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(0:05) Today, I am going to work with you on a new concept, and that is a concept of what we called Electric Field. We'll spend a whole lecture on Electric Field. If I have a charge, I just choose Q, capital Q and plus (+· ), at a particular location, and another location, I have another charge little q, I think that might test charge ( · ). And there is a separation between the two, just r. The unit vector from capital Q to the little q is this vector ( r vector). So now I know that the two charges if they were positive, if the little q is positive, they would repel each other; they would, if little q were negative, they would attract each other. And that is force, the F (F vector), and last time we introduced Coulomb's Law, that force is equal to little q times capital Q, times Coulomb's constant divided by r squares in direction of r roof (Equation). The two have same sign to this direction, or the opposite sign if it's in the other direction.
(1:38) And now I introduce the idea of electric field which we symbolize with capital E (E vector). The capital E (E vector), at that location P, where have my test charge little q at that location p is simply the force that the test charge experience divided by that test charge, so I eliminate the test charge. So I have something looked quite similar, but it doesn't have the little q in it any more, and it is also a vector (Equation). And by convention, we choose the force, such that if this is a positive test charge, then we say the E field is away from Q if Q is positive; if Q is negative, the force is in the other direction, and therefore E is in the other direction. So we adopted the convention that the E field is always in the direction that force is on a positive test charge. Which we've gained now is that you have taken out the little q, in another words, the force here, depends on little q, electric field is not. So the electric field is a representation for what happened around the charge plus Q, it could be a very complicated configuration and electric field tells you something about that charge configuration. The unit for electric field, you can see it's Newton divided by coulomb [N/C], in SI unit, and normally we won't indicate the unit, we just leave that as it is.
(3:28) Now we have graphical presentations for the electric field. Electric field is a vector, so you expect arrows. And I have here the example of a charge +3, so by convention, the arrows are pointing away from the charge in the same direction that the positive test charge would experience the force, and you know that, very close to charge, the arrows are larger, then farther away, that sort of represent, or trying to represent the influence of r square relationship. Of course it cannot be very quantitative. But the basic idea is it of course, it's spherically symmetric, this is point charge, the basic idea is, you see the field vectors and direction of the arrows tell you which the direction, the force would be if it's positive charge, and the length of the vector gives you idea of the magnitude. And here I have another charge of minus 1 (-1), doesn't matter whether the minus 1 (-1) coulomb, or minus micro coulomb, just is a relative representation. You see now the E field vectors, I reverse in direction to pointing forward, the minus charge by convention, and you go further out, they are smaller. And you have to go all the way to infinitive, of course, for the field to become zero because one of our square fields falls off, and you have to be infinitive far away for you to not experience the principle any effect from the charge.
(5:18) What do we do now when we have more than one charge? Well, if we have several charges, here we have Q1 ·, and here we have Q2 ·, and here we have Q3 ·, and here we have Q of i, we have i charges. And now we want to know what the electric field at point p. So it's independent of the test charge that I put here, you can think if you want to, as the force per unit charge, you divided out the charge. So now I can say what is the E field due to Q1 alone, that would be, if Q1 were positive, and this might be a representation for E1. If Q2 were negative, this might be representation of E2, pointing to negative charge, and if this one (Q3) were negative, then I would have here the contribution E3, and so on, and now we use the superposition principle as we did last time which Coulomb's law, that the net electric field at point p is a vector, is E1 influence of charge Q1, plus the vector E2, plus E3 and so on. And if you have i charges, this is sum of all i charges of individual E vectors. Is it obvious that the superposition principle works? No. Does it work? Yes. How do we know it works? Because it's consistent with all our experimental results, so we take this superimposition principle for granted. And that is acceptable. But it's not obvious.
(7:28) If you tell me what the Electric field at this point is, which is vectorial sum of the individual vectors. Then I can always tell you what the force will be, if I bring a charge at that location. I take all the charge I have in my pocket, and I take it out of my pocket, and I put it out there in the location, and the charge of that point might be little q, then the force on that charge is always q times E, doesn't matter whether the q is positive, then it will be the same direction as E, if it's negative, it will be the opposite direction of E. If q is large the force will be large, if q is small the force will be small. So once you know the E field would be the result of the very complicated charge configurations, the real secret behind the concept over E field is that if you bring any charge at that location, and you know what force X at that point on that charge.
(8:33) If we try to be a little bit more quantitative, suppose I have here a charge +3, and here I have a charge -1. Here is -1, and I want to know what the field configurations as a result of these two charges. So you can go to any particular point, you get the E vector which is going away from the +3, you get one that goes to -1, and you have 2 vectors and add the two. If you very close to -1, it's very clear because influence of r square relationship, that the -1 is probably going to win. Let's, in our mind, take a plus charge now. And we put the plus charge now very close to -1, say we put it here; even thought +3 is trying to push it out, clearly -1 is most likely to win. So it will probably be a force on my test charge in this direction (ß), the net results of the vectors of the two. Suppose I take the same test charge and I put it here, very far away, much far away than this separation. What do you think now is the direction of the force of my plus charge? Very far away! Excuse me? (Student answered.) Why do you think it's to the left? You think -1 wins? (Student answered.) Do you really think -1 is through +3, because +3 is pushing it out? And the -1 is trying to lure it because the test charge is positive. (Student answered.) Imaging all this way in Mass. Avenue, you think that this thing matters? Who think the force is in this direction? Who think it's in the direction (to the left)? Very good, you help him, really. The force is obvious in that direction because if you very far away, the field will be the same. If you just have +3 and -1 somewhere here, which is +2. So if you far away from the configuration like this, even if you are here, or if you are there, or away there, clearly the field is likely a +2 charge, and so force is one over r square, so therefore if you are far away and force is in this direction (à). Now look what's very interesting. Here if you close to -1, it's in this direction (ß). Here if you are very far away, maybe I should be all the way here, it's in that direction (à). So, that means there must be somewhere here a point where the E field is zero. Because if the force is here in this direction (ß), it ultimately turns over in that direction (à), there must be somewhere a point where E is zero, and that is part of your assignment. I want you to find that point for a particular charge configuration.
(11:46) So let's now go to some graphical representation of a situation, which is actually +3, -1. Try to improve on the light situation. And let's see how these electric vectors, how they show up in the facility of these two charges. So here you see the +3 and -1, relative unit, and let's take a look at this in some detail. First of all, the length of arrows again indicates the strength, gives you the feeling of strength, not very quantitative of course. So let's first look at the +3, which is very powerful. You see these arrows all go away from +3, and when you go closer to +3, they are stronger which is a representation of influence of r square field. If you're very close to the -1, the arrows are pointing into the -1, because of one over r square, the -1 wins, so you see there clearly go in to the direction of -1. But if you're in between the +3 to the -1 in this line, always the E field will be pointing from the plus (+) to the minus (-). Because the plus is pushing out and the minus is sucking in so the two support each other. But now if you go very far away from this charge configuration, anywhere, but very far away, much farther than the distance between these two charges, so somewhere there, somewhere there, or somewhere there, or here, noticed that the arrows are always pointing away. The reason is the +3 and -1 is as good as +2 if you are very, very far away. But of course if you're very close in, then the field configuration can be very, very complicated. But you see very clearly that these arrows are all pointing outwards. None of them come back to -1, none of them point to the -1 direction, and that is because +3 is more powerful. And then there is here this point, and only one point where by electric field is zero, if you put a positive test charge here, the minus will attract it, the plus will repel it, and therefore it comes to a point where the two cancel each other exactly.
(14:21) Now there is another way of electric field representation, which is more organized, and we call this field lines. So you see again, the +3 and you see there the -1. If I release right here, or I place here, a positive test charge, all I know is that the force will be tangential to the field lines. That is the meaning of these lines. So if I am here, the force will be in this direction. If I put a positive test charge here, the force will be in this direction. And of course, if it's negative charge, the force lifts over. So the meaning of the field lines are, that it always tells you in which direction a charge experiences a force. A force the positive charge always in the direction of the arrows, tangentially to the field line, and negative to the opposite direction. How many field lines are there in the space? Well, they are infinite number, just like the little arrows we have before; we only sprinkle in a few. But in any single point, there is electric field, so you can put in infinite number of field lines, and that will make this representation of course useless. So we always limit way to a certain number. If you look very close to the -1, notice that all the field lines come in to -1, we understand that of course because positive charge would want to go to -1. If you go close to the plus (+), and they will go away from the plus (+), they're being repelled. You can sort of think of these field lines if you want to imagine the configuration that the plus charge is blow out air like hair dryer, and that minus suck in air like a vacuum cleaner, and then you get the feeling for the left side, it is hair dryer where wants blow out stuff, and then there is that little sucker want to suck something in. And it succeeds some degree if not powerful as +3 though.
(16:46) Have we lost all the information about the field strength? We have earlier with these arrows; we have the length of arrow, the magnitude of the field where it represented. Yah, you've lost that, but there is still some information on field strength. If the lines are closer together the density of lines is high, the electric field is stronger than when the density becomes low. So if you look for instance here, look how many lines there are, per few millimeters, I mean when you go further out, these lines flat out that tells you these E field is going down; the strength of E field is going down, it's one over r square of course. If you want to make these drawing, what you could do to make them look good, you can make three times more the field lines going out from the plus in this case, then returns to the -1. So the field lines are very powerful, and we will often thinking in terms of electric fields, and line configurations, and you will have several homework problems that will deal with electric fields and with electric field lines.
(18:01) If the electric field line is straight, so I have a electric field, get some red chalk, say we have fields like this, straight field lines, and I release a charge there. For instance a positive charge, then the positive charge would experience a force exactly in the same direction as a field line because the tangential is in the direction of the field line, it would become accelerated in this direction and always stays on the field line. If I release it with zero speed, it'd start to accelerate and stay in this field line. In a similar way, if we think that the earth has a gravitational field, as the one we never use the gravitational field, but in physics, we think of the gravity also being a field. If I have here, a piece of chalk, the field lines, the gravitational field lines, here 26100, nicely parallel and straight, if I release this piece of chalk at zero speed, it will begin to move in the direction of the field lines and it will stay on the field lines. So now you can ask yourself the question, if I release a charge, will it always follow the field lines? And the answer is no, it's only in the very special case. Let's suppose now the field lines are curved, so here are the field lines, as you've seen in those configurations, it's very common. If now I release a charge here, say I have a point charge here, it will experience the force in this direction. So it will get acceleration in this direction, so immediately abandon that field line. So now if you ask me what is the trajectory rate of that charge, well, it could be very complicated, I really don't know. Maybe it's going like this by the time it reach this point, what I do know is the field force will be tangential to this field line, so will be in this direction. And so as it marches out and pick up the speed, locally it will experience forces represent with the field lines, and so the trajectory can be rather complicated. So field lines are not trajectories, and not even when you release charge with zero speed, only in case that the field lines are straight lines.
(20:42) Let's now look at the field configuration, which match well in itself with great maestros in some of these applications. Put there, with ratio 1 to 4, and rather, it is +4, +1 or -4, -1, is not important because that's just a matter of the direction of the arrows. But, that's why I didn't put arrows in, so I leave it up to you, if +4 and +1, you've to put arrows going onwards. And what you see now here is the air blow effect. Think of both of them being positive. There is +4, try to blow like hair dryer, and +1 is trying to find its own thing, so you get a field configuration, field lines which are sort of, no perhaps easy, but it's sort you can imagine why you have this peculiar shape. If you put a plus test charge in between the 1 and the 4, then the 4 will repel it, but the 1 also will repel it. And so it's going to be somewhere probably close to 1 whereby the two forces will cancel out. Therefore, E will be zero there. In a similar way, between the moon and the earth, there is a point not too far away from the moon where gravitational attraction came from the earth, and the gravitational attraction from the moon exactly cancel out. Those two are similar in this situation. So when you have charges with same polarity, you always find in between somewhere a point where the electric field is zero.
(22:29) Let's now go to a very special case whereby I make two charges, equal magnitude but opposite in sign, and we have a name for that, we call that a dipole. Plus charge (+) is here and minus charge (-) is there. Situation is extremely symmetric, as you would expect because they have equal power, there is one air blower upstairs and one vacuum cleaner downstairs. If you close to the plus charge, notice that all the field lines going away from plus charge; if you close to the minus charge, notice that all the field lines coming to the plus charge, you expect that. If you are far away from this dipole, now you have problem. Before we have +3 and -1, and if you far away, the plus three (+3) wins, so it's like having a +2 charge. If you far away, you always expect the electric field point away from the equivalent charge of +2. But if you add plus and minus and have an equal magnitude, lets say +1 and -1, you get zero. So, neither one wins if you far away. And notice carefully if you are very far away, indeed you do not see arrows in or pointing out, nor pointing in, cause it can not decide there is no one stronger than the other. That makes dipole fields very, very special. In the case of the +3 and -1, if you very far away, it's like having a +2 charge, and E field when you go further and further out, where one falls over r square. With dipole, your intuition sort of tells you that it will probable fall off faster than one over r square. And that is part of homework assignment that you have this week. If fact I can already give you the answer, but you have to prove it if you far away from an electric dipole, electric field falls one over rQ, if go faster, then one over r square.
(24:41) There is no single point in space where the electric field is zero. You can think about that why that is the case. So these field configurations can be rather complicated, and can be very interesting, and each one has its own applications. Are dipoles rare in physics? Not at all, in fact, they are extremely common you cannot avoid them. Remember last time, I told you if you have a spherical atom, or you have a spherical molecule, and you bring that close to a charge, let's now think of it, you bring it to an electric field, its another way saying the same thing. So we have a nice spherical atom or a nice spherical molecule, and we bring it into an electric field. The electrons want to go upstream the electric field vectors; they go against the direction of the electric field. And the positive charge wants to go in the direction, wants to go downstream. And so what are you going to do, the electrons will spend a little bit more time on one side of the nucleus than they would in the absence of that electric field. And therefore, you are through induction turning that atom, turning that molecule into become a dipole. If you have a little bit more charge on this side, average over time, you have the same amount of extra charge plus on that side average over time. So you make dipoles very often whether you like it or not. And later in this course, we will learn more about the polarization of atoms and molecules creating dipoles when we talk about dielectrics. And you see we will have enormous impact on the properties of the material.
(26:36) Could I make you a dipole here in class? Oh yea, that's very easy. To make one of non-conductors is not so easy in class; to make one of conductors is very easy. I am going to do that with these two spheres that we have. Look at these two metal spheres, conductors, free electrons, very easy for them to move. And I am going to bring this rubber rod which I will rub, I think I will get negative charge if I remember correctly, and I will bring that, say close to these two, which I touching each other. So here is one metal sphere, and here is another metal sphere, and here comes the rubber, negatively charged. Ah, what's going to happen? Electrons want to go away, so this becomes negatively charged, and therefore this remains a little bit positively charged. For every one electron that has excess here when I started it's neutral, there will be a positive excess there because charge is conserved, you can't create charge out of nothing. Now what I will do while this rubber is still here, while that rubber rod is there, I separate them. So here they have to be in contact first, they have to be in contact. (Wow, we got some visitors!) So what I do now is while this rubber rod is still in place, I take them apart, and when I take them apart, this negative charge is trapped and this positive charge is trapped. So I have thereby created negative charge on this one, positive on this one, and each with equal magnitudes, so I have a dipole. What I am going to demonstrate to you is that indeed, I have positive charge here, and negative here. That's a difference in polarity between these two, that's the way that I will do the experiment. I will not show you the amount of the charge are exactly the same on each which of course it has to be. So let me give you some better light, we have to get a few graph off the overhead.
(29:02) You see that for the first time of electroscope. We discussed last time. These are aluminum foil very thin with a metal rod next to it. And when I put a charge on the rod, it will also go into the aluminum foil, and they will repel each other, and so the aluminum tensile will go to the right. And the more charge on it, the farther it goes to the right. So let me first put these two together, make sure they are completely discharged. And now I am going to bring these two into an electric field, which is produced by this rubber rod. Have to rub it with cat fur, and I believe it was negative. But if you, you never have to remember it's negative or positive, of course that is not so important. What it is is a name after all. But it happened to be negative. OK, so now, we go here, I bring it here, I hope the noise part will fly over because that will ruin the demonstration, and now noticed what I do? While the rod is here, I separate them. So as I was holding it there, things were going on there that you and I couldn't see, but the electrons, the rubber rod is negative, the electrons were shifting into this direction, and this is now positive and that is now negative. If I take this one, and I touch it with electroscope, you clearly see that there is a charge on this. How can I show you now that there is a charge of different polarity on the other one? Well, the way I will do that is I will approach this electroscope by bring this sphere very close to it, and if this charge is different, then the charge that is on it, the electroscope will, the reading will become smaller. And why is that, why the reading will become smaller? Well, here is the situation of the electroscope now, and here is that ball you see on top, this is upside down there. If this is all negative, that's why it's apart. If now I approach this here with an object which is positively charged and I claim that this one now is positively charged, because this one was negatively charged, then electrons are afraid of positively charge, so more will go, excuse me, electron love the positive charge, so the electron want to come to the positive charge, so the electron drift down again. And so if they come down, fewer will be here, so you will see this. If however, I put here negative rod, then electrons, which are here, want to go further away, they will stream up, and therefore the reading will become larger. So you can always through induction test what the polarity is of your charge. Let's hope this one is still holding its charge while I was talking. So I claim out that if this polarity is different, and if it/s still there when I approach electroscope, come very close that the reading should become a little smaller without even touching it. Let's see whether it works. You see it goes down. You see it goes down, goes down. So through induction, I demonstrated that this indeed has a different polarity from this one. If I approach with this one, it will go further out, unless it is already a maximum. Let's try that. You see it goes further out. So not only have I demonstrated that I could created dipole, but you also see that by means of induction that you can demonstrate that there is a difference of polarity between the two spheres.
(33:11) If I created a dipole, and I put a dipole in an electric field. The dipole will start to rotate. Let's first talk about it why it rotate, then I will try to demonstrate that. By making a dipole, big one, this big, right in front of you, almost as big as the one over there. So let's have an electric field, like so, and I bring in this electric field a dipole, a biggie, here, this is the one I am going to use for demonstration. Ping-pong balls on either side, they are conducting, and they are connected with a rod, which is not conductive. And so here is a dipole, this rod in not conducting, and this is conducting, and this is conducting. And let's suppose this is positive, and this is negative for now, so this is how we get a charge onto it. Well, the positive charge will experience force in this direction, always in the direction of electric field, and the negative charge will experience the force always upstream. And now there is a torque come to this, and as the torque comes toward dipole, it will start to rotate clockwise. And of course, if it overshoots the field lines when it's in this direction, the torque will go reverse. And it's very easy to see. So what you will see is it's going to oscillate, and if there is enough tamping, it will come to hold more or less in the direction of the field lines. And this is something that I can demonstrate.
(35:00) First I will make a dipole of this kind, the way I will do that is following. This is metal ball, this is insulator, and here are these two ping-pong balls, the one on this side is yellow marker, and that side is an orange marker. And I am going to attach them, holding them up against this metal bar. Another words, here is this dipole, it's not a dipole yet, and here is the metal bar and is conductor, which connects them. I am going to turn on the Vandergraph here, and the vandergraph create electric field, so we have vandergraph here. Let's suppose this vandergraph creates positive charge, sometimes vandergraph creates positive charge on the dome, others can be designed to create negative charge on the dome. Remember for now I assume it's positive. What will happen now, electrons want to go into this direction, so this becomes negative, protons, positive charge stays behind, so that become through induction a dipole, because I have them connected, I have connected it with this metal bar. So these electrons can flow through this bar, and end up here. Now, I remove the bar. So when I remove the bar, I have created now a dipole, I have here, an insulating thread, and I have a fishing rod, and in the end of my fishing rod, I have now a permanent dipole. With the permanent dipole, I'm now going to probe the electric field around this vandergraph, I could've chosen the same vandergraph, but there is a reason why I pick this one. And as I walk around this vandergraph, you will see that this fishing rod on the end of this dipole, that the dipole always want to go radially inwards or outwards, depends on how you look at it of these two. I can probe this field, and make you see for the first time, that the reason, somewhere here a strong radio field going in or out of the vandergraph.
(37:25) And now comes something very interesting which I found out this morning the first time when I did this experiment. This, the other vandergraph, there, is also positive when I run it. How do you think this dipole is going to align then, if I walk into it? Will negative ball be closer to the vandergraph, or the positive one go closer to the vandergraph? So I give you thirty seconds to think about it. So I make the dipole, as it here, let's assume this one is positive, this vandergraph, so this side becomes minus, I call that A, and now this side become positive and that is B, I walk to this dipole, I bring this field, and let's assume that the one also positive, we don't know that yet, how will the dipole align of as A goes inwards or A goes outwards? Who says A goes inwards? Very good. Who says A goes outwards? OK, A will go inwards if the two vandergraphs have the same polarity. So if that doesn't happen, that doesn't mean physics doesn't work. It means two vandergraphs have different polarities. We'll see what happens. So, let me first then create a dipole. So here is the dipole, it showed out now, I turn on this vandergraph, so induction takes place, so remember that the yellow is pointing towards the vandergraph, and that the orange is away from the vandergraph. OK, so I induce a dipole, oh, I really should redo that. I don't know what happen to it. Let's remove the field first, ok, yellow was inside, right? Is that the way it was? Ok. Yellow inside, here we go. So now it's creating a dipole through this metal ball here, and I break contact, and this now would be a dipole. Now I turn on the field of the... So if the polarity is the same, yellow will go in. I will try to swing a little. Notice two things: it's going to line up radially, but the yellow is not in, yellow is out, so the two vandergraphs have different polarities. But you will see it rotate nicely, and they end up beautifully radial, and when I go all the way around here, again, they may swing a little, they may oscillate a little, but through temping they will come to halt, and look into it, a beautiful radio, and yellow is ß, so we speak the wrong side. Two vandergraphs have different polarities. So you see how we can create a dipole, and you have also seen how we often can make statement about the specific polarity.
(41:02) I can probe an electric field using grass seeds in oil. Grass seeds are elongated, and when I put grass seeds in an electric field, it will become polarized, there's nothing you can do about it. Here is grass seeds, and electric field is like so, and so the electrons want to go as far away in the direction as they can through induction, and so this side remains positive. And so what is the grass seed going to do? It's going to rotate. It's going to line up with the electric field. And this is the way, then I am going to show you now the field configurations in vicinity of a dipole, and I will also show you then the field configurations in vicinity of two charges, which have equal polarity. You may have seen this in high school with magnetic field with iron file, that's kids stuff, that's easiest thing to do. This is real thing with electric field I bet you have never seen electric fields, which are traced by these mysterious seeds. So I will give you some light that may optimize the demonstration. The seeds first have to be oriented in a way so that it is chaos. The first thing you see, is I am going to make this, I believe it's going to be a dipole first, almost certain. So I am going to charge one positive, and charge the other negative. And we will see how these grass seeds will form each other. Watch closely, there you go. My goodness! That is a wonderful dipole field. Of course we don't know which one is plus or minus because grass seeds have no arrows on them. But you clearly see, these incredible lines, radially inwards or outwards on each one of the charges. And then you see these nice arcs in between. You could see it easily. OK, you got something worth you twenty thousand dollars tuition. Put a little bit more charge maybe, very clear. And now, which is perhaps more interesting, I like to show you the field surrounding two charges, but now the charges are both the same polarity. So we have to undo the memory of the grass seeds. Ok, now we try to make both the same polarity. And watch the hair blow effect that I told you about. I am not sure they make contact. OK, we try again. Come on, it's very funny, you know, it looks like there are some charge hind because it doesnt look as beautiful as we had earlier on the Maxwell view graph. It seems like there is something here on the side, which we put first and therefore the electric field is being distorted. Let me try to discharge it. I am a reasonable conductor, so I should be able to take any straight charge off. Oh, wait a minute. Ha, I have it upside down. (Students laughing.) Oh my goodness, so they were never really in good contact. You ready now? Ah, look at that! Great, now you really clearly see this, these field lines you see in between, how these two air blowers are competing with each other. Very impressive! Alright, so that's the way you see field lines, now electric field lines, and some of you may have seen iron files magnetic field lines.
(45:48) If I have a vandergraph, and I have the vandergraph here, and let's suppose vandergraph is positive, I don't know whether it's positive or negative, and I suppose I'm going to use the one over there. And I am going to stand here on the ground, Walter Lewin, what is going to happen with me? Through induction, the electrons being sucked out of the earth, and coming up because they want to go close to the positive charge, so I become negatively charged. What will the field lines do? Oh it will be extremely complicated, very complicated, but something like this maybe. And something like this, uh, some may come out here, some maybe end up my neck here, some may go here, like so, very complicated field configurations. But I want to probe that field. Somehow a little, get a feeling for what the field is like. And the way I am going to do this is I am going to put a charge balloon, there you see the balloon it's conductor, I am going to put a charge balloon, and put it here, say, well, if it is a positively charged balloon, it will take off in that direction, right? The force is always toward the tangential lines. It will abandon the field lines it won't stay on the field lines, there's a lot of bending on the balloon, that's why I chose balloon, so it will move relatively slowly, and it will ultimately, maybe add up on my head, right here. Once it ends up on my head there, something maybe like this, now it will get negative charge from my head, so it will become immediately negatively charged, and so the force now will reverse, and go in this direction, tangential to this field line, and so it will go back, when it hit vandergraph again, it will get positive charge, reverse its polarity, and it will go back, so it will bounce back and forth between me and vandergraph. And give you some feeling of these field configuration is about, although I want to remind you that the charge does not follow exactly field lines. So I'm going to sit here, and I will be part of this. That is going to be positive, I will automatically become negative, there's nothing I have to do. All I have to do the turn on the vandergraph, and I have to put a little bit charge on that balloon, it will probably do that by itself, but I can always give it a little kick, so it goes to the vandergraph. (Student laughing.) Well, my glasses are good insulator, so I'm going to take my glasses off, so every time it hits me, changes polarity. So this is the way you can do physics and have fun at the same time. See you Monday!
Last Modified 2/22/06 3:43 PM |
