Relation of Charge polarity and voltage polarity

[KKJ20]

Homework Statement

What is the relationship of charge polarity and voltage polarity?

I understand an anion has negative charge and cation ion has negative charge, but does an anion have a positive voltage and the cation have a negative voltage?

Given the above is generally true, when is the above is not true?

Homework Equations

The Attempt at a Solution

 

[haruspex]

does an anion have a positive voltage

Voltage is purely relative.

 

[Douglas Burn]

Yeh when people talk about the charge polarity of an object they are talking about whether it is positive or negative.

For example an electron has a positive charge; a proton has a negative charge.

Voltage on the other hand is not a property of an object.

Voltage is a property of the space AROUND a charged object.

More specifically it is a way of comparing the energy that an imaginary positive charge would have at one point around a charged object with the energy that it would have at a different point around the same charged object.

For example if you had a proton and you put another proton near by,
how much energy would the second proton have? And if you moved it somewhere else how much energy would it gain (or lose)?

The change in energy is what people call the voltage between the two points.

I hope that's helpful.

There is a lot more to it which I can try to explain if you want.

 

[haruspex]

Voltage is a property of the space AROUND a charged object.

No, the charged object itself can also be regarded as having a voltage.

way of comparing

Yes, that is the key point: it is only for forming comparisons.
This is not always clear because of the convention applied in many cases that the voltage at infinity is zero. But in principle you can define any point as having zero voltage, and all other voltages are relative to that.
This is made starkly clear in the case of a uniformly charged infinite sheet. If you make the voltage zero at infinity then everywhere else it is infinite, so in that case the usual approach is to take the sheet itslef as being at zero potential.

 

[Douglas Burn]

No, the charged object itself can also be regarded as having a voltage.

Yes, that is the key point: it is only for forming comparisons.
This is not always clear because of the convention applied in many cases that the voltage at infinity is zero. But in principle you can define any point as having zero voltage, and all other voltages are relative to that.
This is made starkly clear in the case of a uniformly charged infinite sheet. If you make the voltage zero at infinity then everywhere else it is infinite, so in that case the usual approach is to take the sheet itslef as being at zero potential.

Voltage is a measure of the difference in energy that a +1 Coulomb charge would have at two different points in the electric field. So it's a property between two points in the electric field.

You couldn't take a charged object, put it somewhere and definitively say "this object now has 3.7V".

You could say "this object is at a position which has a potential energy of +3.7 joules per coulomb relative to that other point so it is at a position which represents 3.7V relative to that other position."

So you can say that an object is at a position which is at 3.7V relative to some other position but you can't say that the object has 3.7V. The voltage is a property of two different points in a field not of the object that you're putting into it (assuming a test charge which doesn't change the field).

Do you agree?

 

[haruspex]

Voltage is a measure of the difference in energy that a +1 Coulomb charge would have at two different points in the electric field. So it's a property between two points in the electric field.

You couldn't take a charged object, put it somewhere and definitively say "this object now has 3.7V".

You could say "this object is at a position which has a potential energy of +3.7 joules per coulomb relative to that other point so it is at a position which represents 3.7V relative to that other position."

So you can say that an object is at a position which is at 3.7V relative to some other position but you can't say that the object has 3.7V. The voltage is a property of two different points in a field not of the object that you're putting into it (assuming a test charge which doesn't change the field).

Do you agree?

Yes.