# Suppose an electron was kept with an alpha particle at a finite

1. Mar 25, 2013

### ehabmozart

Suppose an electron was kept with an alpha particle at a finite separation x. Why does it have a negative potential ENERGY ... In other words, what does a negative electric potential energy mean? I want an answer relating it with potential energy at infinity which is zero. Secondly, if the separation x is increased, why does the speed of the electron slow down assuming it had an initial positive speed. Moreover, when it reaches to a speed of zero, why does it reverse its path again. Thanks for helping in advance

2. Mar 25, 2013

### mikeph

Potential energy is the energy you can extract from a system. If zero is defined at infinity, and you have to put energy in to separate the two attracted objects to that distance, then the potential must be less than zero.

3. Mar 25, 2013

### ehabmozart

I am still confused. Can you explain this using examples. Consider an alpha particle at a distance from an electron. They are said to have negative potential. What does this mean. Let me say all what I actually know. By nature they will attract, so what does negative potential mean.

4. Mar 25, 2013

### sophiecentaur

It's a matter of definition, which has to be stuck to for all circs. Potential is defined as the work done to bringing an object in from infinity. When there is an attractive force (opposite electric charges or gravity) you get work out - hence negative work needs to be done on the object and, hence negative potential for attractive situations. It nicely fits the mental picture of a 'potential well' into which things will fall, however far they are away (assuming they are not moving tangentially, at all, when they could go int an orbit)

5. Mar 25, 2013

### rcgldr

The change in potential energy between two states is:

negative if force is attractive and the distance between objects is reduced.
negative if force is repulsive and the distance between objects is increased.

positive if force is attractive and the distance between objects is increased.
positive if force is repulsive and the distance between objects is decreased.

The sign convention for electrical potential (voltage as opposed to electrical potential energy) assumes that the object affected by a field has postive charge.

6. Mar 26, 2013

### ehabmozart

Fine, one last question. When two opposite charges are kept together, it is said that q0 has negative U... What does this mean qualitatively??

7. Mar 26, 2013

### ehabmozart

Can you give me such a perfect comparison as you did right now but instead I need the potential energy between two points (++, --, +-) rather than the change in potential with explanation and I would be so thankful!

8. Mar 26, 2013

### rcgldr

Normally the potential energy between two points is the change in potential energy from an infinite distance to the distance between the two points, so a distance of infinity is the "reference" distance for potential energy between points and defined as zero. So for two positive charges, the force is repulsive, and the distance from infinity to some finite distance is a decrease in distance, so the potential energy is postitive. Wiki article with formulas:

http://en.wikipedia.org/wiki/Electric_potential_energy

9. Mar 29, 2013

### ehabmozart

I just love those lines. I owed you bro! That was extremely helpful. I just urge you to give me just like these lines but now, in electric potential and not energy. In the cases of positive and negative charges. Thanks a lot!

10. Mar 29, 2013

### sophiecentaur

What distinction do you thing exists between the two?

11. Mar 30, 2013

### ehabmozart

Apparently they are proportional . But potential is distance-independent.

12. Mar 30, 2013

### sophiecentaur

No it's not. You can alter potential without changing position.

13. Mar 30, 2013

### ehabmozart

I can;t really get it... I need more explanation I guess

14. Mar 30, 2013

### sophiecentaur

Apologies. I missed the "in" in "Independent".

But I still can't understand the confusion / distinction between Potential Energy and your "Energy".
What is there to "get", apart from the facts as described in ehabmozart's last post? Perhaps there's something you are not making clear about what your point is.

15. Mar 30, 2013

### ehabmozart

I know mr. sophiecentaur that this is getting tides. My point here is the confusion I get whenever I read the book. Now, my main question is what is in the definition. I need someone to pick every word in the sentence and explain it to me. PLEASE! The main confusion is how did we define the potential using Energy and it is a single interaction, and not an interaction between two points!

16. Mar 30, 2013

### sophiecentaur

17. Mar 30, 2013

### ehabmozart

Well, not asking for your help. I keep in trying understanding physics which is quite tough cause I spend nights over this topic which may seem silly to you but is really driving me crazy. Such comments really depresses me, so please if you are not willing to help, atleast don't try to add to my dilemma. :)

18. Mar 30, 2013

### WannabeNewton

I think that is very rude ehab. sophie (if I can call him/her that :D) has tirelessly tried to help sort out your confusion(s) in various threads you have started on literally the same topic, over and over. Perhaps you should collect up your confusions and make them more coherent and be more appreciate of sophie's help thus far. Now what exactly is your main set of confusions because you have spread your questions over multiple threads and it is hard to keep track of what has been cleared up and what hasn't.

19. Mar 30, 2013

### ehabmozart

Thanks for clarifying me. I apologize to sophie from here though I can't tag the name overhere. But physics is tough and I am nervous and when I spread my questions, so I am not satisfied. Guys, bear with me. Asking is not bad, isn't it? ... Ok, in less than an hour, I'll post my final thread about this topic involving all my doubts clearly in a systematic type. Thanks a lot guys. I always owe to physics forum and Sophie, I never meant to hurt you or something! THANKS!