What determines the direction and cause of electric field exerted on test charges?

-I understand when there is an on-going electric field, if it is produced by a positive charge, the direction of electric field is away from it etc. And, if we get a +ve test charge, it repels it, and the force is also the same direction as the field. However, I can't understand why and how we know the main electric field is + or - charged? What causes it? And, if electric field is produced by electric charge, how come the test charges don't change or affect the main field? (Sorry, if I make no sense!) For instance, what causes and determines the direction of electric field in a DC power line?

However, I can't understand why and how we know the main electric field is + or - charged?

One way is to place free test charge in the field and see which way it moves.

One instrument that essentially does this is the electrometer.

What causes it?

In physics textbooks, we an study a case where a single charge sits in an infinite universe with no other charge, producing an electric field that radiates away from it in all directions with perfect symmetry.

In the real world, we almost always deal with situations where charge is balanced. There is a negative charge somewhere to terminate whatever field emanates from a positive charge and vice versa.

So if we see an electric field pointing from A to B, there is almost always a positive charge somewhere in the neighborhood of A, and a negative charge near B.

For instance, what causes and determines the direction of electric field in a DC power line?

Whoever designed the power system designed the system to produce either a positive or negative voltage on the line (relative to earth).

If they produced the DC voltage by rectifying an AC source (like a generator), then they can choose the polarity of the DC voltage by how they connect the rectifier to ground.

If they produced the DC voltage from a battery, then the chemical reactions going on in the battery determine which terminal is positive and which is negative. We can connect our power line to either of these and connect the other to earth (and possibly also to a return conductor) to get whatever polarity we want on the DC power line.

• First, thank you so much. But, I wonder the real life situations (in regards of your answer to my first two quoted sentences.) And your 3rd answer. What does exactly negative voltage mean? (how can it have lower potential than the Earth?) And for the DC power line, wouldn't connecting the powerline to the wrong terminal cause damage?
– Vyun
Commented Dec 18, 2018 at 18:09
• It's totally common and not at all unusual to have a potential lower than earth. For half of every 50 or 60 Hz cycle, your AC mains supplies a potential lower than earth. Or make a battery powered device, and connect the positive terminal of the battery to earth. All potentials are relative, and you shouldn't think of earth as a lower limit on the potential, just a reference from which to measure other potentials. Commented Dec 18, 2018 at 18:13
• So, that means the electrons will flow from + terminal to the Earth in a battery powered device? And in that case, isn't the earth the lower potential? I get that AC has a sine wave, but I don't understand how actually that is maintained and that makes it (for - V values) lower than earth, so the polarity reverses.
– Vyun
Commented Dec 18, 2018 at 18:19
• Electrons will flow from the - terminal to the + terminal in a battery powered device. They won't flow through earth unless there are at least 2 connections to earth in your circuit. You could connect either the - terminal or + terminal to earth. That would give your circuit a reference to earth, but it wouldn't cause current to flow to or from earth. Commented Dec 18, 2018 at 18:22
• I tried to mean from - to +, and from + to earth.
– Vyun
Commented Dec 18, 2018 at 18:24