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Thread: Quick and silly question

  1. #1 Quick and silly question 
    Administrator KALSTER's Avatar
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    Electromagnetic induction = basically a magnetic field applying a force to the free electrons in the circuit, moving them along?

    Let's consider a dynamo setup (DC for simplicity's sake). Can the speed of the rotation be directly related to the speed of the moving electrons in circuit? Then basically the potential difference depends on the impulse imparted by the rotation? When the current is then passed through an appliance, the electrons emerging on the other side would be travelling slower than before entering the appliance, having lost energy?

    Also, the higher the amount of free electrons involved, the less speed is transferred to each (with a constant rotational speed), with a maximum wattage being attainable?


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  3. #2  
    Forum Masters Degree bit4bit's Avatar
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    electromagnetic induction is the production of an EMF in a conductor, when the magnetic flux through the conductor changes. This can be when the conductor is stationary, and has a changing current through it, or when the conductor moves through a stationary magnetic field...either way it happens when the conductor cuts the magnetic flux of the field.

    Faradays law says that the magnitude of the emf is proportional to the rate of change of cutting of this flux. i.e. E=dφ/dt, where φ is the flux through the conductor.

    The only reason a dynamo might be considered DC, is because it contains a commutator at its output which gives the effect of a 'rectified' waveform. To get it into steady DC you'd need to smooth it out with a regulating circuit. A generator without a commutator will give an AC waveform.

    For a coil rotating within a stationary magnetic field, such as a generator/dynamo, you can calculate the induced emf, depending on the area of the coil, the number of turns, the magnetic flux density of the permanent magnets, and the angular velocity of the coil.

    I wouldn't try to think of it in terms of the speed of electrons in the circuit, but rather, the rate of change of flux linkage through the conductor. Impulse isn't really something that comes into, since the conductors need not touch each other for inducatnce to happen.


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  4. #3  
    The Doctor Quantime's Avatar
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    You read just like my old physics textbook :P. Ah Electro motive force, thats an old one . Your talking about Lenz's law right? Doesn't that change when it comes to the transformer, or am I mixing that with flux? Loops?

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  5. #4  
    Forum Masters Degree bit4bit's Avatar
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    Well, Lenz's law just states that the induced emf opposes the change that caused it. Faradays law says that the induced EMF is proportional to the rate of flux cutting, or rate of change of 'flux linkage' for a coil, where flux linkage is given by, Nφ, and therefore the emf is given by E=d(Nφ)/dt. Combining Lenz's law with that simply gives the EMF a negative value. E=-d(Nφ)/dt.

    For a straight conductor moving perpendicularly to magnetic flux, at a constant velocity, the induced EMF can simply be calculate dby the product of the length, the magnetic flux density, and the velocity. E=BLv, since this really amounts to the same thing as dφ/dt.

    For the rotating coil, things are much the same, except we consider the horizontal and vertical components of the angular velocity, with respect to the stationary field. And of course, the greater the flux density, number of turns, and area of the coil, the greater the EMF will be. Notice because of the vertical/horizontal components, we use trigonemtry to express the rate of flux cutting, and so the EMF is expressed using a sin/cosine function, which gives you a sinusoidal wave, so you can express the EMF as an 'RMS' (Root mean squared) value. It's also intuitive when you think about whats happening with respect to the angle between coil, and magnetic field lines.

    The situation is the same for transformers, since the primary winding has an oscillating current, which produces an oscillating magnetic field, whihc in turn changes over the secondary winding, producing an oscillating current (Of the same frequency).
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  6. #5  
    The Doctor Quantime's Avatar
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    Where exactly is this part of physics used again? What kind of machines or common household objects use it, I can personally understand it better when I have a purpose towards learning it. Thanks for brushing up my knowledge for me .
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  7. #6  
    Forum Masters Degree bit4bit's Avatar
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    Well, where to start...

    Electric motors
    Electric generators
    Dynamos
    Transformers
    Inductors...having application in all sorts of things:
    -Electronic filters, LR and LC
    --used in radios/audio equipment among other things (EG LC tuned circuit for radio tuning)
    -RF chokes (isolate high frequency noise from one part of a circuit to another) like ferrite beads, that you find on computer mouse/keyboard cables etc..(little cylinder thing)
    -Series and parallel resonant circuits (RLC)
    Induction heaters
    Radio....including...
    -shortwave
    -medium wave
    -longwave
    -microwave
    ....so its' used in mobile phones, wireless internet, wireless anything really to some extent
    Also used in electronic actuators/solenoid valves etc.

    ...and proabably some more I've forgot...
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  8. #7  
    The Doctor Quantime's Avatar
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    So its definatley worth proporly learning it if you want to tinker with that stuff. Right O .
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  9. #8  
    Forum Masters Degree bit4bit's Avatar
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    Yeh definately. Since it's to do with electricity, most of its applications are naturally in electronics, so if you want to play with electronics, its definately good to know, especially when it comes to designing coils for various things. Tesla coils seem popular to build for instance.
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  10. #9 Re: Quick and silly question 
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    Quote Originally Posted by KALSTER
    Electromagnetic induction = basically a magnetic field applying a force to the free electrons in the circuit, moving them along?

    Let's consider a dynamo setup (DC for simplicity's sake). Can the speed of the rotation be directly related to the speed of the moving electrons in circuit? Then basically the potential difference depends on the impulse imparted by the rotation? When the current is then passed through an appliance, the electrons emerging on the other side would be travelling slower than before entering the appliance, having lost energy?

    Also, the higher the amount of free electrons involved, the less speed is transferred to each (with a constant rotational speed), with a maximum wattage being attainable?
    The electrons don't really build up much speed. They are constantly bumping into atoms of the conductor and just kind of migrate along. If it's alternating current, they are changing direction every 1/50 or 1/60 of a second.
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