here that should also be 10 volt in fact i could draw a circle of two centimeters and two set images an example okay and everywhere on that circle the potential would be equal 10 volt so that circle would be an equi-potential surface and since it's a three-dimensional you have to imagine this actually is not a circle but it's a sphere so let me just draw that nicely so i could draw a sphere let's see here it is a sphere and you have to imagine this is a three-dimensional sphere where every on every point of it the potential is 10 volt equal and so this would be my 10 volt equipotential surface can i draw more of course if i go a little farther away maybe two and a half or three centimeters far away i would can draw another sphere that will have another that would be another equipotential surface let me draw that if i go farther away the potential will decrease right so let's say this is another equipotential surface why is this equipotential because every on every point of it the potential is equal and is equal to 7 volt can i draw more yes more spheres every sphere you draw will be an equipotential surface in fact if i if i go a little farther away and i draw another one i might get a nine volt equipotential surface if i go a little farther away and i draw another one i might get an eight volt equipotential surface and so on and so forth now before we continue you may immediately notice that the surfaces are closer here and they're going farther and farther away why is that well it's got something to do with the strength of the electric field close to the charge the field is very strong and that's where the potentials are equipotential surfaces will be closer to each other as we go far away from the charge the field weakens and so the surfaces go further and farther away from each other but why why is it that if the field becomes weaker the equipotential surfaces go farther away can you pause and think a little bit about this all right here's how i like to think about it consider a tiny test charge kept over here on the 10 volt equipotential surface what will happen if i let go of it well electric field will push it and it'll accelerate and will move from this equipotential to another the nine volt equipotential now because the force over here is very strong because you are in a strong electric field region it will accelerate very quickly it will gain kinetic energy very quickly and as a result it will lose potential energy very quickly and it's for that reason in a very short distance it would have reached from 10 volt to 9 volt equipotential surface however what would happen if i were to keep that same test charge over here well now the field is very weak or weaker compared to here and so the force acting on it is very weak and so it will accelerate slowly and so it's going to take more distance for it to pick up the kinetic energy and so it's going to lose potential energies more slowly and as a result it's going to take a longer distance before it reaches uh it loses one volt now and so what do you think will happen for the six volt equipotential it will take even larger distance to reach eq six volts and so it'll be even farther away does that make sense it's kind of like if you take a ball and drop it on say jupiter where the gravitational field is very strong then it will accelerate very quickly and so it will gain kinetic energy very quickly so it will lose potential energy very quickly but on the other hand if you were to drop that same bowling ball on say moon well because the gravitational field is very weak it's going to accelerate very slowly gain kinetic energy very slowly and so therefore lose potential energy very slowly so the weaker field in weaker fields you lose potential very slowly and so the potential surfaces are further away all right let's take another example and i want you to take a shot at drawing equipotential surfaces let's say we have a long infinitely long sheet of charging big sheet of charge which has let's say negative charge then we know we've seen before it produces a uniform electric field can you think of what the equipotential surfaces here would look like can you draw try drawing a few exponential surfaces over here pause the video and think about this use the same approach as we did over here all right just like over here let me go at some distance say about two centimeters from this sheet it'll have some potential because it's a negative charge maybe there is some i don't know negative 10 volt potential now if i go two centimeters from here i should get exactly the same potential as here and the same would be the case over here as well oh that means i can draw connect all these lines and if i do that now my equipotential surface would look somewhat like this so this would be my minus 10 volt equipotential surface i can draw another if i go a little bit farther away maybe i will get another let's say minus 9 volt equipotential surface if i go farther away maybe i get another minus eight volt equipotential surface and so on and so forth over here i hope you agree that the equipotential surfaces will be equidistant because the field lines are all uh the electric field is uniform and again just to reiterate this is not a line this is a surface it's so you have to imagine this in three dimensions and i'll help you visualize that if you could see this in three dimensions so if you look at them in 3d you can now see that now the equipotential surfaces are plane surfaces so over here we've got spheres over here we're getting plane surfaces all right but here's a question these were simple cases but what if we have to draw equipotential surfaces in general what if i have some random electric field line due to like some complicated network of charges something like that i don't know just randomly drawing how would we draw equipotential surfaces then we may not be able to use the same approach like here but what we can try to do is see if there is some geometrical relationship between electric field lines and equipotential surfaces so let's come over here can we see any relationship between these field lines and the potential surfaces if you look very closely you can see that these equipotential surfaces are perpendicular to the field lines and that makes sense right because in general over here the field lines are forming the radius and the radii are always perpendicular to the spheres or circles so here we are seeing that the two are perpendicular to each other hmm let's look it over here hey here also we are seeing that the field lines are perpendicular to the equipotential surfaces interesting so can we say that this is true in general that equipotential surfaces and field lines must always be perpendicular to each other we can't just say that using two examples we could say that might be a coincidence so is this true in general well if you and i were in the same room maybe you would have an interesting dialogue over here but i don't want to take too much time and i'll go ahead and tell you that turns out that this is true in general so let me just write that down equipotential surfaces are always always perpendicular to electric field lines i can just say perpendicular to field or field lines always regardless of how complex the field lines are and again the final question for us in this video is why this is true and i want you to again pause and ponder upon this is a deep question but i'll give you one clue think in terms of contradiction what would happen if the equipotential surfaces were not perpendicular to the field lines what gets broken think a little bit about that like i said it's a deep question don't expect it to get right away and it's okay if you don't get it right away but the idea is just to think a little bit about it before we go forward all right let's see there are multiple ways to think about this uh the way i like to think about is again bring back my test charge so here's my test charge now imagine we move this charge along the equipotential surface say from here to here now because it's an equipotential at every single point the potential is the same that means the potential energy of this test charge will remain the same as you move it right let me write that down no change in potential energy no change in potential energy as you move along the equipotential by definition right okay what does that mean well if the potential energy is not changing it automatically means no work done by the electric field no work by the electric field now think about it for a second why should this be true because whenever electric field does work whether positive work or negative work where automatically potential energy would change for example let's get let's come let's bring back gravity because gravity helps in understanding this what happens when when you drop a ball gravitational field does positive work what happens to the potential energy it loses it what happens when you throw a ball up gravity does negative work what happens to the potential energy it gains it so notice whenever gravity does work this ball would either lose or gain potential energy same would be the case over here if electric field did work the charge would have gained or lost potential energy but we are seeing that it is not changing its potential energy means that as you go from here to here electric field must be doing zero work but how is that possible electric field is definitely pushing on the charges putting a force on the charge and the charge is moving so how can work done be zero oh work done can only be zero if the force and the direction of motion are perpendicular to each other so in short as you move a test charge along the equipotential surface its potential energy should not change that can only happen if the electric field does no work and that can only happen if and only if electric fields are perpendicular to the equipotential surfaces now if you find this a little hard to you know digest this right away it's completely fine it took me also a long time to do that so keep pondering keep thinking about it it'll eventually make sense so long story short this basically means if you have been given some random field lines and if you want to draw equipotential surfaces just start drawing perpendicular drawing them perpendicular to the field lines this is how you might do it and of course nobody's going to ask you to do that but you know or you you usually use computers to do that but that's the idea but equation surface must always be perpendicular to the field line all right let's summarize and i want you to summarize and the way to do that is i'm going to ask you three questions and see if you can explain it to a friend what what are equipotential surfaces that's question one second question why over here these surfaces are going farther and farther apart from each other but over here the surfaces are equidistant and third one why are equipotential surfaces always perpendicular to the field lines