Electrostatic Question

[NubianPrincess]

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So, two point charges of opposite sign will exert forces on each other that are equal in magnitude and opposite in direction according to Newton's third law.

What I don't understand is why, for example, an Na+ ion and a (SO4)2- ion will exert equal but opposite forces on each other when the sulfate ion is twice as negative as something like a Cl- ion. Am I not understanding the meaning of charge q?

Can someone explain this to me?

 

[NubianPrincess]

If it helps, my question is related to question #763 in EK 1001 Physics.

 

[Nuel]

I'll take a stab at this one. I don't use EK, I use Kaplan, and it never explains anything like this, but I will draw on my little knowledge here.

First of all I would look at the possible alignment of charges. For every sulphate anion, there are two sodium cations. So imagine a sulphate ion at the middle of two sodium cations such that it applies a proportional quantity of negative charge to the interaction with both neighboring (adjacent) cations. In that case, each sodium cation and sulphate anion will exert equal and opposite charges on each other. For Mg2+ with two chloride anions the same reasoning applies.

I hope this helps as I had to think about this for a moment.

 

[zepplinfan]

Originally posted by NubianPrincess
So, two point charges of opposite sign will exert forces on each other that are equal in magnitude and opposite in direction according to Newton's third law.

What I don't understand is why, for example, an Na+ ion and a (SO4)2- ion will exert equal but opposite forces on each other when the sulfate ion is twice as negative as something like a Cl- ion. Am I not understanding the meaning of charge q?

Can someone explain this to me?

Princess,

I think ur making it more difficult than its. I have the 1001 and this is my explanation. It's simple electrostatics

F = k*abs(q1*q2)/r^2...........simply coulombs law

Though the question seems to suggest that there are two forces here (there kinda is): one of them is a reaction force. The only quantitative answer to force is given by coulombs law.

Consider the Na+ as q1 and the SO4-2 as q2. It is important to see that it does not matter what q1 and q2 values are......a force calculation is made and this same force is experienced by q1 due to q2 and by q2 due to q1. The important thing to identify finally is that they are in opposite directions.

THIS IS JUST LIKE GRAVITY. The REACTION FORCE to gravity pulling you down onto the earth is NOT THE NORMAL FORCE. It is the force of gravity (due to your mass) pulling on the earth.

This is why the book mentions Newton's 3rd law where Fb = -Fa if they are reaction forces. Finally, you should see why it does not matter what q2 is , meaning it doesn't matter if it is sulfate ion or chloride ion. Though the FORCE CALCULATION would be different, it would not change the answer choice.

Just my thoughts on it. Let me know if this doesn't make sense or I missed something.

~ Zep

 

[NubianPrincess]

Thank you for the replies. After reading a few explanations over and over, it finally makes sense

 

[trauma_junky]

you have to take into accout the atomic radius and that Coulombs law states that the force is ralated to the inverse square of the distance.