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Electric Motors - Which way the force goes.

  1. Jun 8, 2003 #1
    I uploaded the question i have to answer.

    I am given a daigram of a simple DC electric motor.

    I am asked to find which way the coil will spin at certain points

    I have been using the right hand palm rule where my thumb points in the direction of the current and my fingers in the direction of the field. Thus the force exerted on the coil at certain poits will be in the direction of my palm.

    The field always goes from North to south. and the current is shown so at point X the force should be out of the page. But the answers say its into the page.

    is this right?

    Attached Files:

    • q.jpg
      File size:
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  2. jcsd
  3. Jun 8, 2003 #2
    well, your upload thingie dont work and i cant post images so i will give you a link to the question

    http://cummingsiam.steven-sst.com/school/q.jpg [Broken]

    may i sujest you allow images cos it makes it soooo much easyer.
    Last edited by a moderator: May 1, 2017
  4. Jun 9, 2003 #3
    Yes, into the page. I can only think that you've got your fingers the wrong way round or something. First finger field, second finger current as I remember being taught
    Last edited: Jun 9, 2003
  5. Jun 9, 2003 #4
    argh. I use the right hand palm rule where my thumb is the current through the wire, my fingers are the field(from north to south) and the force will be in the direction of my palm.

    Does this way work with motors?
  6. Jun 9, 2003 #5
    I've never heard of the right-hand palm rule as you described it [?].

    I only heard of RH-rule for motors (thumb - movement, first finger - field, second finger - current), LH-rule for generators.
  7. Jun 9, 2003 #6


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    Always remember current flows from negative to positive depending on your battery position. This is your curent direction (I), i see at X that the B (magnetic field)is pointing in a curling upward motion like position C. Your hand will be actually wrapped around the wire since thats the way your thumb points (think placing your hand on the wire like your grabbing it) see the direction now as the magnetic lines curl upward. Since my right hand thumb points towards the positive terminal of the battery current flows in that direction and B is fluxing in that upwardly motion as shown in your pic.

    Cool pic BTW.
    Dx :wink:

    Ask me anything about electronics, its my passion. Oh and computers!
    Last edited by a moderator: Jun 9, 2003
  8. Jun 9, 2003 #7
    umm, i have been tought that electrons flow from the negative terminal to the positive terminal. As electrons are negative charges, this is the negative flow.

    Current is determined by the positive flow - oposite to that of the flow of the electrons Thus going from positive to negative.

    Also, the diagram states the current is in the direction of the arrow (positive to negative)

    this question seems to be vert troubling. My teacher sais that my way is correct......that at X the force will be upwards
  9. Jun 10, 2003 #8
    Yes, DX, convention current flow is positive to negative - think there's an arrow on the diagram. I think when they discovered that it really flowed in the opposite direction they'd already written too many texts with the current the 'wrong' way round they kept it as it was, lol :wink:
  10. Jun 10, 2003 #9
    ie. this is LH-rule (!) Kind of alarming how I'd forgot the difference from not doing it for a few years lol.

    I tapped google and also found a little on this RH palm rule - again disturbing that we were never taught this in school . Check out
    http://www.bearwoodphysics.com/ph4.5.htm [Broken].

    So after all that lol seems like you are right - force is out of the page.

    hope that eventually helps:wink:

    edit:to add some more, the RH palm rule is used to find the direction of field around a current carrying wire, you can see in the link.

    Flemings left/right hand rules are when you have a conductor in a magnetic feld. LH when the wire carries a current and experiences a force due to the B-field, the RH rule to find the direction of the e.m.f induced in the conductor when it is moved perpendicular to the B-field.
    Last edited by a moderator: May 1, 2017
  11. Jun 10, 2003 #10
    well, at least we got all that sorted out. I was looking through past examinations and my way corresponded to the correct answers so i was going to stick to it anyways.

    Thanks for your help tho, this site is very, very handy.
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