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Direction of motion of conductor (force)

  1. Jan 20, 2015 #1
    I can't understand this. When I think of a conductor I think of a cable, and last time I checked they are pretty static. So what is this "force" in practical use?
     
  2. jcsd
  3. Jan 20, 2015 #2

    mathman

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    The force has to do with the motion of electrons within the cable.
     
  4. Jan 21, 2015 #3
    I'm trying to understand EMF, I understand current flow - which is the direction of the flow of electrons, I understand the field which goes from north to south, the only thing I can't grasp is the motion of a conductor, I cannot imagine it in my head, what does it mean?
     
  5. Jan 21, 2015 #4

    DrGreg

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    In an electric motor, current through a conductor (a coil of wire) causes relative motion between the coil and the magnets in the motor.
     
  6. Jan 22, 2015 #5
    Alright, I absolutely do not understand what a relative motion means, you guys must be trolling me. If I may rephrase myself, what does the upwards or downwards motion of a conductor in Fleming's left hand rule means?
     
  7. Jan 23, 2015 #6
    That’s because Fleming’s left hand rule for motors applies to electric motors, not single strips of wire like a cable. The magnetic field around a thin wire is much smaller than the electric field (I think the speed of light times smaller). In general the effect is very small and not noticeable.

    With electric motors it’s a different story. The motor consists of electromagnets and the magnetic effect is very much more powerful. To see how this is done just look up electromagnets..
     
  8. Jan 23, 2015 #7

    sophiecentaur

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    That really isn't true. The basic statement of Flemming's LH rule refers to a single conductor in a magnetic field, with a current running through it. An actual 'Motor' is much more complicated than that but each wire in it will obey Flemmings LH rule. The word "motor" means "causing motion" in this context and not an object you connect up which turns a wheel. (See 'Motor Nerves' in biology)
    The Magnetic Field and the Electric field will each depend upon the Current, in one case and Electric Potential in the other. They can be varied quite independently. It is actually much easier to produce a significant magnetic field in a circuit with a single wire and a low voltage battery than it is to produce a measurable Electric field because this would involve a High Voltage Power supply. Otoh, it is fairly easy to produce an electrostatic charge by rubbing a piece of plastic.
     
  9. Jan 23, 2015 #8
    They are static when they don't move. It does not mean they cannot move.
    Here is an experiment showing how it moves:


    The usual electric cables are not in strong magnetic fields. And besides, they are made from two wires, carrying currents in opposite directions. And most of the time teh current is AC so the force will change direction too fast for conductors to follow.
    But with the right conditions, the conductor will move.
     
  10. Jan 23, 2015 #9
    Yes, of course there is a magnetic field around a single wire (and Fleming's left hand rule applies) as I indicated by pointing out how much weaker the magnetic field is to to the electric field. I referred him/her to look up electromagnets to see how the magnetic effect can be greatly increased by looping the wires many times, and by wrapping the wires around a ferromagnetic core etc.

    I did not know that the magnetic field can be increased while at the same time keeping the electric field low. Thank you. I do need to study electricity/electrictonics more.
     
  11. Jan 23, 2015 #10

    sophiecentaur

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    A 12V car battery will easily put 200A through a single thick wire and it will seriously twitch if next to a similar wire. The Force is BIL, where B is the Field, I is the current and L is the length. To get a similar 'twitch' with two high voltage wires, you need a fair old voltage. Under normally achievable conditions, the Magnetic Motor effect is much more significant - which is why they do not use Electrostatic motors.
     
  12. Jan 27, 2015 #11
    nasu, that is exactly what I needed to see to fully understand emf, a picture is worth a thousand words. Thank you.
     
  13. Jan 28, 2015 #12

    sophiecentaur

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    The same can be said about a Mathematical Equation, which will apply in All Cases. Maths is a language that expresses Physical concepts better than other media. People are only suspicious of Maths when they do no 'have the Maths'. It's more than a aid - it's sine qua non, as Julius Caesar was heard to remark.
     
  14. Jan 29, 2015 #13
    Well, first you need to understand all of those math symbols, get into habit of knowing them and how they interact with each other. I guess it's hard for people to get into math because it takes so much mental resources and leaves so little for the fun of not knowing, this is why people are more fascinated by illusionists rather than mathematicians. Of course I'm not saying that this statement is justified, rather this is just how things are.
     
  15. Jan 29, 2015 #14

    sophiecentaur

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    Of course. But people seem to think that anyone can do anything, despite their ability or how much effort they put in. No one would think they could be a concert pianist without a load of graft and an equal load of ability. There are so many posts from people who 1. Reject Maths as being unnecessary 2. Want a "Physical Explanation" of some phenomenon. That approach just will not deliver the goods. I have no problem with people not having the Maths - just the attitude that, in fact, belittles the whole of Science to the level of a video simulation.
    I like the "illusionists" statement. Spot on!
     
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