Question

Chemistry question related with chemical bonding!

a) As two oppositely charged particles approach each other, the potential energy of the system decreases. Why?

b) If two particles of the same charge approach each other, the potential energy of the system increases. Why?

Please explain. Thank you

Answer 1

Hey @matt101 :D

Answer 2

Haha hey! Let me see about answering your questions...

Answer 3

Cool you got this!

Answer 4

@matt101 it is time for sleep for me. Got to go, but when I come back, I will read your answer.

Answer 5

No problem - just took me some time to find the right words.

Let's start with what potential energy (PE) is. PE is a consequence of an object's relative location in some sort of field...which means it isn't really as intuitive as something like kinetic energy, which you can see in something moving, or even heat energy, which you can feel in something hot. PE usually arises when something is moved away or deviates from some reference point in that field. PE can also be applied to the state of an entire SYSTEM, which is more appropriate for your specific question (as I'll explain in minute).

In your case, you're dealing with an electric field. An electric field forms any time you have a charged particle. The reference point used by convention is either the particle creating the charge or, if appropriate, the "natural state" of the system for more than one charged particle. For multiple charged particles, the PE of the system depends on where it is relative to the first particle AND the CHARGE of the particles because that influences how they interacts with the field itself.

So for your questions: When you have two oppositely charged particles, they're attracted to each other and want to be as close as possible. When they're side by side, it's the natural state for the SYSTEM - the point where there are no deviations. If you move the two particles farther apart, the PE of the system will begin to increase, because you're creating a deviation now from that starting point. Similarly, if you move them closer together again, you decrease the PE because you're restoring the system to its starting point.

If your two particles have the same charge, the opposite is true for PE. Now, the particles repel one another and want to be as far apart as possible. In fact, the most stable configuration of the system is for the particles to be an INFINITE distance from one another, and so we treat this as the reference point instead. That means if you starting pushing the particles towards each other, the PE of the system will increase, because you're creating a deviation from that starting point. PE will decrease for a similar reason if you move the particles further apart once more.

This is all really just a wordy explanation for something you can figure out quite easily just by looking at the equation for electrical PE:

\[PE={kq_1q_2 \over r}\]
If you want me to explain how this equation works in more detail then let me know!

Anyways, hopefully that wasn't too long-winded, just wanted to be thorough. If you have any questions just ask!

Answer 6

Thank you!