If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:10:37

Video transcript

where I left off in the last video we saw if we had a magnetic field coming in from the right and we had this loop of of I guess we called metal or a circuit and it's carrying a current in this where the current is coming in this direction you can imagine positive protons although we know the electrons go in the other direction but the current is coming in this direction and going out that direction we figured out using the right-hand rule and just this formula that the net force of the magnetic field coming in this direction on this arm of the wire or the circuit is net downwards right and on this arm it was net upwards and so it provided a net torque on this circuit or as I said in the last video a paperclip and where this dotted line is the axis of rotation then this is how I showed you it would rotate right where the the magnetic field is essentially pushing up on the right-hand side and pushing down on the left-hand side it has no effect over here on the top and the bottom so it would rotate in this direction and then this was a kind of what it looks like after it rotates a little bit and what I though the whole reason why I did that I said well this arm which is the same as this arm the net force is still upwards out of our screen but that upwards direction is now no longer going to be completely perpendicular to the moment arm distance that's the moment arm distance now the moment arm distance is kind of coming at an angle out of the page so only some of this net outward force from the magnetic field is going to be perpendicular perpendicular to the moment arm and so the torque on it will be less but it's still going to be torque in that same direction kind of coming out of the page on the right and into the page on the left and the same is true of the left hand side and then you go all the way to the point that the coil is actually vertical where this side this side right here is on top and this side is on bottom below the plane of your video screen and at that point the torques actually there there is no net torque and why is that because on this top part when it's pointing straight out at you when it's right here the magnetic field the the force of it the force that it's affecting the the circuit is pushing straight up so there's no longer any net torque right because the force is pushing straight up and that moment arm distance this distance is now also pointing point straight up and torque is also a cross product so you actually care about the perpendicular forces so there at this vertical point there is no net torque and the same is true at the bottom of the of the circuit because at the bottom the magnetic field force is going to be downwards which is parallel with the moment arm distance so there's no net torque and I said well maybe there's a little bit of of angular momentum that keeps the this object rotating and then it will rotate too and let me and this is where it gets interesting I'll draw it neatly then it'll rotate to this point once again I want to have the perspective its perspective will rotate here so let me just make sure I have all so here it was rotating in this direction and in that direction right and then here maybe some that there's no longer any torque on it but it still might on the top be moving to the left and on the bottom moving to the right up to a point that it's going to get into this configuration where this this side is so it's rotated at this point it has rotated more than 90 degrees so this edge is now this edge it had rotated from here all the way it's still pointing out of the screen but if this edge is the same as this edge now this the current direction is going to be like this right because this Edge has rotated down also it's rotated from that position all the way to the back to this position so the current is now coming let me make sure let me draw it on the right the current is coming like that like that like that going up here and to the right up like that right so the current now on this left hand side although it was the former right hand side it's still going in that upwards direction so when you take the cross product what is going to be the net magnetic field on that or the force of the magnetic field well you do this you do the same right hand rule point your index finger up put your middle finger in the direction of the magnetic field this is the palm this is your other two fingers let me draw the fingernails just so they're painted fingernails not that mine are then your thumb points upwards so on this side of the coil we still have an upwards force and if you do the cross-product where you do the right-hand rule on the on the bottom side or the behind side you could have even imagined it you're still going to have a net downward force so now all of a sudden you could imagine the thing had rotated so it had rotated in the way I drew it here where it pops out on this side and it goes in on that side and it had done it all the way to the point where we had rotated more than 90 degrees but now all of a sudden the net force through the magnetic field is going to reverse right because the side that has the current going upwards is now the left-hand side so now the force from the magnet field is out on this side and you're going to you're going to want to rotate in the opposite direction hopefully that makes sense just think about what happens visualize this coil rotating so what is essentially going to happen is you're going to rotate you're going to rotate like I did here on the top maybe once you get to this level you're going to have a little bit of angular momentum that will keep you rotating or rotational inertia that'll keep you rotating until you're in something like this configuration maybe you go all the way back to this configuration where the sotware it's essentially a complete 180-degree turn right and then B then since on this side the current is going to be going up and on this side the currents going down because you've essentially flipped this thing over then the effect of magnetic field is going to say well upwards on the Left downwards on the right right and so it's going to turn the other way so if you think about it it's going to keep oscillating let me draw it from let me draw it from a well I don't want to drop from that hang because I want to confuse you so we have a problem if we wanted to turn this into some type of electric motor and keep it spinning we would either have to reverse the current once you get into this configuration or either turn off the magnetic field or maybe you could reverse the magnetic field to get it going in the other direction and actually you have another problem which is a slightly lesser problem is this if this was a circuit you just kept turning over and over the circuit the wires would get twisted here so you couldn't do it indefinitely so the solution here is something called a commutator a commutator so let me draw a commutator so what if let me I have let's see the same same circuit which I have now drawn Messier but it has these two leads has these leads that essentially curve you can imagine them curving out of the page and then we have a circuit it can you can imagine leads here to and there this this round thing and this thing or they're touching each other the whole time so current could pass through it right let me draw my battery this is positive and this is negative so up here on the circuit the current is always going to be flowing in this direction it's always going to be flowing in this direction it's always going to be flowing up and like this right now when you're in this configuration what's going to happen well the current is going to flow down here that's going to be I and that's going to be I and when you do your right-hand rule we have the same magnetic field I haven't changed the magnetic field coming in from the left so just like we did before I cleared the screen you use the right-hand rule and you'll figure out well the net force from the magnetic field is going to be upwards here and downwards here and that's what's going to create that net torque and you're going to rotate this part so this part of this contraption is going to rotate you could imagine maybe there's like a little pole here maybe it's a non-conducting Pole so that none of the you know and it can it's connected to an axle somewhere so you can rotate along taxes right so the force of the magnetic field is going to create a torque we're going to rotate up on this side up out of the page on that side and into the page on that side and then behind the page and then back out of the page right that's what the net for torque would be and then we would get it and it would keep doing that until you get to the vertical configuration so so at the vertical configuration this is the circuit on the top stays exactly the same so the vertical configuration and I'm trying my best to draw this properly at the vertical configuration one of two things can happen and probably the best thing is is that we actually lose contact with the two leads so maybe the SIRT the actual current stops flowing when we're in the vertical configuration I'll do it in the same color so when we're vertical we just see you know the top we see this and then we see you know it pops out a little bit and then we see this arm right there and then we see that pole that's maybe holding it or that it's helping it rotate all right but we're still having some a you know the current has seized so there's not going to be any torque no force through the magnetic field because we've lost touch at that point right because these things kind of point out hopefully you kind of you you you you could visualize how to build such a thing and we're still rotating in this direction because of some type of rotational inertia then this is what the interesting part is what happens when we rotate more than ninety degrees and I just realized that I'm pushing over ten minutes so you can think about that a little bit while I stop here and continue this in the next video see you soon
AP® is a registered trademark of the College Board, which has not reviewed this resource.