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

Question about voltage polarity and power

  1. Oct 29, 2016 #1

    If a positive charge is moved from point b to a, then we say a is positive to point b, and assign negative polarity to point b.
    If a negative charge is moved from a to b, we still say the same.

    Why? Please explain in simple terms if possible. Another question, if current and voltage polarity are in opposite directions of a circuit component it supplied power to the circuit, and if negative it takes power..why?
  2. jcsd
  3. Oct 29, 2016 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    If you have a positive charge - then wherever that charge is, is positively charged. Wherever it used to be, if there was nothing else there, is neutrally charged. Everywhere the charge is not, is neutrally charged.

    If you have a negative charge - then wherever that charge is, is negatively charged. Wherever it used to be, if there was nothing else there, is neutrally charged. Everywhere the charge is not, is neutrally charged.

    If we have two neutrally charged materials, A and B, however, and we take a positive charge from B and place it on A, then, A becomes positive because we added a positive charge and B becomes negative because we removed a positive charge (an extra negative one was left behind). If we had, instead, removed a negative charge from A and moved it to B, then B becomes negative because we added a negative charge, and A becomes positive because there is now a positive charge left behind that was previously cancelled by the negative charge we just moved away.

    We are not "assigning" the polarity here - what happened is a consequence of there being an equal number of positive and negative charges to start with.

    That help?
    Last edited: Oct 29, 2016
  4. Oct 30, 2016 #3
    Yes, it was an excellent explanation!
    thank you. :smile:

    But about polarity, this is what scams outlines says..that if moving a positive charge from b to a results in work done, then a is assigned positive voltage polarity and b negative. I dont completely understand why.
  5. Oct 30, 2016 #4
    In particular I never understood this...when we say in conventional current that the positive charges flow to the negative, do we mean:

    1. The positive charges are repulsed by the positive concentration towards the negative.
    2. The negative charges attract positive charges towards them.
    3. Both.

    I know voltage is with respect to a point in the circuit, usually the source of voltage. Why is it called voltage rise when a positive charge goes from point b to a? (b is negative while a is positive polarity?) does the polarity of voltage mean higher potential vs lower potential>

    I understood this: The passive sign convention states that for any device the direction of voltage (- to +) is opposite to current flow, which fits in with the model of the charge. I just want to know why because I feel very confused.
    Last edited: Oct 30, 2016
  6. Oct 30, 2016 #5


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    You need to avoid getting into generalisations here. A Current, in the form of a flow of charged particles can be in the opposite direction to the applied voltage, if the PD is only managing to slow down an already high speed beam of charged particles.
  7. Oct 30, 2016 #6


    User Avatar
    Science Advisor

    Voltage * Current = Power. Power is the rate of energy transfer in Joules per second.
    The signs determine which way the energy is travelling.

    We usually ignore the signs because we know what is happening, but the polarity signs are really very important as they indicate the direction of energy flow.
  8. Oct 31, 2016 #7
    ok..I think I got it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted