# Electric Potential Difference

• annaphys
In summary: So the voltage difference is between the dome and ground. The wire is necessary to make the measurement. In summary, measuring electric potential difference involves selecting two points and using a voltmeter to measure the difference in voltage between them. The distance between the points does not affect the potential difference. This is similar to measuring distance, where two points are needed to determine the distance between them. In the case of a Van de Graaff generator, the voltage difference is between the dome and ground, and the wire is necessary to make the measurement.

#### annaphys

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.

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.

annaphys said:
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'

sophiecentaur said:
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?

anorlunda said:
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?

annaphys said:
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.

annaphys said:
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.

sophiecentaur
annaphys said:
So the ground is 0 but where is the other point then?
The dome of the generator, where the charge is accumulating.

## 1. What is Electric Potential Difference?

Electric Potential Difference, also known as Voltage, is the difference in electric potential energy between two points in an electric field. It is measured in volts (V) and represents the amount of work that must be done to move a unit of electric charge from one point to another.

## 2. How is Electric Potential Difference calculated?

Electric Potential Difference is calculated by dividing the change in electric potential energy (measured in joules) by the amount of charge (measured in coulombs) that is moved between the two points. This can be represented by the equation V = ΔPE/q, where V is the potential difference, ΔPE is the change in potential energy, and q is the charge.

## 3. What is the relationship between Electric Potential Difference and Electric Current?

Electric Potential Difference is directly related to Electric Current, as it is the driving force that causes charged particles to move through a conductor. The greater the potential difference, the greater the electric current. This relationship can be represented by Ohm's Law: V = IR, where V is the potential difference, I is the current, and R is the resistance of the conductor.

## 4. How does Electric Potential Difference affect the flow of electricity?

Electric Potential Difference is the driving force that causes electricity to flow through a circuit. The greater the potential difference, the more energy is available to move charged particles, and the stronger the electric current will be. In other words, the higher the potential difference, the more easily electricity can flow.

## 5. What factors affect Electric Potential Difference?

The factors that affect Electric Potential Difference include the distance between two points, the amount of charge present, and the type of material the charge must move through. In addition, the presence of other electric fields or sources of resistance can also affect the potential difference. Ultimately, the potential difference is affected by any factor that can alter the amount of energy required to move charged particles between two points.

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