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Rule of thumb

  1. Jan 22, 2010 #1
    I have repeatedly run across the right hand rule of thumb but rarely have i found anything on the left hand rule of thumb. any ideas as to why this is?
  2. jcsd
  3. Jan 22, 2010 #2
    Welcome to Physics Forums.

    Do you mean the rule relating the direction of the magnetic field around a wire carrying a current? This is sometimes known as the Right Hand Corkscrew Rule.
    There is also a rule called Fleming's Right Hand (Dynamo) Rule that tells you the direction of the current induced in a wire when moved in a magnetic field.
    Furthermore there is Fleming's Left Hand (Motor) Rule that tells you the direction of the force on a wire carrying a current in a magnetic field.
  4. Jan 22, 2010 #3
    If this question is about the rules learned in school during a unit on magnetism: Unfortunately, definitions depend on where you learn it. The procedure in New York State is that you have left hand rules for all purposes in high school, where the variable that flows in a conductor is always "electron flow", and right hand rules for all purposes in college courses, where the variable that flows in a conductor is "conventional current." Beyond that, they're usually numbered the same: rule #1 for the magnetic force on a moving charged particle or a stream of particles, rule #2 for the circular magnetic field produced by a straight current-carring wire, and rule #3 for the magnetic field produced by a current-carrying loop or coil -- but depending on the particular school you put the word "right" or "left" in front of the word "hand." In all other geographical locations besides New York, I have no idea what they do.
  5. Jan 23, 2010 #4
    This explains why students over here in the UK, when they google for answers to their questions on electromagnetic induction and the motor principle, get really confused when they find answers on American sites.
    Thanks for the info, it will help me explain this next time.
  6. Jan 23, 2010 #5
    In the instance of a strait wire when considering electron flow all one would have to do is change the direction of the flow to have the left hand rules apply?

    I'm primarily interested in coil to permanent magnet interaction. both motive and induced under ideal conditions. Ideal being permanent magnets within a electromagnetic field such as a solenoid driving the magnets through induction coils.

    Here is one scenario: two solenoids playing catch with a magnet. a tube two solenoids and an induction coil between them. barring the magnet from flipping to align opposite poles. The magnet would travel the length of the tube relatively freely. each solenoid is to arrest momentum and repulse the magnet to the other solenoid. Keenly putting the magnet through the induction coil in both directions.

    Would the solenoids have to be one wound left and the other right? Or might simply changing polarity be sufficient? Also would the induction coil be beneficial to be wound in a manner to utilize both directions of travel of the magnet? Is the flux field cutting across the induction coil more efficient in one direction than the other?

    That was just a scenario of two solenoids playing catch with a magnet.
  7. Jan 25, 2010 #6
    If you reverse the polarity applied to the solenoid, it will reverse the north and south directions of the magnetic field.

    If you replace the solenoid with another one whose winding is the mirror-image of the winding of the first solenoid, it will reverse the north and south directions of the magnetic field.

    If you make both of those changes at the same time, reversing the polarity and also reversing the direction of the winding, then the north and south directions would be unchanged.

    To determine who attracts or repels whom, you can pretend that you have all bar magnets, and not worry about having a mixture of bar magnets and solenoids.
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