Here's the question:

2 point charges q1 = +5.00 nC and q2 = -3.00 nC are seperated by 35.0 cm.

 

What is the potential energy of the pair?

 

The answer I got was 3.85 * 10 ^ -7.

I ignored the signs (+) & (-) of the charges and reasoned that it's b/c the charges attract each other.

 

But the answer in the book says it is -3.85 * 10 ^ -7 since energy must be added to seperate the charges.

 

I thought potential energy is the energy needed to bring a particle from point A (infinity) to some finite distance B. If it's (-), then that means work needs to be done to bring these 2 together. But I reason that since they are opposite charges, they d o not need any additional energy to keep them together.

 

Well, thanks in advance for any replies.

 

:/

Hi so i can help can you please tell me what equation you used for your calculation whether it was PE = k q Q / r or not .... ty

sup NSX! i think the problem lies in whether you are a Chemist or a Physicist. The reason being as follows...

 

If you are a Chemist, then you take a "system-centric" point of view. That is, if the system looses energy, then you treat it as a negative sign. Thus, in chemical reactions that release heat, we say that they have a negative delta "H". On the other hand, if you were to have to input energy into the system, then you would say that the process has a positive sign for the energy, as the system gains energy. Our physicists friends are not quite so nice.

 

If you are a physicist, then you take a "observer-centric" point of view. Thus, energy that is extracted from a sytem can be gained by an external object and they say that energy is positive. Thus, if you can extract energy (for instance, leltting two oposite charges come together) then you would label that as a positive energy. Becuase we can extract this eneregy from the system. However, to move the particles away from eachother energy is required. It is assumed that "we" will be supplying the energy to do this and so we will be loosing energy to the system. Thus, we call this a negative energy change.

 

 

WEll, i hope that helps, it is all really just do to a sign convention and the fact that chemists and physicists cannot agree on it.

 

Cheers

The existence of potential energy in regards to particles suggests instability. That is this instability can be stabilized through conversion to kinetic energy or other forms of energy.

 

Remember that energy is not a vector quantity so do not confuse direction the particles are moving with energy.

 

In regards to your question, you should think in terms of change in potential energy and not just potential energy itself. So in order to found out whether the change in potential energy is negative or positive use Coulumbs law (your posted equation) and add the desired magnitudes in the distance variable.

More simply (perhaps): the PE at infinity is zero. If the force is attractive, you have to add energy to separate the particles, so they have negative PE. You could have extracted this energy as the particles came together.

 

One of the implications of this convention is that negative mechanical energy (KE + PE) implies a bound system.