Question about the polarity of a voltage (PHY 2 for engineers)

In summary, according to the author, batteries move negative charges from its negative terminal to its positive terminal. This creates a REPULSION force onto the negative charge, which is backwards according to the logic stated in the original question.
  • #1
mhrob24
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My problem is understanding how the polarity of a voltage makes sense. For example, In my textbook, its states that "batteries move negative charges from its negative terminal to its positive terminal. In terms of potential, the positive terminal is at a HIGHER voltage than the negative terminal". Now, reading that just doesn't make any sense to me, and ill now explain my rationale:

If batteries move negative charges from a NEGATIVE terminal, then considering how Coulomb's law works; this means that the field from the negative terminal will produce a REPULSION force onto the negative charge (like signs repel). In terms of potential energy of the negative charge, this means that the negative charges being repelled from the negative terminal are GAINING kinetic energy and LOSING potential energy (If I manually took a negative charge being repelled by the negative terminal and moved it TOWARD the negative terminal, its would GAIN potential energy).

So with this logic in mind, as the negative charges are repelled toward the positive terminal...then the negative charge being moved by the battery has LOST potential energy as it moves from its negative terminal to its positive terminal. If voltage is the change of potential energy, how the hell is the voltage at the positive terminal HIGHER than the negative terminal if the negative charge underwent a NEGATIVE change in potential energy as it traveled from its negative terminal to its positive terminal?

That seems backwards to me but I'm not the brightest bulb in the box so maybe I'm overthinking this and confusing myself somewhere, but I have read that the topic of voltage is one that tends to be quite confusing.

Thanks in advance to anyone who helps!
 
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  • #2
mhrob24 said:
If voltage is the change of potential energy, how the hell is the voltage at the positive terminal HIGHER than the negative terminal if the negative charge underwent a NEGATIVE change in potential energy as it traveled from its negative terminal to its positive terminal?
Voltage (V) is the change of potential, not the change of potential energy. The change of potential energy is the voltage times the charge (QV).

So going from the negative terminal to the positive terminal V is positive, and for a negative charge Q is negative so the change in potential energy QV is negative.
 
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  • #3
mhrob24 said:
"batteries move negative charges from its negative terminal to its positive terminal.
If it really says that, it's wrong. A battery can cause current to flow from the positive to the negative terminal through an external circuit. A positive current is opposite to the direction of the flow of electrons. Double check your book. Is it talking about charges inside the battery or charges in the external circuit? (If you want to know who to blame for that horrible sign convention, it was Benjamin Franklin. A millenium from now, people will still be confused by that.)

But to understand a battery well, takes more than charges. It takes a little chemistry. Try this article, and see if it helps.

https://en.wikipedia.org/wiki/Electrochemistry
 
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We have encountered that problem a couple of times, and it is necessary to distinguish between an electric field that is part of an EMF,( oftentimes written as ## E_{induced} ##, but it can also be part of a chemical potential in a battery), and an electrostatic electric field. ## \\ ## For the EMF, the voltage at B, (calling point A as zero voltage), is ## V_B=\int\limits_{A}^{B} E_{induced} \cdot dl ##. ## \\ ## For the electrostatic electric field, ## V_B=-\int\limits_{A}^{B} E_{electrostatic} \cdot dl ##.
 
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  • #5
anorlunda said:
If it really says that, it's wrong. A battery can cause current to flow from the positive to the negative terminal through an external circuit. A positive current is opposite to the direction of the flow of electrons. Double check your book. Is it talking about charges inside the battery or charges in the external circuit? (If you want to know who to blame for that horrible sign convention, it was Benjamin Franklin. A millenium from now, people will still be confused by that.)

But to understand a battery well, takes more than charges. It takes a little chemistry. Try this article, and see if it helps.

https://en.wikipedia.org/wiki/Electrochemistry
You are correct. I did not accurately quote my textbook. This was the case of a CIRCUIT connected to a battery moving a NEGATIVE charge from its negative terminal to its positive terminal (the circuit was a headlamp). So, if I truly do understand this time, the voltage is the potential difference of a charge moved from A to B DIVIDED by the charge (V = change in U/Q) Hence, independent of the charge, aka only dependent on the DIFFERENCE.

So really, the polarity of the voltage is just a matter of where you choose your origin (start point) to be. So in the example of a battery moving a charge in a circuit from its negative terminal to its positive terminal, the potential DIFFERENCE is positive because we defined our origin (start point) to be the negative terminal. So as the negative charge moves to the positive terminal, there is a positive DIFFERENCE (like going from 0 to 10). The only way for the Voltage in this case to be NEGATIVE is if the flow of the negative charges from the negative to the positive terminal is reversed. This would mean that our negative charge would be moving closer toward our origin point (the negative terminal). So like, going from 0 to 3 instead of 0 to 10. There is LESS of a DIFFERENCE from 0 going to 3 than there is from 0 to 10, so that would be a NEGATIVE potential difference aka a negative voltage...?
 

1. What is polarity in the context of voltage?

Polarity refers to the direction of the electric field created by a voltage. It is a measure of the distribution of positive and negative charges within a circuit.

2. How is polarity determined in a circuit?

Polarity is determined by the placement and orientation of the voltage source and the direction of current flow within the circuit. The positive terminal of a voltage source is typically marked with a plus sign (+) and the negative terminal with a minus sign (-).

3. What is the significance of polarity in circuit analysis?

Polarity is an important factor in circuit analysis as it determines the direction of current flow and the behavior of components within the circuit. It also helps in determining the voltage drop across components and in calculating the overall voltage in a circuit.

4. Can polarity be reversed in a circuit?

Yes, polarity can be reversed in a circuit by switching the connections of the voltage source. This will result in a change in the direction of current flow and may affect the behavior of components in the circuit.

5. How does polarity affect the overall voltage in a circuit?

The overall voltage in a circuit is determined by the algebraic sum of the individual voltages. If all the voltages have the same polarity, they will add up to a higher voltage, whereas if they have opposite polarity, they will cancel each other out and result in a lower voltage.

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