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Thread: Electromagnetic induction (faraday's law)

  1. #1 Electromagnetic induction (faraday's law) 
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    Hi, i am a secondary student and i have some doubts on Electromagnetic induction.

    It is clearly stated in Faraday's law that 'The induced electromotive force or EMF in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.'

    Now is there a way to explain in terms of electron or particles as to why this phenomenal?

    Secondly, as i searched on wikipedia, the explanation for lenz's law is that ' If the magnetic field associated with this current were in the same direction as the change in magnetic field that created it, these two magnetic fields would combine to give a net magnetic field which would in turn induce a current with twice the magnitude. This process would continue creating infinite current from just moving a magnet; a violation of the law of conservation of energy'

    I am unable to understand this explanation, can anyone please help me explain these 2 laws in greater detail?


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  3. #2  
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    Rest assured, induction is proven, and has been for years.

    I was looking for the animation on inducing current, but I can't find it anywhere. Basically you induce current by moving a wire through a magnetic field.

    It's hard to explain, but flux is like a liquid moving through some object. At any given point you have a certain amount flowing through the cross-section at that point. That amount is called the flux. Due to many factors, magnetic fields are not all shaped exactly the same, so when you move a wire through them, the amount of "field lines" you pass through changes. The more field lines you pass through, the higher the induced current is. There is one small caveat. Instead of current in the direction of the field lines, you get current in the opposite direction. This is the "equal but opposite reaction."

    Lenz's law is simple. "If you are inducing a current in an object, the current will be in the direction opposing the motion that created it."

    The bit that you're asking about is an explanation, poorly worded. What they should have shown is the math. Basically, induction cannot work if the induced current is in the direction of the motion. This is because if it were in the direction of motion, it would create a larger field in the same direction, which would create a larger current, which would create a larger field, until eventually it hit the mechanical limit. This breaks the conservation of energy law because you are creating energy.


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  4. #3  
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    oh i get it now. Thank you very much. Basically, it must oppose the motion so energy is not created. But is the energy lost in anyway?
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  5. #4  
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    Quote Originally Posted by tikai
    oh i get it now. Thank you very much. Basically, it must oppose the motion so energy is not created. But is the energy lost in anyway?
    Basically the electromotive force (voltage) around a closed loop is equal to the tiime rate of change of the magnetic flux through the area enclosed by the loop.

    A proper explanation of this requires the machinery of vector calculus. You will see it done when you study Maxwell's equations after you have the necessary mathematics.

    This is the principle by which electric generators work. Energy is never created, but mechanical energy (say from steam expanding in a turbine) is used to move a coil through a magnetic field and that converts mechanical energy into electrical energy.
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  6. #5  
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    Yes, some energy is lost. That is the nature of real systems.

    It's really hard to explain and understand without understanding 3D calculus in vector form. I'd suggest learning how to do that, then picking up Principles of Electrodynamics by Schwartz. It's a really good book, fairly cheap, and it goes in great detail.
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  7. #6  
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    thank you very much for all your reply. I have gained at least the basic idea on why how electromagnetic induction works. I have yet to learn any higher level mathematics and i will do so once my o'level examinations ends.
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