# What is negative electric potential energy?

1. Jan 18, 2006

### matt85

I am trying to get this concept, but I can't figure out what exactly this means. Can anyone explain what it means if something has negative electric potential energy?

2. Jan 18, 2006

### ZapperZ

Staff Emeritus
Do you understand negative gravitational potential energy then?

Zz.

3. Jan 18, 2006

### matt85

Does it just mean that it is surrounded by a negative potential? I'm pretty lost on this concept overall. I have never dealt with negative potential energy before.

4. Jan 18, 2006

### Staff: Mentor

Potential energy is relative. Relative to some datum where the potential energy is zero. If you add work to the charge to move it against the forces (from fields), you are increasing its potential energy with respect to that datum. If you extract potential energy from the charge by letting it fall with the force, then the potential energy is getting lower than the datum value, and hence negative. Just remember that potential energy is with respect to a datum. Think about ZapperZ's question in this context, and think about the tradeoff between potential energy and kinetic energy, for example.

5. Jan 18, 2006

### matt85

Thanks for the help.

6. Jan 23, 2006

### Robine

We usually think of (gravitational) potential energy at the earth's surface to be zero. If we raise an object we consider it to have gained potential energy because we did work against the force of gravity. But dig a hole on the beach and move the object into it and it's now at negative potential to where it was at the earths surface. It's all relative.

7. Jan 24, 2006

### DaTario

Energy is a concept for which only the differences between two well specified situations play relevant part. Force, for instantce, is defined as the spatial derivative of energy, which clearly means that global addition of a certain fixed amount of energy will cause no physical result.

Nevertheless, as we can always sum a constant value to the energies of all positions (potential energies in the case of a conservative field) we may, for convenience, decide to attatch negative values of energy to those situations where the particle is confined and positive energy to non confined states.
Therefore, if in a pure gravitational situation, a particle have total energy negative, then it means that there will be a moment where all the energy will be in the form of potential energy due to gravitational force and the it will be at rest and/or at a limited distance from the gravitational center.
But if the particle have zero or positive total energy then, even subjected to the gravitational pull, the particle will be likely to be observed at arbitrarily great distances from the gravitational center.

In short: negative energy is just a matter of convenience to represent confinement. You can always redefine it to set some negative value to positive or zero.

Best Regards

Last edited: Jan 24, 2006
8. Nov 8, 2009

### sudar_dhoni

Does it just mean that it is surrounded by a negative potential? I'm pretty lost on this concept overall. I have never dealt with negative potential energy before.

dont think that energy is negative
think like this
there is a pt A where the charge has potential energy of 10J
now when it is moved against an electric field u supply energy by doing work and move it to anothere pt B .U have supplied 5 J so now potential energy of the charge is 15J.
which means that work done =increase in potential energy

now consider ur case
if the charge is at A with 10J and u allow it to move in the direction of E field in this case in order for the charge to move it has to spend its energy so it moves to a pt C such that it has used up all its 10J.but it still continues to move by using some internal energy and now energy decrease and becomes less that 0 and finally the charge settles at a pt D.here u find that energy is -5J.
the meaning of -5J is that it has a deficit of 5J and has to make up its deficit in order to be in the base level ( 0 J)
An analogy for this is that in order for a student to get promoted he has to score min (base level) of 30/100.if he scores 25 he has to make up his deficit of 5 marks to be in the base level

i.e base level is 0 J it has to make up its deficit and somehow has to attain the base level.
THerefore from nowonwards the -ve sign in energy indicates deficit i.e it is below base level and has to make up its deficit. Hope u understood

9. Nov 11, 2009

### SpartanG345

wat about negative Ke, i don't think its possible to have it, at least that's what my textbook said in a problem