# Electric Potential concept

1. Sep 29, 2008

### 00PS

ok, So I am in an entry level physics class (electricity & magnetism) and I am having trouble grasping the idea of voltage/electric potential. Wikipedia describes it as "electric pressure" but that is still pretty foggy. My professor relies heavily on mathematical derivations and barely says a word about topics conceptually. The mathematics has never been the problem its just I am such a visual learner and must be able to "visualize" what is happening at the most basic level. Now I am we are doing DC circuits and I am totally lost, because I can't understand what is happening on the atomic level.

2. Sep 30, 2008

### cabraham

On the atomic level is a more advanced topic. Are you planning on taking e/m field theory soon? That will help a lot. I'd recommend focusing on your class material and text. If you still need more info, chack out the university library. This is a great source.

In a nut shell, the voltage from a to b, is simply the work done transporting a charge from a to b divided by the charge. The unit is joule/coulomb.

Forget "pressure" and all analogies involving fluid flow. They are misleading. Wikipedia contains much truth, plus some untruth. Which is which is not easy to distinguish for a beginner. BR.

Claude

3. Sep 30, 2008

### 00PS

No. this is the only electrical class in my curriculum (Mechanical Engineering)

I realize its pretty complex, but what about more of "elemental" level. Not necessarily individual atoms, but perhaps a picture of some sort. I am really struggling. Can you explain it in the context of a circuit? Do bigger charges require more work? Could it be described as how much "push" an amount of charge gets going through a circuit?

4. Sep 30, 2008

### Defennder

There are a number of assumptions circuit theory makes. For one thing it assumes that the potential along any point on a wire is the same if you don't pass over any circuit elements.

Secondly the concept of electric potential is a little irrelevant. We don't speak of electric potential, only potential difference for DC circuits. Remember that the electric potential is the amount of work done in bringing a unit of charge from infinity to a given point. But in circuit theory, we only speak of relative potential differences across circuit elements.

So a potential difference of 5V across a circuit elements could mean two things. One, it could mean that each Coulomb of charge gains 5J of energy as it passes through the circuit element. On the other hand, it could be that each unit charge loses 5J of energy when it passes through the circuit element. So you can see from here, that it's kind of true that bigger charges (more accurately called current since it's moving charges) dissipates more energy when it passes through a circuit element. Whether it gains or loses energy depends on whether it's a voltage source or voltage drain.

Circuit elements like resistors absorb energy when charges pass through them, so we always speak of a voltage drop across resistors. Batteries are voltage sources, so we speak of energy being given to the charges as it passes through the battery. So unlike resistors, when current enters from the positive terminal and exits the negative, current enters a battery from negative and exits through positive. That means the charges are being powered by the battery.

We don't speak of charges because we're not concerned about the electric field due to these charges unlike in electrostatics. Instead we talk about current, since ultimately moving charges is current. You should think of potential difference as the amount of energy supplied/depleted to/from each unit of charge as it passes through that circuit element per unit time.

5. Sep 30, 2008

### granpa

how does that differ from the concept of 'pressure'?

6. Sep 30, 2008

### Defennder

Well I don't know the difference because I'm not familiar with the mechanics of water pressure. But it goes without saying it's always better to be directly acquainted with the concepts of a theory without having to understand them through the lens of an analogy which will break down sooner or later.

7. Oct 3, 2008

### granpa

now that I think about it the pressure at any point in a liquid would be equal to its potential energy. moving from an area of pressure A to an area of pressure B will release energy proportional to A-B. so voltage as potential energy and voltage as electric pressure are the same.

8. Oct 4, 2008

### Staff: Mentor

In this particular case, the main difference mathematically is that pressure is two dimensional whereas voltage is one dimensional. Otherwise, they work pretty much the same.

Since electron flow is not something you can see, and so people often have trouble visualizing it, I favor using an analogy until they can get over that hump. To learn something unfamiliar, you can use something familiar as a bridge.

And though you say you don't know the mechanics of water pressure, you do. You just don't know you do. Haven't you ever put any thought into what happens when you put your thumb over a garden hose to create a nozzle? Or flushed a toilet while your sister was in the shower to hear her scream? Or gone white-water rafting and thought about how the rapids work? These are the basic concepts that people - knowing the equations or not - have an intuitive feel for.

9. Oct 4, 2008

### granpa

2 dimensional? 1 dimensional?

you mean wires are one dimensional? so are hoses.

Last edited: Oct 4, 2008