Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Correct electric charge/current flow & left/right hand rule?

  1. Feb 20, 2015 #1
    Hello there, people! I am quite new here; in fact, this is my first post. I am quite strange to this place. Feeling kinda welcome, though.

    Okay, so short to the point. I am an 8th grader, and my school just started teaching us about electricity. Well, the basics. I knew much about it, much further than what they have taught. It is all cool until I read about those conventional and real flows, right and left hand rules online (self study) started messing with my logics.

    Let me nail it; please correct me if I am wrong.

    Electric Charge

    In conventional flow, the charge flows from positive to negative.
    In electron flow, the charge flows from negative to positive. ( < I prefer this much more than the former)

    A positive terminal has holes, or molecules/atoms with missing electrons, especially metallic substances.
    A negative terminal has extra electrons, or molecules/atoms with excess electrons, especially, non-metallic substances.
    Thus, the electrons from the negative terminal flows to the positive terminal to fill up the holes.

    Well, this is very clear, right?

    Electric Current

    This was where it started to get confusing.
    I mean, electrons flow from negative extras to positive holes, from negative to positive, right? Then, of course, the current, too, must flow from (-) to (+). BUT why the other way around? I see many people online claiming that it flows opposite to the charge, but no one has explained why.

    The Left/Right Hand Rules

    I was stubborn and way too curious. Without clearly knowing the rules of current flow, I decided to step into the left/right hand rules. Which, of course, just lead to more confusion.

    What are Fleming's left/right hand rules, exactly? Left hand is used in motors, right in generators. But why? What is the difference? Does it depend on the current flow? Can you depend the direction on electric charge flow?

    This is really confusing:
    >http://goo.gl/2nK2tv
    >http://goo.gl/PF4kwH

    Plus, as we all know, there is a force field extended across a wire if a current pass. The force is shown by some arrows and circles. But what does that mean? Is the pointed direction of the arrow... the north pole? Or the south? Or none at all?

    Another thing. The winding of a wire over a core. Does winding clockwise/anti-clockwise matter? Does it affect anything?



    I know, that was a ton of questions, probably with bad English, which I please you to forgive (not a native speaker; I am from SE Asia). You can answer any. I am really desperate to feed my brain.

    External URLs are appreciated.

    Thanks a ton, in advance! Have a nice day! :D
     
  2. jcsd
  3. Feb 20, 2015 #2
    When current flows through a conductor (something that has free electrons excess like a metal), a magnetic field is produced around it (i.e., the conductor acts like a magnet). This is the principle that is used in electromagnets (those huge magnets that are used in cranes in dumpsters). The fleming's left hand rule is a rule that shows in which direction the conductor with current flowing through it would move if you put a magnet near it. If you hold the fingers in such a way that your center finger points towards the direction of flow of current, your forefinger points the direction of the magnetic field produced by the magnet, then the conductor would move in the direction that your thumb finger points.
    For more information I would suggest you to take a look at chapter 12 and 13 of the book whose link I have add Here:

    http://www.ncert.nic.in/ncerts/textbook/textbook.htm?jesc1=13-16

    Next the arrows in the current flow in just an internationally accepted convention there is (as I belive ) no special reason to do so
    Thirdly, If you are interested in electricity, you should also know magnetism they are interconnected like a huge fishing net. If you are interested in electricity the automatically and most probably also interested in magnetism that is the reason they call this study electromagnetism
     
  4. Feb 20, 2015 #3

    Thanks a lot mate! That sure was a lot helpful! The link was very nice too.
     
  5. Feb 20, 2015 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    The electron wasn't discovered until 1897 and by Murphy's law turned it out to have negative charge,of course after the pioneers had already developed their formulas around positive..

    You'll want to become conversant if both types of current, negative and conventional. That's because in your career you will encounter people who've been trained one way or the other, so if you are to be effective with both groups you must be "bilingual" in that regard.
    It gets easy with practice.

    Very early transistor manuals (like 1959) have pages dedicated to helping engineers make the switch.

    And be careful about water analogies. Since water always falls back to earth many people assume (mistakenly) that electricity has some magical affinity for ground. The "gravitational potential energy" analogy to voltage reinforces that mistake.
     
  6. Feb 20, 2015 #5
    Exactly what I thought about the water analogy the other day! Maybe teach kids that the ground is... a positive terminal and the water tower is the negative? And that the height of the tower= charge, and amount of water= current? That was a good thing you mentioned.
     
  7. Feb 20, 2015 #6
    The thing is... why does the current flow the other way around? Or is it because people still use this, due to how people back in the day used these kind of conventional flow rules and made laws like left/right hand rules, and that we don't want to adapt into a newer, more accurate version?
     
  8. Feb 20, 2015 #7

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Left and right hand rules dont work for me because i'm too , well, shall we say challenged? to remember which finger is what property and which type of current left or right applies..

    When you take calculus(i'm told nowadays they teach it in high school !) you'll learn vector cross products, which are easier than the name implies....

    watch this:

    Lorentz in words:
    "Force F on a charge Q moving at velocity V in field B is Q V cross B",
    in mathspeak:
    F= Q X (V cross B) , and (V cross B) is called the "vector cross product"

    Since that's an academic definition you'll apply it to the charge used in academic circles, conventional aka positive, charge.

    Commit this sentence to memory:
    "Direction of a vector cross product is that taken by a right handed screw rotated in direction of first vector into second ."


    In this picture
    vvec5.gif

    Observe that a right handed (conventional) screw when rotated counterclockwise as shown , will move in direction indicated by the black arrow.
    If it helps you to imagine this, think of arrows A and B as indicating the initial and final directions of the slot in the screwhead.
    That's a powerful word picture that'll serve you well. At least it served me well.

    F = QV cross B
    Conventional charge,conventional screw, rotate V into B. Practice it.


    Using just that you can explain why a motor makes torque and why as it begins to turn it develops counter EMF.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Correct electric charge/current flow & left/right hand rule?
  1. Right hand thumb rule (Replies: 21)

Loading...