# How can Voltage have polarity? How can they be negative or positive?

Voltage is potential difference between two terminals of a power source that causes the electrons to flow, right? How can that be positive or negative? What is positive voltage and what is negative voltage?

Thank you,

NascentOxygen
Staff Emeritus
Potentials are relative. So if one wire in a circuit is of a greater voltage than a second wire, we can say the first one is positive with respect to the second. Or, equivalently, we can say the potential of the second wire is lower than that of the first, i.e., the second wire is negative with respect to the first.

nasu
Gold Member
Voltage is potential difference between two terminals of a power source that causes the electrons to flow, right? How can that be positive or negative? What is positive voltage and what is negative voltage?

Thank you,

Do you understand how, in general, a difference can be positive or negative?

UltrafastPED
Gold Member
Voltage is potential difference between two terminals of a power source that causes the electrons to flow, right? How can that be positive or negative? What is positive voltage and what is negative voltage?

Thank you,

Current flows from the + terminal to the - terminal; if you have a galvanometer hooked up in the middle, and switch the leads at the terminals, it will read the same current, but with the opposite sign.

This is the test of polarity. For an AC generator, the sign changes 60 times per second, so you better put the meter into AC mode!

Batteries are also labeled + and -.

PS: Note that "standard current" is the flow of positive charge carriers; electrons actually flow in the opposite direction, but that is simply a confusing fact which is due to the history of how + and - charges were defined by Ben Franklin!

rcgldr
Homework Helper
You could also consider the voltage between two points near some charged object that generates an electrical field, using a third point as a reference point.

For the potential from a charged point or sphere (for the field outside the sphere), a convenient reference distance from that charged source would be ∞, with the potential defined as zero at ∞ distance, and increasing (towards +∞) as distance from the charged source decreases. Note that voltage is potential energy per unit charge, so for a negatively charged particle, the electrical potential energy would become increasingly negative as distance from the charged source decreases.

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nasu
Gold Member
You mean that the potential difference will be positive between any two points in that field?
How can this be? The sign and value of the potential difference between the two points depends on the two points, including the order of the points.

rcgldr
Homework Helper
You mean that the potential difference will be positive between any two points in that field?
No, only that the potential of both points relative to a third "reference" point could both be positive. The potential difference could be positive, zero, or negative. Note - I cleaned up my previous post.

For the potential from a charged point or sphere (for the field outside the sphere), a convenient reference distance from that charged source would be ∞, with the potential defined as zero at ∞ distance, and increasing as distance from the charged source decreases. Note that voltage is potential energy per unit charge, so for a negatively charged particle, the electrical potential energy would become increasingly negative as distance from the charged source decreases.

For the potential between two oppositely charge plates, the surface of the negatively charged plate could be used as a reference where the potential is defined to be zero, and potential would increase as distance from the negative plate increases, until the positive plate was reached.

For the potential from an infinite plane or disc with a uniform charge per unit area, the surface of the plane or disc could be used as a reference point where the potential would be defined as zero. For a positively charged infinte plane or disc, the potential would become more negative as distance from the plane or disc increases.

For the potential from an infinitely long wire with uniform charge per unit length = λ, the potential at radius r versus a reference radius r0 is - 2 k λ ln(r / r0), so choosing r0 = 1 depending on the unit of distance (meter, foot) would result in a potential of zero at r = r0.

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nasu
Gold Member
Yes, it's true that potential of all the points in the filed can be positive in respect to a reference point.
But you mentioned the "voltage between two points" in your previous post. It was a little confusing. rcgldr
Homework Helper
Yes, it's true that potential of all the points in the filed can be positive in respect to a reference point.
But you mentioned the "voltage between two points" in your previous post. It was a little confusing.
I cleaned up that previous post.

Potentials are relative. So if one wire in a circuit is of a greater voltage than a second wire, we can say the first one is positive with respect to the second. Or, equivalently, we can say the potential of the second wire is lower than that of the first, i.e., the second wire is negative with respect to the first.

Thanks, but could you explain as to how a wire in the circuit creating potential difference could have more voltage than the other?

Khashishi
If you attach two wires to opposite ends of a battery, and complete the circuit by attaching the other ends of the wires across a resistor, then one wire will have a higher voltage than the other. This is because one end of the battery attracts electrons more strongly than the other end. This has to do with the chemistry of the elements that make up the cells of the battery.

sophiecentaur