# Homework Help: Electric potential

1. Aug 13, 2007

### MIA6

I am so confused with 'electric potential energy', 'electric potential' and 'potential difference'?!! What's their difference? The first one is an energy, the second one is a work? How about the third one? Hope you can explain to me, thanks so much.

2. Aug 13, 2007

### Dick

Energy is the same as work. If you change the last one to 'electric potential difference' (as opposed to say, 'gravitational potential difference') they are all the same concept. The last one simply refers to a 'difference' of potential between two points, of course the first two are also implicitly differences between the potential at some point and an arbitrary fixed reference point. There is no notion of a potential independent of reference to some other point. So really, they are all the same.

3. Aug 13, 2007

### Mindscrape

To an extent they are all the same. Electric potential energy and electric potential, which really are the same because often people just drop the energy part of the name, you can think of as the electrical version of gravitation potential. The gradient of the potential, the rate of change of the potential in all dimensions, gives the electric field.

Potential difference is what you are thinking about with work. It is described as the amount of work it would take for a charge to move from one spot to another under a given force. Work is the integral of a force dotted with a displacement vector over a curve, or path, that the object follows. Work energy theorem says that the amount of energy transfered in a system equals the work done.

They are all related to one another in the general concept of potential energy. Try this helpful link:

http://hyperphysics.phy-astr.gsu.edu/hbase/pegrav.html#pe

4. Aug 14, 2007

### MIA6

My book seems to talk about electric potential as a work, is that okay?

5. Aug 14, 2007

### Dick

Technically, it is work (or energy) per unit charge. Moving 2C of charge through the same potential difference as 1C involves twice the work.

6. Aug 14, 2007

### learningphysics

Electric potential energy is not the same as electric potential... they are related, but you shouldn't think of them as the same...

electric potential*charge gives electric potential energy...

Suppose you have a charge q1... the potential due to that charge a distance r away is kq1/r. If you place another charge q2, at that point r then the energy between the two is kq1q2/r. So potential is Energy/charge... similar to how electric field strength = Force/charge

The potential difference is just the potential at one point - the potential at the other...

If two points have a potential difference V... that means that a charge q will gain q*V in potential energy by going from the point of lower potential to the point of higher potential...

Potential = energy/unit charge... Field = force/unit charge

7. Aug 14, 2007

### MIA6

okay, I got it. Is a neutral object who contains equal amounts of positive and negative charge a charged object? Or a charged object must have different amounts of positive and negative charges, as in different numbers of electrons and protons.

8. Aug 14, 2007

### rootX

no, it's only charged when it has excess or deficiency of electrons.

9. Aug 14, 2007

### MIA6

10. Aug 14, 2007

### rootX

It depends on the valence electrons.

So, metallic object would move away from the charged object.

11. Aug 14, 2007

### MIA6

I thought a neutral object would redistribute the electrons and protons inside, so it will usually first attract to the charged object. Here is a question about induction, If the object is then disconnected from the ground while the positively charged rod is nearby, the object is left with a net negative charge. What does that mean by left with a net negative charge.

12. Aug 14, 2007

### rootX

But all neutral objects are not capable of doing so.[non-conductors]

So, the positive rod would pull all electrons to one side leaving the other side positive. And the ground has plenty of electrons that are attracted by that positive side of the object...
and so in the end the object would be negatively charged.

13. Aug 14, 2007

### learningphysics

Have a look at the animation in the middle of this page:

http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/estatics/isop.html [Broken]

It shows this exact process, except leaving a net positive charge on the object instead of negative.

Last edited by a moderator: May 3, 2017
14. Aug 14, 2007

### MIA6

Is pith ball a conductor or not? Since it was written on my book that it rearrange its electrons and protons. If electrons from ground/earth attract to the positive side of the object, then the protons in the positive side will go into the ground? but it is disconnected? Does net negative charge or net positive charge means that an object has more electrons than protons; an object has more protons than electrons?

Last edited: Aug 14, 2007
15. Aug 14, 2007

### rootX

=I think it's a conductor then.
=protons don't move :)
=negative charge = excess of electrons

16. Aug 14, 2007

### MIA6

Then how come you said that the ground has plenty of electrons that are attracted by that positive side of the object...
and so in the end the object would be negatively charged.
If the object changes from neutral to negatively charged and disconnected to the ground, where do other protons go?

17. Aug 14, 2007

### rootX

protons just stay there.
the object gets more electrons, so in other words, the object's mass would be increased slightly.

18. Aug 14, 2007

### MIA6

so the object gets more electrons from the ground? but it's disconnected.

19. Aug 14, 2007

### rootX

yes, if there's a contact.
oops, I missed that part.
So, the object can get electrons from the ground only if there's a contact.

20. Aug 14, 2007

### MIA6

But on my book, it says If the object is then disconnected from the ground while the positively charged rod is nearby, the object is left with a net negative charge. Well, of course this is a neutral object. So it means electrons are more than protons eventually, but how come? If it is disconnected from the ground, then there is no way to gain electrons.