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Intro to torque on a dipole in uniform electric field

When an electric dipole is placed in a uniform field, it experiences a torque. This torque tries to align the dipole in the direction of the field. Let's explore more. Created by Mahesh Shenoy.

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  • blobby green style avatar for user pallavi
    how do the electric field lines of the electric field and dipole interact with each other? does one distort because of the other? does it have no effect? doesn't the force created by electric field tear the dipole apart?
    (6 votes)
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    • starky sapling style avatar for user Dev
      So yeah! It's an amazing question!!

      How do the electric field lines of the electric field and dipole interact with each other ?
      → Well as the Electric field of the oven keeps on flipping, the electric field lines of the dipole tries to get aligned with the electric field lines of the electric field. (You will understand it with your next question)

      Does one distort because of the other ?
      → Well, first of all I wanna know what you mean by distorting of electric field? Do you mean that one electric field vanishes other? No, that doesn't happen.

      → Well, what actually happens is that the electric fields by the oven and the electric field of the dipole never intersect (cross each other) and that's why the dipole arrange itself with the electric field such that it experiences minimum torque, or you can say that dipole tries to attain such a position in which it attains maximum stability.

      Does it have no effect ?
      → No, it does have an effect, if consider any instance (of changing electric field due to oven) you will notice that the electric field due to oven and electric field due to dipole doesn't intersect each other but they change each other's path.

      Well, what I actually mean ?
      → To understand what I mean, visualize two positive charges kept close to each other and try to think, how does the electric fields of both charges interact with each other? Does the electric field line intersect each other? No, they don't intersect each other but they repel each other. In the same way, the electric fields due to oven and the dipole doesn't intersect each other but orients themselves in such a way that they attain the maximum stability.

      → To visualize it better, consider an electric field. (You can say that it originates from the positive charges and ends at the negative charges) And now, Let's introduce a dipole in the electric field. Now, what actually happens is that (the like charges repel each other, and opposite charges attract each other) and so the positive charge of the dipole turns towards the direction of electric field (towards negative charges which had created the electric field) and similarly the negative charge of the turns towards the origin of electric field (towards positive charges which had created the electric field) and this way, the dipole orients itself to attain the maximum stability.

      But it was all just about an instance, in just next instance the electric field changes and the process gets repeated, over and over again, until the food gets ready to serve :)

      Doesn't the force created by electric field tear the dipole apart ?
      → No, that doesn't happen because the electric field of the dipole also applies the same amount of force as the electric field of the oven. (By, Newton's Third Law)

      → But I guess you mean that, doesn't the strength of the electric field is such massive that the dipole can't withstand it? well, that's entirely different thing it's related to dielectric medium and break-down of dielectric medium or dielectric strength.
      (4 votes)
  • blobby green style avatar for user festavarian2
    The electric field is said to be "uniform". How can this be possible if it's a wave form flipping polarity a ~billion times/sec?
    (2 votes)
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  • blobby green style avatar for user Isteak Ahamed Imon
    Sir, what if I try to measure the amount of torque acting here at , if the distance between two charges is 2s then will the net torque be (s*qE)+(s*qE)=2sqE ? The thing I want to know is from where do I need to count the distance of charge or the force acting on both charges? Is it the middle point between positive and negative charge?
    (1 vote)
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  • blobby green style avatar for user Dan Frank
    Another awesome video
    (1 vote)
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Video transcript

my favorite way to heat up a leftover burger or any food for that matter is to use a microwave oven but have you wondered how does it work how does it heat up things without fire well it turns out that these ovens produce electric fields and burgers or any food items contain moisture which have water molecules and these water molecules are electric dipoles but wait how does that explain anything for that we need to investigate what happens when dipoles are kept inside an electric field and that's exactly what we're going to do in this video and towards the end of it we're going to figure out exactly how a microa1 works so let's begin so let's say we have a uniform electric field inside which we keep an electric dipole and just to quickly recap what's an electric dipole well two equal and opposite charges separated by some distance is a dipole and what is this p thing well remember dipoles themselves create this beautiful electric field and the strength of the field everywhere depends on the product of the charge and the distance between the charges and that product is what we call p the dipole moment the dipole moment represents the strength of the dipole and why is it from negative to positive charge that's it's a vector why is it this way because it kind of represents the direction in which it creates the field you can see the field that dipole creates is away from the positive and it's coming into the negative and so the dipole moment kind of represents that now we've talked a lot about this in our previous videos so if you need if you feel you need a refresher feel free to go back and check that out but we don't have to worry about the feel created by the dipole in this video so don't worry about that my question to you is what's going to happen to this dipole when i keep it inside an external electric field think about the forces that are going to be acting on this dipole and see how the dipole may be moving so can you pause the video and think about what would happen all right so i know positive charge experiences a force in the same direction as the electric field and so it's going to get pushed this way and negative charge experiences the force in the opposite direction of the electric field and so it's going to get pushed this way and since the two charges have the same strength and it's in the same electric field the two forces will be exactly equal and opposite and therefore i know the net force acting on the dipole or the total force acting on the dipole is zero so this means what happens to our dipole well our dipole is not going to accelerate so if it was at rest it's going to stay at rest it's not going to move it's going to stay over here but because the two forces are acting at two different points we can kind of feel it in our bones this is going to make it turn think about it if you imagine these were strings attached and you pull it what's going to happen it's going to make it turn you can feel that right it's going to make your dipole turn this way and this turning effect provided by the force is called a torque so although there is no net force acting on it there will be some total torque that acts on our dipole i know in a future video we'll calculate exactly what that torque depends on and we'll derive an expression for that but for now let's see the effect of that torque so the torque is going to make our dipole turn the two forces are going to make it turn and what i want to know now is will the dipole keep turning forever so what i want to know now is as the dipole turns what happens to this torque does it stay the same does it increase does it decrease will its direction change what would that depend on so for that we need a little bit about what does torque depend on well torque definitely depends upon the strength of the force if the forces are larger we'll have more torque but the torque also depends upon the distance between the two forces so torque also depends upon this distance if this distance is high if this system is far if the forces are farther apart torque is high it's for the same reason when you look at a bicycle the bicycle handles are far apart from each other because when you put a turning force on them using your hands these are these are these are the hands okay the forces are far apart and so the torque becomes very high it becomes easier to turn the handle but imagine if this uh if this handle was not there okay and then you have to apply that same turning force directly on the rod now the forces will be so close to each other the distance would be very small the torque would be very small it becomes so difficult to turn this you can feel this right so torque also depends upon the distance between the two forces and i want you to think about that as the dipole turns what happens to the distance will it increase decrease stay the same so pause the video and think about it and let me get rid of this epic drawing okay let's see a second later our dipole is going to turn and the force has come closer to each other can you see that the distance between them has decreased so this means as the dipole turns this torque becomes smaller and the more it turns the closer the forces get the strength of the force stays the same but the torque becomes smaller and smaller eventually the dipole gets perfectly aligned and the forces are also along the straight line so the torque goes to zero because now there is no torque now these forces are just pulling the dipole apart not producing any turning effect so now our torque has gone to zero so i'll just delete that and so long story short what happened let me go back when the dipole is not aligned like when it's now perpendicular to the field the torque acts until it gets aligned now in reality even though even though right now the torque is zero as it turned it gained momentum it gained some speed and so because of its inertia it might overshoot a little bit and i want you to think what if it overshot this way what would now happen well now again there will be a torque but this time look the torque would be in the opposite direction so again we're seeing that the dipole is trying to get aligned in the direction of the field so again the moral of the story is dipoles experience a torque that aligns them or tries to align them in the direction of the field so let's quickly check our understanding what if i have an electric field to the left and i kept a dipole this way what direction will be the torque clockwise or anti-clockwise and how will it align can you think okay since p tries to align towards e this time the torque will be anti-clockwise and once it aligns the torque disappears let's try one more what will happen over here okay again p wants to align towards e so this time the torque will be oh clockwise again once it aligns torque disappears one more how will the torque be again p tries to align towards e and so this time the torque will be again anti-clockwise once it aligns torque disappears okay now let's really start having fun so again keep a dipole in electric field talk acts on it until it aligns right and it overshoots because of its inertia but the torque take care takes care of it and aligns it now here's my question what if what if as the torque acts as it overshoots before it has time to align i flip my electric field what happens now well now the dipole says okay again i have to align so it'll continue to turn and again before it has time to align itself and when it overshoots what if i flip my field again what's gonna happen oh can you see what's happening if i flip my electric field at the right time i can keep that dipole turning and that's exactly how your microwave ovens work so what do microwave zone do well they produce microwaves of course but what are microwaves they're electromagnetic radiation and we'll talk more about them in the future videos but it's important is that these microwaves contain flipping electric fields and they're actually flipping at billions of times per second and when you keep any food item inside your microwave pizza burger anything at all you'll find that these food items always have some or the other moisture which means water molecules not so giant ones but tiny water molecules and the important thing is these water molecules are electric dipoles and you learn more about this in chemistry it turns out that oxygen and hydrogen are sharing electrons and the shared pair of electrons are pulled more towards oxygen compared to hydrogen as a result oxygen becomes slightly negative hydrogen becomes slightly positive we have a dipole turns out that this dipole moment is very very tiny but that's all that that's all that we need that tiny dipoles these tiny dipoles when they come inside a flipping electric field they will keep turning and as they turn they will make other they will hit other molecules they will make them vibrate pretty soon all the molecules are jiggling and as a result your burger is going to get hot very very quickly this is how microwave ovens work incredible isn't it to think that we use we use the idea that dipoles turn inside electric fields and we can use that to heat up food that is mind-boggling for me it's pretty pretty amazing if you ask me so long story short keep a dipole inside a uniform electric field you will find that the total force acting on it is zero so the dipole will not accelerate but there will be a torque acting on it and what does the torque do it will always try to align the p vector or the dipole moment in the direction of the electric field