Subject: differential mode in voltage response: electrical terminal polarity?

Question

For only a given voltage difference between two points, how can we determine which point has a polarity of positive or negative?

Answer 1

This sounds like a word problem and not a real world one, so I'm going to assume that volt meters and oscilliscopes are out of the question for an answer.

Tech Dude is right that given only what you have you cant know the voltages at either node.

But polarity is always a referenced concept of voltage. Usually we reference it to the 0V level and that is how we get >0V is positive and <0V is negative. If you change the reference you can change what is 'positive' and 'negative' in regard to polarity.

So, if your example gives you a positive voltage measured from point A to point B then point A is your 'positive' node. Like wise if the voltage from point A to point B was negative then your point B is the 'positive' node.

If you are not given the voltage difference in terms of point A to point B, you can't actually say which is 'positive' or 'negative'.

Answer 2

The point that has an excess of electrons is negative. The direction of positive current flow is from the positive point to the negative point. The direction of electron flow is the opposite direction. Electrons flow from negative to positive.

To find out which point is more positive, simply find which direction the electrons want to flow. A volt meter works great if it's a real world problem.

If your equipment will only measure from a zero reference (usually referred to as ground), then you can measure the voltage between each point and ground. Then you do some simple math to find out the voltage difference between the two points.

Answer 3

If the only information you have is the voltage difference between two points, it is impossible to determine whether either point is positive or negative. They could both be positive, or both negative, or one positive and one negative.

For example, say the voltage difference from point A to point B is +1 volt. This is satisfied by each of the following conditions:

A = +2 volts, B = +1 volt (both positive)
A = -1 volt, B = -2 volts (both negative)
A = +½ volt, B = -½ volt (A positive, B negative)

There is also a rule in mathematics for the solution of linear equations, which requires that the number of unique equations must be greater than or equal to the number of unknown variables. For this example and your problem, you've got two variables (Vᴀ and Vʙ) but only one equation Vᴀ - Vʙ = +1. So it can't be solved.

Answer 4

Use a voltage meter.