Signs (pos/neg) with electricity and physics

Discussion in 'Homework Help' started by poopscoop, Oct 3, 2013.

  1. poopscoop

    Thread Starter Member

    Dec 12, 2012
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    One day I'm going to get caught ignoring signs until the end of the problem.

    When dealing with electric force on a charge in an electric field, what direction is considered to be positive for work and force?

    Is there a general rule on electrics for positive and negative direction of the field and forces?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    The signs drop directly out of the definitions.

    From a fundamental view, you want to focus on the force that is applied to charged particles as a consequence of the presence of other charged particles. Then you just need to keep in mind that a force acting on an object is always in the direction that the object would accelerate if that were the only force acting on it.

    For work, a force acting in the direction of a displacement does positive work.
     
  3. poopscoop

    Thread Starter Member

    Dec 12, 2012
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    Would I be correct in saying that a particle accelerating AWAY from a positive field would be in the positive direction, and a particle moving TOWARDS a positive field is in the negative direction?
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    What do you mean by a "positive field"?

    Also, wouldn't it depend on if the particle in question was positive or negative itself?
     
  5. studiot

    AAC Fanatic!

    Nov 9, 2007
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    The direction of an electric field is from positive towards negative (or less positive).

    So it is outwards from a positive charge and inwards towards a negative charge.

    For magnetic fields the direction is from north to south. Or more correctly from a north seeking pole towards a south seeking pole.

    Bearing in mind that fields are space filling and vary their direction in space, 'positive' and 'negative' means with the field or against the field and may vary with direction in space.

    The force is given by coulombs law and is attractive for unlike charges and repulsive for like charges, and actually acts on both charges.

    We sometimes consider one charge 'fixed', say the positive charge, and call the other a 'test' charge.

    Then the field will be away from the positive charge.
    If the test charge is negative it will feel a force towards the fixed charge ie against the field direction.
    Conversely if the test charge is positive it will feel a force in the same direction as the field (away from) source charge.

    This convention gives the correct signs when substituted into Coulombs Law.
    It also requires the calculation of work or potential to include a negative sign.

    Does this help?
     
  6. poopscoop

    Thread Starter Member

    Dec 12, 2012
    139
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    That does help quite a bit. Many thanks.
     
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