3. Electric motors(2)
· 0:00Where I left off in the last video we saw that if we had a
· 0:03magnetic field coming in from the right and we had this loop
· 0:06of-- I guess we call it-- metal or a circuit, and it's
· 0:10carrying a current where the current is coming in this
· 0:13direction--.
· 0:14You can imagine positive protons, although we know the
· 0:16electrons go in the other direction.
· 0:17But the current is coming in this direction and going out
· 0:20that direction.
· 0:20We figured out using the right hand rule and just this
· 0:24formula, that the net force of the magnetic field coming in
· 0:27this direction on this arm of the wire or the
· 0:31circuit is net downwards.
· 0:34And on this arm, it was net upwards.
· 0:36And so it provided a net torque on this circuit.
· 0:39Or, as I said in the last video, a paper clip.
· 0:41And where this dotted line is the axis of rotation.
· 0:44And this is how I showed you it would rotate.
· 0:46Where the magnetic field is essentially pushing up on the
· •Current transcript segment:0:49right hand side and pushingdown on the left hand side.
· 0:51It has no effect over here on the top and the bottom.
· 0:54So it would rotate in this direction.
· 0:58And then this was kind of what it looks like after it rotates
· 1:01a little bit.
· 1:02And the whole reason why I did this, I said, well, this arm--
· 1:06which is the same as this arm-- the net
· 1:08force is still upwards.
· 1:09Out of our screen.
· 1:11But that upwards direction is now no longer going to be
· 1:14completely perpendicular to the moment arm distance.
· 1:19That's the moment arm distance.
· 1:20Now the moment arm distance is kind of coming at an angle out
· 1:23of the page.
· 1:23So only some of this net outward force for the magnetic
· 1:27field is going to be perpendicular
· 1:30to the moment arm.
· 1:31And so the torque on it will be less, but it's still going
· 1:34to be torque in that same direction.
· 1:36Kind of coming out of the page on the right and into the page
· 1:39on the left.
· 1:39And the same is true of the left hand side.
· 1:41And you go all the way to the point that the coil is
· 1:44actually vertical.
· 1:45Where this side, this side right here, is on top.
· 1:49And this side is on bottom, below the plane
· 1:53of your video screen.
· 1:53And at that point, the torque-- actually, there is no
· 1:59net torque.
· 1:59And why is that?
· 2:00Because on this top part, when it's pointing straight out at
· 2:03you, when it's right here, the magnetic field-- the force of
· 2:09it, the force that's affecting the circuit-- is pushing
· 2:13straight up.
· 2:14So there's no longer any net torque because the force is
· 2:17pushing straight up and that moment arm distance-- this
· 2:19distance-- is now also pointing straight up.
· 2:22And torque is also a cross product, so you actually care
· 2:25about the perpendicular forces.
· 2:28So there, at this vertical point, there's no net torque.
· 2:30And the same is true at the bottom of the circuit.
· 2:33Because at the bottom the magnetic field force is going
· 2:36to be downwards, which is parallel with the moment arm
· 2:38distance, so there's no net torque.
· 2:40And I said, well maybe there's a little bit of angular
· 2:45momentum that keeps this object rotating.
· 2:49And then it will rotate to-- and this is where it gets
· 2:52interesting.
· 2:53I'll draw it neatly.
· 2:55Then it'll rotate to this point.
· 2:58Once again I want to have the perspective.
· 3:08It'll rotate here.
· 3:10So let me just make sure I have all of it.
· 3:12So here it was rotating in this
· 3:14direction and in that direction.
· 3:20And then here maybe some-- there's no longer any torque
· 3:22on it, but it still might on the top be moving to the left,
· 3:26and on the bottom moving to the right.
· 3:28Up to a point, then it's going to get into this configuration
· 3:31where soon. this side is-- so at this point it has rotated
· 3:38more than 90 degrees.
· 3:40So this edge is now this edge.
· 3:42It had rotated from here all the way-- it's still pointing
· 3:46out of the screen.
· 3:47But if this edge is the same as this edge, now the current
· 3:51direction is going to be like this.
· 3:53Because this edge has rotated down.
· 3:56So it's rotated from that position all the way back to
· 3:59this position.
· 4:00So the current is now coming-- let me make sure, let me draw
· 4:02that right.
· 4:04The current is coming like that, like that, like that.
· 4:08Going up here, to the right, up like that.
· 4:13So the current now on this left hand side, although it
· 4:15was the former right hand side.
· 4:17It's still going in that upwards direction.
· 4:20So when you take the cross product, what is going to be
· 4:22the net magnetic field on that?
· 4:25Or the force of the magnetic field?
· 4:27Well, you do the same right hand rule.
· 4:31Point your index finger up.
· 4:32Put your middle finger in the direction of
· 4:34the magnetic field.
· 4:35This is the palm, this is your other two fingers.
· 4:38Let me draw the fingernails, just so they're painted
· 4:40fingernails.
· 4:40Not that mine are.
· 4:42Then your thumb points upwards.
· 4:44So on this side of the coil we still have an upwards force.
· 4:51And if you do the cross product, or you do the right
· 4:53hand rule on the bottom side, or the behind side, if you
· 4:56could imagine it, you're still going to have a
· 4:59net downward force.
· 5:00So now all of a sudden you could imagine--
· 5:02the thing had rotated.
· 5:04So it had rotated in the way I drew it here, where it pops
· 5:09out on this side and it goes in on that side.
· 5:12And it had done it all the way to the point where we had
· 5:14rotated more than 90 degrees, but now all of a sudden the
· 5:17net force through the magnetic field was going to reverse.
· 5:21Because the side that has a current going upwards is now
· 5:24the left hand side.
· 5:26So now the force from the magnetic field is out on this
· 5:28side and you're going to want to rotate in
· 5:31the opposite direction.
· 5:34Hopefully that makes sense.
· 5:34Just think about what happens.
· 5:35Visualize this coil rotating.
· 5:38So what is essentially going to happen is you're going to
· 5:42rotate like I did here on the top.
· 5:46Maybe once you get to this level you're going to have a
· 5:48little bit of angular momentum that'll keep you rotating.
· 5:52Or rotational inertia that'll keep you rotating until you're
· 5:55in something like this configuration.
· 5:56Maybe you go all the way back to this configuration, where
· 6:00it's essentially a complete 180 degree turn.
· 6:04And then since on this side the current's going to be
· 6:08going up and on this side the current's going down, because
· 6:10you've essentially flipped this thing over, then the
· 6:14effect of the magnetic field is going to say, well, upwards
· 6:17on the left, downwards on the right.
· 6:19And so it's going to turn the other way.
· 6:20So if you think about it, it's going to keep oscillating.
· 6:23Let me draw it from-- well, I don't want to draw it from
· 6:27that angle, because I don't want to confuse you.
· 6:28So we have a problem.
· 6:30If we wanted to turn this into some type of electric motor
· 6:33and keep it spinning, we would either have to reverse the
· 6:37current once you get into this configuration, or either turn
· 6:41off the magnetic field.
· 6:42Or maybe you could reverse the magnetic field to get it going
· 6:46in the other direction.
· 6:47And actually you have another problem, which is a slightly
· 6:49lesser problem, is if this was a circuit and you just kept
· 6:52turning over and over the circuit, the wires would get
· 6:55twisted here.
· 6:56So you couldn't do it indefinitely.
· 6:58So the solution here is something called a commutator.
· 7:03you So let me draw a commutator.
· 7:10I have the same circuit which I've now drawn messier.
· 7:23But it has these two leads.
· 7:27It has these leads that essentially curve.
· 7:29You could imagine them curving out of the page.
· 7:31And then we have a circuit.
· 7:35You could imagine leads here, too.
· 7:36And this round thing and this thing are touching each other
· 7:41the whole time, so current could pass through it.
· 7:47Let me draw my battery.
· 7:50This is positive and this is negative.
· 7:52So up here on the circuit the current's always going to be
· 7:55flowing in this direction.
· 7:56It's always going to be flowing in this direction,
· 7:58it's always going to be flowing up and like this.
· 8:02Now when you're in this configuration,
· 8:04what's going to happen?
· 8:05Well, the current is going to flow down here.
· 8:08That's going to be I and that's going to be I.
· 8:12And when you do your right hand rule, we have the same
· 8:15magnetic field.
· 8:16I haven't changed the magnetic field coming in from the left.
· 8:21So just like we did before I cleared the screen, you use
· 8:24the right hand rule and you'll figure out, well, the net
· 8:26force from the magnetic field is going to be upwards here
· 8:32and downwards here.
· 8:34And that's what's going to create that net torque.
· 8:36And you're going to rotate this part.
· 8:39So this part of this
· 8:41contraption is going to rotate.
· 8:43You could imagine maybe there's like
· 8:44a little pole here.
· 8:45Maybe it's a nonconducting pole so that none of the-- and
· 8:48it's connected to an axle somewhere.
· 8:50So you can rotate along that axis, right?
· 8:53So the force of the magnetic field is
· 8:55going to create a torque.
· 8:56We're going to rotate up on this side, up out of the page
· 9:00on that side, and into the page on that side.
· 9:03And then behind the page and then back out of the page.
· 9:07That's what the net torque would be.
· 9:09And then we would get it, and it would keep doing that until
· 9:12you get to the vertical configuration.
· 9:19So at the vertical configuration, the circuit on
· 9:22the top stays exactly the same.
· 9:29I'm trying my best to draw this properly.
· 9:31At the vertical configuration one of two things can happen,
· 9:34and probably the best thing is that we actually lose contact
· 9:37with the two leads.
· 9:38So maybe the actual current stops flowing when we're in
· 9:42the vertical configuration.
· 9:43I'll do it in the same color.
· 9:44So when we're vertical we just see the top.
· 9:48We see this.
· 9:50And then we see it pops out a little bit.
· 9:51And then we see this arm right there.
· 9:55And then we see that pole that's maybe holding it or
· 9:57that's helping it rotate.
· 10:00But we're still having some-- you know,
· 10:02the current has ceased.
· 10:03So there's not going to be any torque, no force through the
· 10:05magnetic field, because we've lost touch at that point.
· 10:09Because these things kind of point out.
· 10:10Hopefully you could visualize how to build such a thing.
· 10:14And we're still rotating in this direction because of some
· 10:17type of rotational inertia.
· 10:20Then this is what the interesting part is.
· 10:23What happens when we rotate more than 90 degrees?
· 10:27And I just realized that I'm pushing over 10 minutes, so
· 10:29you can think about that a little bit while I stop here
· 10:32and continue this in the next video.
· 10:34See
4. Electric motors(3)
· 0:00So where I had left off is we had the circuit.
· 0:02We had these little leads here.
· 0:04This was kind of our innovation.
· 0:05And this is actually called a commutator, where this part
· 0:08that's connected to our rotating piece, that's the
· 0:10commutator.
· 0:11And these are the brushes.
· 0:12So you could imagine, you could design them as brushes
· 0:14that always stay in touch.
· 0:15Kind of like the brushes on a, what was that?
· 0:20What are those cars at the amusement park?
· 0:22Bumper cars, right?
· 0:24On the bumper cars you have a pole behind your bumper car.
· 0:27I'll draw that for fun.
· 0:27So let's say this is your bumper car.
· 0:30Looks like a shoe a little bit.
· 0:32This is you driving your bumper car.
· 0:34And they have a pole.
· 0:36And at the top of the pole, you'll see these brushes that
· 0:39are touching the ceiling, right?
· 0:41You could view that as the same type of brush.
· 0:43And what it allows is a constant electric current to
· 0:46flow through the ceiling.
· 0:48I don't know what direction it's going in.
· 0:49But it allows a current to flow through the ceiling.
· 0:52And maybe your car is grounded so the current can flow down
· 0:55to ground, so that your car could be powered by the
· 0:57ceiling and not have to carry a battery in every car.
· 0:59Which would be kind of a waste of energy and probably some
· 1:01type of a health hazard and safety risk,
· 1:03et cetera, et cetera.
· 1:04So those brushes on your bumper cars might not be all
· 1:07that different from the brushes that are touching the
· 1:10commutators here.
· 1:12Just a little bit of terminology.
· 1:13And it never hurts to introduce bumper car
· 1:16references.
· 1:18I probably should have done them earlier when we were
· 1:20learning about momentum and things.
· 1:22But anyway.
· 1:24So what was happening here?
· 1:25So going back to our first video.
· 1:27We have the current going down like this.
· 1:30And then if you use your right hand rule with the cross
· 1:33product, you know that the net force from the magnetic field
· 1:36is going to be downwards on the left hand side, upward on
· 1:39the right hand side.
· 1:40So you have a net torque rotating it like that.
· 1:42Rotating the right out of the page, the left into the page
· 1:46or into the video screen.
· 1:47Up to the point that you've rotated 90 degrees and now
· 1:49you're looking kind of, so this side right here.
· 1:52Let me do it in a different color so you can see it.
· 1:54This side is this side, right on top.
· 1:58And this side is on the bottom, below the page.
· 2:00This side is now above the page.
· 2:03If this distance is r, this side is now r
· 2:05units above the page.
· 2:07And I said ideally maybe your commutator loses touch with
· •Current transcript segment:2:11the brushes at this point,right?
· 2:12Because they're popping out a little bit, so when you're
· 2:13vertical, you actually lose touch with the brushes.
· 2:15So you have no circuit flowing, so you save a little
· 2:19battery energy.
· 2:20And you just let a little bit of the angular momentum carry
· 2:25this whole rotating contraption further a little
· 2:28bit to the point that your configuration
· 2:31will look like this.
· 2:35So I know I keep changing colors, but the whole
· 2:40contraption will now look like this.
· 2:51OK, that's my positive, negative, positive, negative,
· 2:56current flows like this.
· 2:57Now we assume that the commutator has
· 2:59gotten back in touch.
· 3:00And let me color code this.
· 3:03So if this side is this color, right?
· 3:07Now this is when we're looking at top on, where it's popping
· 3:10out of the screen, where it's above the screen.
· 3:12And now we've rotated 180 degrees and this side is on
· 3:14this side, right?
· 3:18Let me pick a suitable color.
· 3:19If this side was green.
· 3:21Now this side, we flip the whole thing over 180 degrees.
· 3:25And now something interesting happens.
· 3:26Remember, before we had this commutator and everything, if
· 3:29we just flipped it over, the current, because before when
· 3:33we didn't have the commutator, the current here was flowing
· 3:38down here, up here.
· 3:39And before the commutator, we had the current flowing down
· 3:42here and up here.
· 3:43And so we were switching directions.
· 3:45And so you would have had this thing that would never
· 3:47completely rotate.
· 3:48It would just keep flipping over, right?
· 3:50Which may be useful for, I don't know, if you wanted to
· 3:53flip things.
· 3:54But it's not useful as a motor.
· 3:57So what happens here?
· 3:58Now this side, all of a sudden instead of being connected to
· 4:01this lead, is now connected to this lead.
· 4:03And this green side is now connected to this lead.
· 4:07So something interesting happens.
· 4:09Now the current on the left side is still flowing down,
· 4:12right, and the current on the right side is
· 4:14still flowing up.
· 4:16So we're back to this configuration except that this
· 4:18contraption has flipped over.
· 4:20The brown side is now on the left and the green side is now
· 4:23on the right.
· 4:24And what that allows is that those net torques are still
· 4:27going in that same rotational direction.
· 4:29Use your right hand rule.
· 4:30The current is flowing down here.
· 4:32So if your magnetic field is coming to the left, then the
· 4:36net force is going to be down there and it's
· 4:38going to be up there.
· 4:40And so we can continue indefinite, and we solve our
· 4:43other problem.
· 4:44That we will never keep twisting these wires here.
· 4:47So now using the commutator, we have essentially created an
· 4:50electric motor.
· 4:51And remember I drew that little thing, that could be
· 4:53like the axle.
· 4:54Maybe that turns the wheels or something.
· 4:57So if you have a constant magnetic field and you just by
· 5:00using this commutator which, as soon as you get to that
· 5:04kind of vertical point, it cuts the current, and then
· 5:07when you go a little bit past vertical, a little bit past 90
· 5:10degrees, it switches the direction of the current.
· 5:13So on the left hand side you always have the current coming
· 5:15down, and on the right hand side you always have the
· 5:17current going up.
· 5:18So that the net torque is always going to be pushing, is
· 5:23always going to be rotating this contraption down on the
· 5:28left hand side and up on the right hand side.
· 5:31Coming out of the page on the right hand side and then down
· 5:34on the left hand side.
· 5:34And you could actually turn a wheel now.
· 5:37You could create an electric car.
· 5:39So that is the basics really of how
· 5:42electric motors are created.
· 5:45Well, there's another way you could have done it.
· 5:46You didn't have to use the commutator.
· 5:47One methodology you could have used is you could have had the
· 5:51magnetic field going until you get to this point, and then
· 5:54you turn off the magnetic field, right?
· 5:56And maybe you wait for this situation to go all the way
· 6:02180 degrees and then you turn the magnetic field back on
· 6:05again, right?
· 6:06That's one possibility.
· 6:07But that's maybe not as efficient cause half of the
· 6:10cycle you're not powering it.
· 6:11Or maybe you switch the direction of
· 6:13the magnetic field.
· 6:14Or another option, you don't have to use a commutator.
· 6:16Maybe you use some other contraption to switch the
· 6:17direction of the magnetic field.
· 6:19But this is probably the simplest way to do it.
· 6:21And I think it gives you a general idea of how an
· 6:24electric motor can be created.
· 6:25And then we could play around with the mechanics of
· 6:28innovations on it.
· 6:29But all electric motors are essentially some variation of
· 6:32what you have learned in this video.
· 6:35Isn't it neat to learn something useful?
· 6:37See you in the next video.