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Why does the right or left hand rule work? PLEASE read description

  1. Aug 12, 2013 #1
    I have seen a few threads on this already on the website,but the explanations kind of confused me. I know that this rule is used to predict the direction of a magnetic force. However this approach just seems so random to me. How come when we just orient the fingers on our right hand or left hand, we can "magically" predict the direction of a magnetic force and why do magnetic forces orient themselves in such a way. Also, though magnetic force is a cross product of 2 vectors, and the cross product produces a vector perpendicular to the other two, how do we know and why is the magnetic force is indeed perpendicular to the other two vectors. I get the convention part. For example the positive x direction is to the right and the negative x direction is to left, this aspect all just a matter of convention and can be done differently. However my question is WHY the left hand or right hand rule can be used to predict the direction of a magnetic forces and why magnetic forces orient themselves in such a way. I don't understand why when arrange your fingers in a certain orientation, you are able to predict the direction of a magnetic field. Your fingers don't know anything about magnetic fields. For me, using the rule hand rule is not logical and not a scientific way of predicting the magnetic field, if there was a scientific approach for predicting the magnetic field i would understand this concept of magnetism. I need to know the science and intuition of behind why certain things happen. For example, why is 250 x245=61250. Though there are certain techniques for multiplying large numbers fast, the basic principle is that if you add 250, 245 times you get 61250. This explanation of why 250 X 245 =61250 is a logical way of obtaining this solution. However, i don't get the intuition behind the right or left hand rule. How do we know and why is the direction of a magnetic force the same as a certain arrangement of fingers. Also, though a cross product gives a vector that is perpendicular to other 2 vectors how do we know and why is that the magnetic force is perpendicular to two vectors? The only answer I could come up with to these two questions is because of experimental research and observation we can confirm that magnetic forces orient themselves in the same manner as the way we orient our right or left hands. I don't have any answer though to why magnetic forces orient themselves in such a way and why they are perpendicular to other vectors.
     
  2. jcsd
  3. Aug 12, 2013 #2

    mfb

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    Physics cannot answer "why" questions. This is just an experimental result.
    If the force would not be perpendicular to velocity and the field direction, energy would not be a conserved quantity. We don't know why energy is conserved, but it seems to be a very fundamental conservation law.

    This is not a prediction. We observe that particles behave that way, and our fingers can be bent in such a way that they show the correct direction.

    Sure, but your mind knows which hand to use in which way.
     
  4. Aug 12, 2013 #3
    You have more or less answered your own question in the last couple of lines of your post.
     
  5. Aug 12, 2013 #4

    Dale

    Staff: Mentor

    This is also just a matter of convention. Physics would work just as well with a left-hand rule:

    Suppose that you have a straight wire carrying a current and suppose that you have a positive charge outside the wire moving parallel to the current. The wire and the charge identify a plane, orient the plane so that you are looking normal to the plane the charge is on the right side of the wire.

    Right hand rule: Using the right hand rule we determine the direction of the field, the magnetic field goes away from us through the plane. Now, using the right hand rule again we determine the force on the charge, the force is towards the wire.

    Left hand rule: Using the left hand rule we determine the direction of the field, the magnetic field now goes towards us through the plane, opposite of previously. Now, using the left hand rule again we determine the force on the charge, the force is again towards the wire.

    So the left hand or right hand rule is just a convention for keeping directions straight. The physics doesn't care about our hands and either convention works as long as we use it consistently.
     
    Last edited: Aug 12, 2013
  6. Mar 11, 2014 #5
    " For me, using the rule hand rule is not logical and not a scientific way of predicting the magnetic field, if there was a scientific approach for predicting the magnetic field i would understand this concept of magnetism. I need to know the science and intuition of behind why certain things happen."

    That is very right and right hand rule is not the reason why the magnetic field lines point in a certain direction. It's the experimental results with the help of compasses whose needle is a North pole and points in the direction of the field, and the only convention being magnetic field lines travel from North to the South pole. Thus when Faraday and Oersted did the experiments they figured out a connection between the orientation of the magnetic field with respect to the direction of the flow of the current. After having discovered the connection, they invented the right hand rule as it seemed to consistently work as it would help figure out the direction of either of magnetic field or direction of the current flow given the other. The experiments involve the patterns formed by the iron filings and compass needles deflecting and changing rapidly with reversing the current etc., Therefore, you are on the right track when you say that right hand rule is not a good enough explanation but nevertheless, it's a good tool to figure out directions when you are working in a lab as you know it to be true and tested now.
     
  7. Mar 11, 2014 #6
    You already answered your own question. Here's the quote
    Do you have any answers to why masses attract each other according to newtons gravitation theory? Does that bother you? Physics is an empirical science. No matter how sophisticated your theory at some level you must state that the theory is a good theory "because of experimental research and observation confirms it" to use your own words. That's is how physics is done.
     
  8. Mar 11, 2014 #7
    You do realize that physics is all about coming up with these rules to describe and predict the way the world behaves, right? That is the scientific way. Usually the rules take the form of equations (though it's not obvious a priori why mathematics should be useful at all). Sometimes we can simplify the maths or partially avoid it, instead using simpler rules such as the right hand rule, Smith charts or even Feynman diagrams.
     
  9. Mar 11, 2014 #8

    mfb

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    "global" did not visit this site in the last months, I think the thread is done.
     
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