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Lenz's Law of Electromotives

  1. Dec 8, 2012 #1
    The Law: The direction of the induced current creates an induced magnetic field that opposes the motion of the inducing magnetic field.

    Application: When the north pole of a magnet goes INTO a solenoid, the opening where the magnet is entering from becomes South. As it exits, the same opening becomes N.

    So what I understand is that electrons will move in order to oppose the intruding magnetic field in order to cancel it out? Is that correct?

    And in the application, how does creating a south pole cancel out the intruding north pole? And vice-versa?

    Someone before directed me to a great java applet and diagrams to explain ostred and faraday's principles. Those would be helpful if you know of any.

    I need to build a motor and knowing how one works would be nice before I make it.
     
  2. jcsd
  3. Dec 9, 2012 #2

    Drakkith

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    The magnetic field of the magnet induces a current by applying a force on the electrons causing them to move. It isn't the presence of the magnetic field by itself, but the CHANGE in the magnetic field as the magnet is inserted that creates the electromotive force (EMF). As the electrons are accelerated and start to move they produce a magnetic field of their own. As this field grows it applies a force on the electrons that is opposite of the magnets force and causes resistance against the change in current. (Because the current is increasing as we insert the magnet) This is known as counter electromotive force (CEMF or Counter EMF)

    The key thing to understand is that the magnetic field from the coil doesn't oppose the magnet's field itself, but the change in current flow through the coil. As the magnet creates current the current creates a magnetic field that opposes the current in the first place! This is known as self inductance.

    If you simply insert the magnet and stop, the induced current will initially be zero and slowly increase as if the coil had lots of resistance. As the rate of increase in current falls, the rate of change of its magnetic field does so as well, leading to less counter EMF, which leads to less resistance against the increase in current. The result is that the current increases to 63.2% of maximum over 1 "time constant". After another time constant the current has increased another 63.2% of the remaining, leading to a total current of about 86.5% of maximum. After another time constant current will be 95% of maximum. After 5 time constants current can be said to be maximum. I believe the amount of time that one time constant takes depends on the inductive reactance of the inductor. (The coil) Generally it's a very short amount of time, perhaps millionths of a second.

    Once the magnet is inserted fully its magnetic field is no longer changing, and no longer creates current flow in the coil. As the current flow starts to drop, the magnetic field it produces starts to fall as well. Uh oh! A changing magnetic field! This changing field is the same polarity as it was before, because current is still flowing in the same direction, but this time the field is falling off in strength instead of increasing. Because of that the EMF it produces is opposite from before. It now pushes the current in the same direction as it is already flowing instead of the reverse direction. This means that current flow doesn't immediately stop, but instead decays over time JUST like it took time for it to build up in the first place! How long? Exactly the same amount of time as before with our time constants! After 1 time constant the current has fallen 63.2%. After 2 time constants its fallen by 86.5% and etc.

    When you remove the magnet from the coil the same thing happens except the induced current is now opposite, which means the field it creates is opposite as well.

    Browse around this site: http://www.magnet.fsu.edu/education/tutorials/java/electromagneticinduction/index.html
     
  4. Dec 9, 2012 #3

    sophiecentaur

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    Sorry to be putting a downer on the poor old electrons again but, if you insist on discussing what happens to electrons in a generator / motor then you are assuming that erroneous model of electrons all going in the same direction through a conductor. We all know that the electrons have a very wide velocity distribution (all directions) so you would need to include what happens for all possible vector values of velocity. Why not stick to good old Electric Current or, perhaps, Moving Charge? Is it really so intellectually challenging to do things in the conventional way?
     
  5. Dec 9, 2012 #4

    Drakkith

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    What are you talking about?
     
  6. Dec 9, 2012 #5

    sophiecentaur

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    I'm talking about the fact that people seem to prefer a dodgy model to one that 'all the experts' use. The number of times that 'electrons' are used in serious discussions about electrical matters is very small indeed. Why do you think that is?
    The only reason I can think that people want to talk in terms of electrons for this sort of topic must be that it is somehow more comforting, concrete, familiar (?) and that the conventional description is too intellectually challenging to use. In fact, it's not particularly difficult; when I was at School, we just talked 'current' and got on with it. The particle model, (as with light, also), is full of pitfalls and can let you down very easily.
    This is not a specific objection and is certainly not 'personal' so please don't be offended but, unless the conventional model is too hard, why not use it and why promulgate a less satisfactory one? Does it really help when people are aiming to talk at the level of PF?
     
  7. Dec 9, 2012 #6

    Drakkith

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    What conventional model? This is literally the way I was taught last week. Well, mines a little more in depth than how I was taught, but it's pretty much the same.
     
  8. Dec 9, 2012 #7

    sophiecentaur

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    Does Maxwell discuss electron flow or Current? The very fact that it turned out that electron flow is in the opposite direction to conventional current direction yet everything works fine according to his rules shows how electrons are just not necessary.
    If you are being taught electricity in terms of electrons then treat it with a bit of skepticism because a time will come when, at the very least, the sign will give you a problem.
     
  9. Dec 9, 2012 #8

    Drakkith

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    I have no idea, I've never studied maxwell. And what sign are you referring to?
     
  10. Dec 9, 2012 #9

    sophiecentaur

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    When (negatively charged) electrons flow from left to right (+x), the conventional Current (in terms of positive charge flow) flows from right to left (-x). If, in one breath, you talk of Current and the next you talk of electron flow, then one day, your well designed motor will go the wrong way round or your battery charger will discharge your battery.
    When they start to sell Current Meters with 'electron flow' marked on them and the reading comes out in 'number of electrons per second, I may reconsider my attitude. Twenty million flies can't be wrong and neither can twenty million Electrical Engineers.
     
  11. Dec 9, 2012 #10

    Drakkith

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    I'm sorry Sophie I really don't see the issue here. Current is the flow of charges, and electrons are charges. How could that possible cause a problem? I was just taught that electrons flow from positive to negative in a circuit, and as long as you follow all the normal rules and orient everything the right direction everything works out fine. Is that correct?
     
  12. Dec 9, 2012 #11

    Bobbywhy

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    At the referenced website http://www.magnet.fsu.edu/education/tutorials/java/magwire/index.html
    on the page called “The Magnetic Field Around a Wire,” it is written:

    “Plus and minus signs indicate the poles of the battery (not shown) to which the wire is connected. The conventional direction of current flow is indicated with a large, black arrow. (As convention dictates, the current flow opposes the actual direction of the electrons, illustrated in yellow).” The yellow electrons are flowing from negative to positive.

    Cheers,
    Bobbywhy
     
  13. Dec 10, 2012 #12

    sophiecentaur

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    You have totally proved my point here, D. Electrons flow from Negative to Positive because they are negatively charged. The Sign has confused you as I predicted!
     
  14. Dec 10, 2012 #13

    Drakkith

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    Bleh, I'm not confused, I just can't think when I'm confused. Wait...
    Anyways, typos aside, I was taught current flows from negative to positive, and current is electrons. If that's wrong I blame the Air Force for teaching it that way!
     
  15. Dec 10, 2012 #14

    sophiecentaur

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    I can only hope they got their navigation directions the right way round. I think you need to.brush up on that stuff about conventional current. Wiki will help, I'm sure.
     
  16. Dec 10, 2012 #15

    Drakkith

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    I know about conventional current thanks to my own reading prior to this. I just thought you meant something else initially when you said "the conventional way". Seeing as how my own training doesn't use it and multiple books, including my Basic Electronics book I bought a year or two ago uses moving electrons as current I really don't think there's a problem here unless the OP simply fails to learn the basics of electronics at all. Honestly I think you're getting too worked up over something tiny that isn't even incorrect. This IS how I have been taught, and it's the same way that multiple sources teach it, so I fail to how the "conventional way" is somehow more correct. (Especially since you need to know electrons move in a diode and transistor which we just covered)

    HOWEVER, I admit that my experience in electronics is far far behind yours, so I may be underestimating how often confusion over the two different explanations happens. But I have to ask, why teach it two ways in the first place?
     
  17. Dec 10, 2012 #16

    sophiecentaur

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    If you don't use conventional current, but a version of your own, which is backwards, you get equations like
    R=-V/I
    for instance, which you would not, presumably, say was correct and you would never use.

    If you have heard of Flemming's Right Hand Generator Rule, you would have to rename it the Left Hand Generator Rule (which is the Motor Rule). It would be total mayhem. You must surely realise that the 'direction' that things (numbers and mechanisms) go is very relevant.

    Imagine, for instance, and to take a topic with which you are familiar, that someone decided that a larger (i.e. more in the positive direction) Magnitude for a star meant that it was brighter to look at. You would, understandably, pick them up on it because it's a totally misleading way to use the term 'Magnitude'. They'd be getting the sign wrong.
    Getting the sign of electric current right is at least as, if not more important than getting star magnitudes right. You can't have current going either way, according to taste and I would bet that your memory is misleading you when you say you were actually taught that electrons flow from positive to negative. Have you some notes that you could refer to? What happens in a cathode ray tube or an old fashioned radio valve? Are electrons released at the Cathode of the Anode? If you don't have an answer to those questions then you should really accept that I'm right about this and run away quickly to verify it.

    It is not just me being picky. I know I can be a grumpy old sod but I really do have a valid point here. Find me just one proper reference that says the electron flow is the same as the current flow and I will show you an armful that will put you right.
     
  18. Dec 10, 2012 #17

    Bobbywhy

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    Drakkith,

    Just to reinforce your argument, I was taught electronics by the United States Navy. Electrons within a circuit flowed from negative to positive, and that was called “current flow”. Furthermore, in vacuum tubes (I know, now you can guess how long ago that was!) current flow was from cathode to anode. That same rule applied to a silicon diode and a transistor. They taught us current flow is “against” the arrow. It all seemed logical since we believed electrons were negatively charged.

    Many years later I read that “conventional current flows opposite electron flow”. I have never understood why.

    Cheers,
    Bobbywhy
     
  19. Dec 10, 2012 #18

    sophiecentaur

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    I think that the US services must be functioning 'inspite of' what they teach their staff and not 'because of' what they teach them, if what you both say is really true. I presume that they do not supply service personnel with 'special' ammeters, with the Red and Black terminals reversed so how do they ever manage to check that things were working properly?

    I repeat my challenge to Drakkith to produce a credible reference with this daftness in it. PF is supposed to be above this sort of thing. How could we discuss Cosmology and Quantum Theory if we were so 'approximate' in our ideas?
     
  20. Dec 10, 2012 #19

    Drakkith

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    Why would I use negative volts? We didn't learn it like that and I don't see how current being electrons could make it negative anyways.

    Of course it's important. That's why it's important to understand that whatever rule you are using is either in "conventional" current, or electron flow. (Or whatever you want to call it)


    No, it was a simple error when I typed that. Electrons flow from negative to positive as I said in my other post. That is how we were taught.

    Cathode to anode obviously.

    Uhhh...everything that explains what current is??
    http://en.wikipedia.org/wiki/Electric_current

    Sophie you have me seriously confused...
     
  21. Dec 10, 2012 #20

    Drakkith

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    Perhaps we have a misunderstanding here. CONVENTIONAL CURRENT is defined to be from positive to negative. The electrons are NOT flowing this way. They are flowing from negative to positive. We did not learn CONVENTIONAL current. We learned just "CURRENT", and that it is electrons flowing from negative to positive. Nothing changes, you don't switch your leads or anything else.
     
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