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Current time:0:00Total duration:14:59

Video transcript

when I turn on this switch and electric current starts running and our fan spins such devices which convert electricity into rotation we give a name to it we call it the electric motor and these devices are used in many places like in your washing machines your drilling machines or even your electric cars but the big question is how does an electric current which is basically moving electrons push something as big as a fan and make it spin the secret to these motors are magnets in a previous video we had seen that a current carrying wire can be pushed by magnets or more technically magnetic fields so an electric motor uses this magnetic push to spin a current carrying conductor so in this video let's explore exactly how it works so let's start with a couple of magnets which will generate a magnetic field that's going to push our current carrying wire and instead of just introducing a single wire carrying current we will introduce a current carrying rectangular loop now it can be the loop can be of any shape but if you take it as a rectangle it'll be easier to analyze this and if you're wondering why are we considering a loop will soon understand why and as of now let's not worry about where the current is coming from where the battery is we'll deal with that later now each side of these of this rectangular loop is a current carrying wire inside a magnetic field that means each side will experience a force and as a result this coil might start moving now to figure out how this coil will move we need to figure out the direction of the force acting on each side of this Loup and to do that we've seen before we can use the left hand rule where you take your left hand you stretch your fingers like this I remember FBI but thumb gives the direction of the force the forefinger gives magnetic field and the middle finger uses the direction of the current so to figure out the force on any side of the wire we just have to align our fingers in according to the current and the magnetic field and the thumb will tell us what direction the force is and so what we need to do next is apply this left hand rule to each side of the wire and so can you try and do this yourself first so great idea to pause the video and see if you can apply this yourself and figure out the direction of the force all right let's do this let's start with the pink wire the current is inwards and the magnetic field is to the left so if we align our left hand it will look somewhat like this and so notice the thumb is pointing upwards and as a result the force is upwards now if we apply the same left hand rule to this side we see that the force is acting downwards and if you look at these two remaining sites the current is parallel to the magnetic field and we've seen before whenever our electric current is parallel to the magnetic field the field does not push it and so there will be no forces acting on this wire or on this wire so there have been upward force here and a downward force over here and as a result our coil will start moving but how exactly will it move now well here's my board let's let's imagine this is our rectangular coil notice on left side I'm gonna push it up on the right side I'm gonna push it down and as a result notice it starts spinning so just like this our coil will also start spinning and in fact over here it's gonna spin in the clockwise direction so the same thing will happen over here our coil will start spinning in the clockwise direction now as it spins though forces on this wire and this wire will remain the same because the magnetic field is still in the same direction the current is still into the screen over here that has not changed so the force over here will still be up but and the force over here will still be down but if you're curious you may wonder what happens now to the forces on this side and this side earlier they were parallel to the magnetic field but now they are no longer parallel to the magnetic field so when there be forces acting on these wires now as well yes there will be one those forces affect our rotation no they won't why not let's see why well if we apply the left hand rule to this wire notice the magnetic field is to the left the current is sort of downwards so if you apply the left hand rule we see that the force will be into the screen now similarly if we apply the left hand rule on this side over here the force comes out of the screen so there are forces acting on these wires but but what are these forces doing they're just pulling this coil apart it's just like taking this board and pulling it like this these two forces are not going to do anything they're just going to cancel out they're not going to provide any rotation and as a result we can totally ignore these forces they will not affect our rotation at all so throughout the entire rotation we will ignore the forces acting on this side and this side because they're just gonna cancel out and it will not affect our rotation so yay our coil is now spinning but pretty soon we run into a problem you see if we bring back our board if we want this board to continuously keep spinning in the clockwise direction let's say then notice the force on the right must always be down and the force on the left must always be up it should always follow that rule over here once the pink wire comes to the right notice the force on the right side has become up and the force on the left side has become down this will spin my coil back in the anti-clockwise direction so you know what's gonna happen next next my coil will spin back like this now again the coil will spin forward in the clockwise anti-clockwise clockwise anti-clockwise this is bad this is not a motor or at least not a good motor because it's not completing even a single rotation so how do we fix this well to make sure it continuously keeps spinning in the clockwise direction we have to somehow make sure when when this position comes the force on this wire should become downwards and the force on this wire should become upwards right only then it will continue spinning in the clockwise direction so we need to reverse the direction of the forces but the question is how do we do that well again let's bring in our left hand so to reverse the force say over here we want that thumb to point downwards now there are two ways to do that one we can flip our we can reverse the forefinger that is the magnetic field keeping the current same that's one way to do that but we will not do that because flipping the direction of the magnetic field is not all that easy we have to change the poles and we cannot do that automatically but another way in which we can achieve this is we can keep the magnetic field the same direction but we can reverse the direction of our our middle finger our current like this and as a result our force will now point downwards over here so if we flip the direction of the current everywhere then the force direction will change everywhere so the force over here will be downwards and the force over here will be upwards and if you're wondering well how can we change the direction of the current just like that we'll see how to do that a little bit later but that's the most important thing over here in this position we have to change the direction of the current so let's get rid of our hand flip the direction of the current and now the forces flip and as a result our coil will continue to spin in the clockwise direction yay this will happen until again once the blue wire flips over now once the blue wire comes to the right and the pink wire comes to the left again we have a same problem now again on the right hand side the force is up left hand side the force is down and again we need to do the same thing we need to change the direction of the current it is change once we change the direction of the current the coil will continue to spin and so by changing the direction of the current every half a rotation the direction of the forces will change and our coil will continuously keep spinning finally we have converted electricity into rotation now of course in this animation I'm pausing every time the current is changing so that we can see the forces flipping but of course in reality the the thing will keep on spinning continuously the last thing we need to explore is how do we attach a battery to this and make sure that the current Direction reverses after every half a rotation so let's say we bring in our battery attach a couple of wires to it and let's say we directly attach these wires to our coil then as the coil starts spinning because the current starts flowing through it you will see that the wire starts twisting and tangling and whatnot but now more importantly at this position we won the current to reverse how do we do that well we could either flip the battery or we could flip this connection we can make this wire come here and this wire come here then the current direction will change the forces will reverse and the coil will continue to spin but how do we make this happen automatically that's the question right how do we make it automatic well it can be done by using a device called a commutator a commutator consists of split rings which are basically two half metallic rings with some gap in between and a couple of carbon brushes so the wires are attached to these brushes and these brushes are just in contact with these rings they are not connected to them because of the contact electricity starts flowing through it and as the coil spins notice the ring spins with them but the brushes just stay there so the brushes are literally brushing against the Rings maintaining the contact and making sure the electricity flows through this solves our problem of wires tangling and everything but now more importantly let's see how this reverses the direction of the current every half a rotation so right now the pink ring is in contact with the positive terminal and the blue ring is in contact with the negative terminal but notice what happens once the coil spins enough once the coil spins and comes to this position now notice the pink ring has come in contact with this brush the negative terminal of the battery and the blue ring has come in contact with this brush the positive terminal of the battery so now notice due to this commutator automatically current will start flowing from here into this coil in the reverse direction coming out here into the negative terminal and so the current direction flips and so the forces also flip making sure the coil continues to spin in our clockwise direction until until again notice now again they will change contacts now again the pink will come in contact with the positive terminal the blue income blue will come in contact with the negative terminal and as a result the current will now again flow this way again reversing the direction of the forces and so using a commutator we have now achieved complete motor action so what did we learn in this video well we learned that a motor consists of a couple of magnets and a current carrying coil the magnetic field pushes the coil making it rotate and the important thing is every time our coil comes perpendicular to the magnetic field like this we want the current to tane change directions that is achieved by a commutator which is basically to split metallic rings and a couple of carbon brushes so changing the current changes the direction of the force and ensures that the motor keeps spinning in the same direction and so this is how we can now convert electricity into rotation and if we have enough current in this and if the coil and the magnets are strong enough then we can attach anything we want to it like a fan or washing machine electric drilling machine and make it spin using electricity