# Reference point of voltage for an electric field

1. Jun 2, 2014

### Bengo

When you are told that V is the voltage at a given point in an electric field and voltage is the difference in potential energy between two points/charge, where is the other point supposed to be? Thanks

2. Jun 2, 2014

### Staff: Mentor

When someone says that "V is the voltage at a given point" and doesn't tell you what the other point that's relative to is... They're assuming that it is clear what they mean from the context. Nearly always, the other point will be ground.

3. Jun 2, 2014

### Bengo

I also don't quite understand what is meant by voltage is a property of the field or that it is position dependent.

4. Jun 2, 2014

### Staff: Mentor

Consider the electric field between two charged electrodes placed 1 meter apart. One is negatively charged, the other is positive. A positive charge is placed near the positive electrode (Point A) and another is placed midway between the electrodes (Point B). Both will accelerate towards the negative electrode (Point C), with the charge at point A gaining more energy before it hits the negative electrode than the charge placed at point B. So we can say that a charge placed at point A has more potential energy than one placed at point B. Since voltage is a difference in electric potential, point A has a greater voltage than point B, when both points are referenced to point C.

Make sense?

5. Jun 2, 2014

### sophiecentaur

In an electric circuit, the reference is usually taken to be Earth (an actual connection to the Earth) or a local 'Ground', which is designated to be Zero Potential. This, latter, is arbitrary. Engineers can be sloppy in this regard and tend to take this for granted but, when dealing with small Voltages, it can be very relevant.
Car electrics used to have the + terminal of the battery connected to the car body and it was called a 'Positive Earth' system. Nowadays, it is universal (??afaik) to use a 'Negative Earth' in cars. Everything about the functioning of the circuitry would be the same but the wire connections might be different in the two cases because the body is often used for an "Earth Return"