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الموضوع: Lenz's Law

  1. #1
    مشرفة الاقسام الاكاديمية الصورة الرمزية تمارا احمد
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    افتراضي Lenz's Law

    Lenz's Law


    Energy Considerations - Where does all the Ek go





    If you roll a metal bar into a magnetic field it will slow and stop due to the force opposing its motion (the force that Lenz says will be set up).
    Where does all the kinetic energy of the bar go?

    Well in fact, the Ek is transformed into electrical energy.
    So this is the source of the emf, transferred from other energy into electrical energy.
    Other Applications of Lenz


    Pull this magnet out of the coil. This produces a changing flux in the coil and therefore induces an emf and current in the coil (so long as the coil forms part of a circuit). The emf will oppose the direction of the moving magnet.
    Question:
    What direction is the induced current?
    Answer:
    It opposes the movement of magnet. How? It sets up a magnetic field in coil with a North pole at the left hand end to attract the South pole of the magnet back into coil.
    If the magnet moved back into the coil then the current would have to flow the other way, so that the coil would be repelling the South pole of the magnet.
    Look at this - another common example...
    If you spin this wheel, it slows and stops quite quickly.
    Movement of the wheel through the magnetic field induces an emf and therefore a current in the metal wheel.
    Lenz's Law says that the induced current opposes the motion of the wheel (because it's this motion which is setting up the current).
    Which direction is the current?
    To understand this example, think of one 'spoke' of the wheel as it passes through the magnetic field. The 'spoke' moves through the field just as if it were a piece of wire moving through a field. The 'spoke' moves into the screen, the field direction is from left to right, so using Fleming's Right Hand Rule, line up your thumb (motion) going into the screen, your first finger (field) going from left to right and your second finger (direction of conventional current) should be pointing downwards.
    We've just used Fleming's Right Hand Rule to predict that a current flows from the axle to the rim of the wheel.




    Exam-style Questions


    1. A bar magnet is fixed on the end of a string to make a simple pendulum, as shown in the diagram.

      a) If a flat sheet of copper is placed just below the swining magnet, what happens inside the copper?
      (1 Mark)
      b) What effect does this have on the swinging magnet and why does it have this effect?
      (2 Marks)
      c) What happens if a glass plate is used instead of copper?
      (2 Marks)
      (Marks available: 5)
    2. A magnet is dropped through a wire coil. As it goes through a current is generated in the coil which produces a magnetic field near the coil.

      a) Which magnetic pole (N or S) is produced at X as the magnet goes into the top of the coil?
      (1 Mark)
      b) Explain your answer to a)?
      (2 Marks)
      c) Which magnetic pole (N or S) is produced at Y as the magnet leaves the bottom of the coil? Why?
      (2 Marks)
      (Marks available: 5)

    Answer


    Lenz's Law




    This is a 'stroppy' law. Basically, it rebels against everything! Lenz's Law can be described as:
    The emf is induced in a direction which opposes whatever causes the induction.
    Example:


    The thing causing the emf to be induced is movement of the bar to the left. So the emf induced will oppose this movement. Stop the bar and emf disappears.
    How could you oppose this movement?
    With a force to the right, of course!
    What could produce the force?
    A current flowing in a conductor in a magnetic field will produce a force. So, all we need is a current in the bar in the correct direction.
    Here's where Fleming comes to the rescue. Fleming's Right Hand Rule allows you to work out the direction of the current induced. (Very similar to his Left Hand Rule, but this time we are generating a current not a force.)
    As with left hand rule:
    Thumb = direction of movement or Motion
    First finger = Field direction
    Second finger = induced conventional current
    In the case of his Right Hand Rule, the second finger is the important one. It shows the direction of the induced current (conventional current) if you are moving in the thumb direction through a magnetic field in the first finger direction.
    You have to remember that:
    The Left Hand Rule is for the motor effect (it tells you what force will be produced on a current carrying conductor in a field) - remember motorcars produce forces and drive on the left!
    The Right Hand Rule is for the generator effect (it tells you in what direction a current is induced in a conductor moved through a magnetic field).
    Apply the Right Hand Rule to the diagram...


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  2. #2
    مراقب عام الصورة الرمزية Eiman
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    افتراضي رد: Lenz's Law

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  3. #3
    عضو مميز الصورة الرمزية قمر بلحاج
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    افتراضي رد: Lenz's Law

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