# Question about voltage polarity and power

Hello,

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?

Simon Bridge
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:
Logical Dog
If

That help?

Yes, it was an excellent explanation!
thank you.

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.

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:
sophiecentaur
Gold Member
2020 Award
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.
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.

Logical Dog
Baluncore