 So on this right hand real question hands up. I've given you a few questions to try right now The homework is all of them Every question is fair game. This is going to be do the day of your test along with the review I'll put an answer key online Sometime in the next couple of days and I'll also write down the answers just as letters And I'll give you a copy of that because normally my answers are also attached so that you don't have the answers have to go Online necessarily and I don't have that for this But I will start by saying where there any of these because it was part of your homework that you were trying and you were going I have no idea what the heck the answer is or if I'm right. I'll happily talk about this Number 12. I'd love to do number 12. Okay, what would be the direction on the force of the wire? This is a nice realistic actual. Hey, there's the circuit example, which I kind of like I got to be honest First thing I'm noticing here. I've got to really analyze this diagram This looks like the positive right there. So I think the current is going this way. It's going down I think down below the current is heading away from me Is that okay and I see a great big horseshoe magnet see it Which way do magnetic fields always point from what to what? So if I draw magnetic field lines in They would look like that So I have a current going this way magnetic fields off to the right now. I can use my right hand rule I would point my thumb in the direction of the current. I Would extend my fingers straight like this and I would move my hand So my fingers were pointing in the direction of the magnetic field which for me Sally requires me to do this My palm points which way I Think the answer is B That's absolutely the kind of multiple choice question you'll get on your test There's going to be probably Anywhere from six to eight maybe even more right hand real questions One for the current carrying wire right hand rule One for the holding the solenoid right hand rule and then one for the cross product Multiplying two vectors together right hand motor rule, which is this one here the toughest one Okay, I also gave you I'll do a few more as the days progressed and I will get an answer key for you I'll try and email it to you. I'm surprised. I don't have one. I think I might have deleted it accidentally I also gave you some questions to try from lesson to basically it was skip four skip six But the rest were fair game. This also is totally fundamental to what's going to be on your test What you need to know So I'll start out by saying from your homework here any of these you would like me to go over three B Okay, what do they want us to find? Magnetic force on a wire or on a particle Okay, so it's not bill that was on a wire. It's QVB Do I know the magnetic field? 6.4. Do I know the velocity 6.0? What's the charge on an alpha particle two of those see it As a charge of two protons, so it's going to be two times one point six times ten to the negative 19 Velocity of six times ten to the seventh Magnetic field of six point four. That's it Okay, by the way alpha particles are neat. They're nerdy if they give you a weird particle question They will always tell you its charge or its mass or whatever it is that you need to know in order to solve it You don't need to know what an alpha particle is but for what it's worth an alpha particle. What is an alpha particle? Helium minus its electrons I Ionized helium can't remember That's chemistry. I teach physics Love to okay. I Like number nine. I like number nine For several reasons here is because we're towards the end of the year now now It's time to start reviewing everything and this is going to pull some stuff out of electrostatic. So here's what it says Magnetic field of strength blah blah blah makes electrons deflect into a circular path So for number nine, I would say I guess that means that magnetic force equals circular force Where magnetic force is q v b and Circular force is m v squared over r nothing really new there We did that a few times last day Sally. Oh, and we said yay We can cancel out one of the V's and they want us to find the mass of the electron So if I get the m by itself M is going to be move the r up move the b q B r over V Nothing new there either and I would say Ooh Electron 1.6 times 10 to negative 19 b. Tesla's yay are yay. Ooh They didn't give me the velocity. You know those questions really asking me to find the velocity So I mean the masses incident like to but really I'm gonna do most of my work in finding The velocity now we're gonna have to fall back to electrostatics and electrostatics We said if you were sending Charges through parallel plate voltage We said the potential energy contained by the charge when they were in the plate became kinetic and Now you'll remember this next equation. We said this Sally the potential energy between plates was q V and this is where I had to put the wings on my voltage Remember this one from a couple of units ago and I say ah if I want to find the velocity The velocity is gonna be to q v divided by the mass square root Let's see the accelerating voltage used to get electrons up to a maximum speed was 200 volts Okay, let's get the letter Well, the problem is If I get the V by itself, I'm gonna have an M in my equation And I don't know M So I'm gonna get really really clever. I'm gonna get the M by itself over here So I'm gonna times by two and divide by V squared that okay M equals to q Voltage V divided by I'm gonna scroll up so you can see Sally V squared now. How does that help me? You know what? I take this back number nine I don't quite like this question. This is a bit of a higher level I would feel okay with this being like the nasty multiple choice. I'm not giving this to you as a written What is this equal say the word letter M? What is this equal say the letter M? Then can I not reasonably say? q b r over v equals to q Voltage over V squared they equal each other and Sally I noticed low and behold my charge cancels which is kind of cool and one of my velocity Cancels which is kind of cool and I think now I can find the velocity Which as I said was this question really is want me to find the velocity the velocity is going to be V equals to voltage over Burr Do I know the voltage yep 200 do I know the magnetic field oops, I've scrolled down I Still have to four point three four times ten to negative three another radius 1.1 times ten to negative two and once I had the velocity I could plug it in there and then solve for the mass Okay, now. Here's what I do like about this question Dylan. I think it is fair game It is totally fair game For me to Give you the voltage Make you find the velocity go over here, but instead of asking you to find the mass Sally Maybe asking you to find the radius Giving you the mass which is which you need it on both sides that would be fair game. Is that okay? And that hopefully will work. I think and that's one way you could calculate the mass of an electron by the way The original one was done through charged oil droplets. I believe Suspending no, that was how they found the charge on electron. This may be how they found the mass of an electron Certainly they didn't put it on a scale any others So all today is going to be is neat cool nerdy zany applications so Today Dylan we want to look at some nerdy cool applications of this right-hand rule Combined with solenoids and magnetic fields. What can you do? One of the first applications is called the current balance. This is a very Very fine and accurate scale that can measure very very small masses So figure 7.2 shows the essential components of a current balance You have some kind of voltage source battery plug Some kind of variable resistor really here's what you need to know so this is the Circuit diagram, and then I'll show you a better picture of one on the next page, but a Plastic teeter-totter right here is carefully balanced at the opening at one end of the solenoid A Current is passed through this conducting strip. I'm gonna mark up my diagram You guys don't need to but this strip here is a conducting strip Inside this solenoid, which way is the magnetic field going? Well, let's see if we can figure it out In this solenoid the current is going this is positive right here. So the current is going this way. Oh The current is going Downwards over the top Ryan So imagine holding the solenoid right now all of you curling your fingers so that the current is going downwards Over the top all of you right hand rule right now put your pens down Look at your solenoid in front of you imagine Holding your fingers. I have to hold my hand this way. Yes. Can't hold it this way curling over the top Which is the North Pole inside this magnet? Right here, which is the South Pole inside this solenoid right here. Which way is the magnetic field? The magnetic field runs this way the magnetic field runs that way Okay, it's much easier if you look at the simplified top view So let's look at this diagram here Justin in this diagram first of all, let's look at the solenoid the solenoid the battery has a current going this way going this way So as I follow this through it looks like the current in the solenoid is going I think this way Draw some arrows like that on your solenoid How do I know it's going that way? Well because the current comes underneath this way back to the battery So it must be going over the top away from me Tyler and underneath towards me. Is that okay? Now let's use our right hand rule for this diagram. Hold the solenoid in your hand pretend to right hand rule Means get your right hand up curling your fingers in the direction of the current Which is the north side here in the left side or the right side? Yep, this is going to be the North Pole over here This is going to be the South Pole. Which way do magnetic field lines point from what to what? So inside here the magnetic field is going to be going this way Now this little rectangle here is a little teeter-totter But half of the teeter-totter has a piece of wire on it like that And what's going to happen is we're going to send current. We're going to send current through that wire Going this way going this way going this way going this way going this way going this way That's coming from this battery here or plug Regan this stretch of arm right here is Parallel to the magnetic field and since it's parallel to the magnetic field there won't be a force No right hand rule because remember we said the force on a wire or a charge had to be perpendicular The velocity or the direction of the current had to be perpendicular to the magnetic field and Regan same with right here. This here is parallel to the magnetic field but this little tiny stretch right here is is Megan exactly 90 degrees to the magnetic field Which way will this little stretch of wire feel a force point your thumbs up? Which way is the magnetic field inside the solenoid pointing to the So point your extend your fingers to the right which way is your palm pointing? Into the page Here there is a mass that's pulling this end down Here there is a force pushing this end into the page and basically what you have is A balance where this end is inside the solenoid you hang a mass on this and it wants to go down But by increasing the current which increases the magnetic field you can gradually get this exactly in balance and then by looking at the current on your battery and Knowing what type of a solenoid you have you can work out how much force is here and you now know mg over here indirectly And you can solve for him Because we did say last date Tyler that I could adjust a solenoid magnetic field very specifically it was Mu naught n over i times l well n's not going to change l's not going to change ah But I the current is going to change and that I can completely control with a little dial I can do a very very accurate balancing scale Another view of the current balance. Here's a three-dimensional view to help you visualize it. So here This mass will be pulling it down Justin But I think if I walk through this solenoid the force on this wire would also be down So you can get balance Your pivot your teeter-totter right there put a bigger mass on Simply increase the current here Simply increase the current here Until you get to the balance and then to figure out the mass you would say well Fg equals fb mg equals Bill the mass of this very light object would be the magnetic field times the current times the length divided by g Which length this length of wire here now? I noticed they used a capital L So I'm going to actually change this to a capital L Which magnetic field Jordan the magnetic field from the solenoid? Where the magnetic field is from yesterday? Mu naught and I Over l Where do you remember from yesterday? What did the capital N stand for as a variable? number of turns I was now This is going to be I in the solenoid. I'll put a little subscripted s there and This is going to be L the length of the solenoid. This is going to be L But all of these are things we have control over a current balance Simple device. I Don't think we have one here at the school. I looked I Used to have just the balance part, but when I moved classrooms. I lost them teeter-totter part can't find it a better application a Better application is the cathode ray to televisions two units ago when we looked at the cathode ray to the CRT I Told you that we use parallel plates to deflect electrons. We actually don't what we use is two big solenoids Here's our diagram Okay, here's my handy-dandy three-dimensional diagram So we have electrons coming out of the page towards us We're gonna use our right-hand rule here, but our right-hand rule does not work for electrons What does our right-hand rule work for Steven? Say protons Protons caught you napping there. So Electron coming towards me like that is the same as a proton going away from me like this I would point my thumb in that direction Now this solenoid right here Can generate two types of magnetic field it can put a north pole here and the south pole there or It could put a south pole there and the north pole there It depends Ryan on which way the current flows. That's why I've got the current flowing in both directions, but here's my point Which way do magnetic fields flow from? North to south I'm gonna tell you that the magnetic field from this guy right here is either Straight down or straight up Depending on whether there's a north pole or a south pole there It's one or the other that's gonna deflect this electron Which way Let's find out point your thumbs Into the page in the direction that a proton would be going and Let's pretend Connor that the magnetic field is straight up. So point your fingers straight up Which way will this electron get deflected to the to the right or? What if the magnetic field was down it could also get deflected awkward bend But it could also get deflected to the left this vertical solenoid here Solenoid M can deflect the electron Now this solenoid right here this horizontal one it can either have a north pole going this way or it can have a Magnetic field going that way depending on which way the current goes And I don't know which way the current is going Sally here is my point this one solenoid M Let's point our thumbs up the page again point your thumbs up the page extend your fingers to the left Which way will this electron get deflected up or if the magnetic field was to the right? Which way would it get deflected? That's really how a cathode ray tube works They don't use parallel plates They use solenoids with magnetic fields because we can control that magnetic field so accurately just by changing the current and By changing the current stronger magnetic field bigger deflection weaker magnetic field not as big a deflection Says show magnetic deflection. I already showed you the video of the magnet deflecting the TV screen, right? so What kind of multiple choice usually multiple choice questions will they ask you what this cathode ray tube? Says an undeflected electron beam strikes the center of a cathode ray tube So initially we're hitting dead center initially. We're hitting right there a Solenoid is placed right there and it causes the electron beam to strike at position X So right now when we turn this thing on Dylan the electron is getting deflected downwards That okay What changes to the magnitude and the direction of the current in the solenoid would cause the electron to strike at y? Well, first of all, let's look at the direction instead of getting deflected downwards. How was this electron getting deflected up? Up how could you do that? What would change that here? Reversing the magnetic field how could you reverse the magnetic field? Foot the current right instead of this way this way So I'm gonna say no. Yes. No. Yes Jordan is that okay? Now Jordan is this thing deflected more or deflected less Than it was in X in terms of the distance when it got deflected to y was it deflected further than when it got deflected to X Yes, definitely right almost twice as far What could do that a stronger magnetic field and I said that the magnetic field We said last day the magnetic field in a solenoid was equal to that It's the same solenoid so the number of changes the sort of number of coils didn't change and the length didn't change How could you make a stronger magnetic field Ryan? Increase the current increase increase. No. No, what's the correct answer here? Be example to with the electromagnet turned off Electrons in a cathode ray tube strike the center of the screen as shown So with this turned off with this solenoid turned off they hit dead center Now we're gonna turn this solenoid on you know what I need to zoom into this diagram a little bit So I will what I need to do is figure out which way this electron will get deflected by the way The electron is coming towards me I can't point my thumb towards me an electron coming out of the page towards me is the same as a proton doing what Evan Heading away, so I'm gonna point my thumb kind of into the page at an angle. I'll do that in a second I need to figure out the magnetic field from here This is positive right here so Megan we're gonna draw little arrows like this It looks like the positive is going over the top if I zoom in on this diagram Is it not so hold your hand vertically as though you're holding the solenoid So that your fingers curl over the top towards the right Do that it means all of you eat the heck yeah get your hands of it, right? Which way is your thumb pointing Tyler? Which way is your thumb pointing? That's the North Pole This is the South Pole so the magnetic field would look like this from North to South Right here, which way is the magnetic field pointing up that that's what we needed to get We needed to look at this solenoid and analyze it so that we could figure out what kind of magnetic field the electron was hitting So you're ready? Electron coming out of the page is the same as a proton going which way Point your thumbs into the page right thumbs which way is the magnetic field right there? Up so up the page so point your fingers up the page What direction will this electron get deflected to one two three or four Now I got a pause. I'm seeing about five people doing this You're killing yourselves if you're not trying this if your right hand is not participating with me What you're really saying is I'm gonna get 40% of the test wrong and I guess that's your decision But it's silly. Yes. I'm looking at the back row there Uh-huh. Okay, scratch the head. We're good. Here we go. Ready? Point your thumb in the direction of a proton so Jordan it would be this way Point your fingers in the direction of the magnetic field right there right there the magnetic field is Tyler What direction what location will this electron get deflected to one two three or four? Four the cathode ray tube revisited and then By the way If you haven't looked at that right hand rule sheet, you're killing yourself right now like I've given it to you for a week You need to have gone through some of it. I'll get an answer key for you shortly For some reason I did this as a separate handout. Here's the last and neatest application The electric motor all of you have devices that have little electric motors in them. So figure Seven point well here this diagram shows an electric motor. You ready because we're going to be using the right hand rule here like crazy Here is how an electric motor works Right now we have a battery right there Which way is the current leaving this battery to the left or to the right? To the left so all of us put a little arrow right there, please all of you and There's already an arrow pointing up a little arrow right there in This now here's some terminology Just in every single electric motor has a magnet in it and It has a rotating arm a coil of wires now that coil of wires is called the armature What we do is we now this is a very very simple armature. It's only one wire Normally it's hundreds of wires all wrapped together. We're not going to deal with that just yet We're looking at a simplest case. So one wire the current is going to flow up up up and on the left side The current is pointing up See it Which way is the magnetic field to the? Right because magnetic fields always point from what to what? North to south now this section of arm is perpendicular to the magnetic field That means it's going to experience a force because you have a current running perpendicular to the magnetic field Which way is the current going? Up the page point your thumbs up the page Which way is the magnetic field to the right point your fingers to the right? Which way is your palm pointing? Down into the page if you looked at an end view this arm right here Which has the current moving away from me is going to get forced down. Oh? Not only that as the current comes through the armature as it comes down this arm length This arm here is going to have a current flowing down the page So now point your thumbs down the page Magnetic field still pointing to the right. I'm bending my hand, but on the right hand signed Megan My palm is pointing And you know what that's going to do it's going to make this thing spin That's how your toy cars work the wheels spinning the electric motor. It spins Sound electric motor works so Let's read through this The coil in a motor is called the armature that's a word you need to know so let's underline it Of course a real motor has many many turns of wire Sally some electric motors will just have a permanent magnet some will have an electromagnet right there So the external or field magnet may be an electromagnet rather than a permanent magnet most of them are just a little permanent magnet The external magnetic field exerts a torque on the armature causing it to turn Because there's a force down right there a force up right there, and so it's going to spin Now There'd be a bit of a problem here if we had direct current if we were running from direct current As soon as this arm got to this side it would come to a grinding halt Just get it because I really need your attention for this Thank you And you put the blackberry away, so it's not a temptation at all. Thank you Megan if This was a direct current when this arm flipped over Now I would have current going down the page over here And it would suddenly come to a grinding halt because it will be being forced up over here You would have it go like this. We're just stop So what we need is a way to turn direct current into Alternating current so it says and this is going to seem a little weird relax. It's going to become clear It says note the importance of the split ring commutator huh When the armature makes a half turn the split ring commutator Makes the current in two halves of the coil change direction So that the current is always flowing upwards on this side always and always flowing downwards on this side So that we always have a downwards force on this side, and that we always have an upwards force on that side It says figure 7.29 shows the construction of a typical laboratory Demonstration motor called the st. Louis motor. Oh For a nice animation see let's see if I can find this animation here Let's cause my recording Here is a nice diagram of an electric motor So I'm going to go right here, and I'm going to go pause Okay Right now The current is flowing This is the positive on the top arm the current is flowing. Can you see to the right? So point your thumbs to the right Using my screen as your piece of paper the magnetic field is downwards So extend your fingers downwards, and yeah, you have to bend kind of funny But I think you're getting that the force on this is let's see To the right downwards the force is in the direction of the arrow up there. Okay, and In the bottom arm we have the current coming boom boom boom in the bottom arm the current is coming to the left If you point your thumb to the left Fingers down the page at the bottom. It's getting forced Towards you. This is going to rotate in this direction. Okay, the problem is pause If I didn't have a gap right there if this was continuous Then when this arm got to the bottom the current would be going to the right And it would come to a stop because then you would have it suddenly being forced in the wrong direction So we have what we call the split ring commutator. It's like a hamburger patty cut out of this circle and What happens is Can you see the current is changing direction in the arm if you watch very very closely so that the top the current is always going to the Right and in the bottom the current is always going to the left and then for a split second Tyler right now There's no current, but the momentum of it is continuing it enough so that it can continue and connect the circuit again This is an electric motor speed it up a little bit if I want to change direction. All I do is I reverse the current boom or I could reverse the magnetic field But electric motor now I'm going to see if I can download this so that is right click Save link as will it work? Nope chrome document. Let me know how you can download a Java app from the web page. I'll ask later So back to our notes Okay Have I got a good I think I do bear with me for a moment physics electricity Generator We don't have an electric motor on here. I thought we did I guess we don't always most so here's a side view again and One way you can make this motor go faster is to make your armature a coil of wires That's what this diagram has The field magnet from here and here could be an electromagnet You send a current going through the armature and you'll get it to rotate now right now the current is The the black part here is on the left here. It's on the right. It allows you from a direct current source To get an alternating current or you can just plug it in megan to an alternating current source But you can't just run off a direct current without that split ring commutator I'm sensing some of you were kind of going huh, and I don't know if I've done a great job of explaining this I Will recover but let's keep reading here. I'll find a better app for you guys for next class The split metal cylinder in the armature is called this is in the split ring commutator Current direction is from the brush at a to the left half of the commutator then to the coil of wire on the armature Current direction is such that the left end of the armature is a north pole and the right end is a south pole Current leaves the right side at B and returns to the battery I'll let you read the explanation just a few notes Electric motors come in all sizes some are very tiny and drive toys others are huge and drive trains or buses But this is how every single electric motor works You'll find electric motors and toy trains automobiles cloaks wheelchairs street cars can openers cement mixers furnace fans robots And well pretty much anything that has something that rotates Which is lots of things? Okay, the motors described operate only on direct current alternating current works Essentially on the same principle, but you don't need a split ring commutator because you don't need to flip the current back and forth The current is already alternating Brushless motors. Yeah, I'm simplifying it. Yeah, and I'm starting with the basic concept of an electric motor And and so Evan said they're called brushes motor as a matter of fact The diagram that I gave you like this with a single flat arm most tiny electric motors Actually have a three-part triangular arm and that way you can never stop and be in a stalled Situation where by a fluke you stop where there's no magnetic field and no for yeah It is more complicated than that, but the basic concept is what I'm trying to get across What's your homework? Done work on the review