 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 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 gonna do in this video and towards the end of it We're gonna figure out exactly how a microwave oven 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? Remember dipoles in 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 video 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 field created by the dipole in this video So don't worry about that my question to you is what's gonna happen to this dipole when I keep it inside an External electric field think about the forces that are gonna 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? Alright, so I know a positive charge experiences a force in the same direction as the electric field And so it's gonna get pushed this way and negative charge experiences a force in the opposite direction of the electric field And so it's gonna 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 gonna accelerate So if it was addressed it's gonna stay at rest. It's not gonna move. It's gonna 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 gonna make it turn Think about it. If you imagine these were strings attached and you pull it what's gonna happen. It's gonna make it turn It's you can feel that right. It's gonna 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 and in the future video will 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 gonna 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 with its direction change? What would that depend on? So for that we need to 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 distance 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 the hands. Okay The the forces are far apart and so the torque becomes very high It becomes easier to turn the handle, but imagine if this If this handle was not there, okay, and then you had to apply that same turning force directly on the rod Now the forces would 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. Let me get rid of this epic drawing Okay, let's see a second letter our dipole is going to turn And the forces come closer to each other. Can you see that the distance between them has decreased? Oh, so this means as the dipole turns this torque becomes smaller 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 But 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 now 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 Ooh, 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 anticlockwise and how will it align? Can you think? Okay, since P tries to align towards E, this time the torque will be anticlockwise And once it aligns the torque disappears. Let's try one more What will happen here? Okay, again P wants to align towards E. So this time the torque will be 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 anticlockwise Once it aligns torque disappears Okay Now let's really start having fun So again, keep a dipole in electric field, torque acts on it until it aligns, right? And it overshoots because of its inertia, but the torque 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 when it overshoots. What if I flip my field again? What's gonna happen? Ooh, 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 microwaves own 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 What is 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 We'll 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