Why does potential difference cause current?

Question

I'm doing a physics course in my final year of high school, and I'm finding electricity and magnetism conceptually hard.

Everywhere online says that potential difference is the difference between electric potential (duh). I understand it as "the amount of work required to move a positive test charge from one point to another".

However, I'm failing to understand how potential difference causes current.

In lots of contexts, sources will way something like "The potential difference causes a current". For example, when learning about Faraday's Law we learned that a change in magnetic flux causes potential difference, and thus a current.

But why does potential difference cause a current? How does the value of the work needed to move a positive test charge between two points cause a current? Am I misunderstanding something fundamental?

Answer 1

A potential difference does not cause a current in general. It only causes a current for a specific class of materials called conductors. For other materials, such as insulators or dielectrics a potential difference does not cause a current, and of course a potential difference does not cause a current in vacuum.

Conductors are characterized by having charge carriers that are very loosely bound to the bulk material. This means that they are free to move when a force is exerted on them. A voltage difference implies that there is an E field, which exerts a force on the charge carriers, and they move in response to that force.

Answer 2

One nice analogy is:

Take two buckets, one placed at 10 meters and another on the ground. Now pour water onto the top bucket. If they are connected by some pipe, the water will immediately flow towards the lower bucket , right? The flow will continue as long as there is water in the top bucket.

This is because water in the top bucket has higher potential energy, given by P.E=mgh. When it flows down, from the formula, we see that it's potential energy drops to zero (since h=0).

From this, we conclude that water always likes to prefer to be nearer to the ground and whenever possible, it will go down. This is a fact of nature and is also common experience.

So, we say something like "in case there is a differential in the potential energies between any two points(not necessarily taking one point as the ground), water is sure to flow, if there is a pipe connection between those points."

Then, to predict if there is going to be a flow of water or not between two points, we just ask "What are the positions of the two buckets?" from which we can calculate the "Potential difference" and conclude if water will flow or not.

In electric circuits, the potential difference is provided by the battery (the negative charges in the negative terminal have a higher potential energy and they move towards the positive terminal, losing their excess energy in the process) and the wire is the pipe.