# Electric Potential Difference

Hello,

I'm struggling to understand how the electric potential difference is measure especially when a distance is not given. For instance in Serway, on the explanation of the Van de Graaff Generator the authora write, "Van de Graaff generators can produce potential differences as large as 20 million volts."

But what does that exactly mean? Where is this potential difference considered? I mean, the distance between two the electric potentials could be very far away or very close....so I'm not seeing the purpose of the statement. It seems too arbitrary to make sense.

anorlunda
Staff Emeritus
The operative word is "difference". A voltmeter always requires two wires to make a measurement.

Voltage as used in circuits, always means a difference between two points.

So a Van de Graf generator supplied by a power supply powered by AC voltage from a grounded circuit, the "difference" is relative to ground.

sophiecentaur
Gold Member
how the electric potential difference is measure especially when a distance is not given
The distance (length of wires etc. has no effect on Potential Difference.
There is an analogy with Gravitational Potential Difference. Imagine two roads from a village in the valley to a ski station at a height of 1000m. Travelling up by the short, very steep road and the other zig zag road will still give you the same Gravitational Potential above the village.
PD represents the Energy given to or delivered by a unit charge (of One Coulomb) in a circuit.
One Volt of PD corresponds to One Joule per Coulomb. The Volts on a Van de Graaff generator are alternating because they are produced with a high frequency transformer. When a 'Voltage' is stated rather than a Voltage Difference 'Across' two points, it is assumed that the voltage is referred to Ground (or Earth) at an assumed zero potential.
In a similar way, Gravitational Potential is sometimes assumed to be referenced to 'Sea Level'

The distance (length of wires etc. has no effect on Potential Difference.
One Volt of PD corresponds to One Joule per Coulomb. The Volts on a Van de Graaff generator are alternating because they are produced with a high frequency transformer. When a 'Voltage' is stated rather than a Voltage Difference 'Across' two points, it is assumed that the voltage is referred to Ground (or Earth) at an assumed zero potential.
'

So the ground is 0 but where is the other point then?

The operative word is "difference". A voltmeter always requires two wires to make a measurement.

So a Van de Graf generator supplied by a power supply powered by AC voltage from a grounded circuit, the "difference" is relative to ground.

What does the wire have to do with the voltage?

sophiecentaur
Gold Member
So the ground is 0 but where is the other point then?
It's the point where you specify the voltage (relative to ground). To measure Voltage you use a Voltmeter which has two wires to measure the voltage difference between two points (often on a circuit).
Have you looked anywhere else for guidance on this topic? Google will throw up many alternative links for you to read and that way you can get familiar with the way Voltage is referred to and used in practice.
Edit: PS did you read what I wrote about the analogy with Gravitational Potential and different heights? There is the clue about two points being needed.

anorlunda
Staff Emeritus
What does the wire have to do with the voltage?

For example, to measure the voltage of a Van de Graf generator, you attach one voltmeter wire to the top of the generator, and the other wire to ground.

The key point to learn is that voltages (potential difference) are between two points, not absolute. So the two wires of the voltmeter select the two points to measure the difference.

It is like distance. It make no sense to ask "What is the distance of New York?" We say the distance between some other point and New York.

anorlunda
Staff Emeritus
• sophiecentaur
vela
Staff Emeritus