# Find the magnitude of the attractive force

[SOLVED] Coulomb's Law

## Homework Statement

Find the magnitude of the attractive force exerted by the electrons in a cup of water on the protons in the second cup of water at a distance of 10 m. Assume that each cup contains 250g of water and that 6x10^23 water molecules (one mole) weighs 18 g. Do two cups of water exert any net force on each other? Why?

F = k*(qq/r^2)

## The Attempt at a Solution

250g H2O / 18g H2O * 6.022^23 = 8.36^24 molecules of water

I don't understand how to proceed after that. Do I need to find the number of electrons/protons in all H and O atoms separately and add them? Then multiply by 1.602^-19C (electron/proton charge) and plug those numbers into the formula?

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You can approximate the all the electrons in one cup as one big charged point. Ditto for the protons.

dynamicsolo
Homework Helper

## The Attempt at a Solution

250g H2O / 18g H2O * 6.022^23 = 8.36^24 molecules of water

I don't understand how to proceed after that. Do I need to find the number of electrons/protons in all H and O atoms separately and add them?
You will need to find how many electrons or protons are in that many water molecules (you know they'll be the same total) and then, as genneth says, treat the total electron charge in one cup as a "lump" and the total proton charge as a "lump" in the other cup.

You will need to find how many electrons or protons are in that many water molecules (you know they'll be the same total)
Right, that's where I'm stuck.

How many electrons are there in a water molecule? How many protons?

1+1+8?

dynamicsolo
Homework Helper
1+1+8?
Roger that! So what is the total proton or electron charge in each cup?

Sounds about right. And you already know the number of molecules in the beakers... so -- what's the number of electrons/protons per beaker? Can you finish the question now?

8.36^24 * 10 * 1.602^-19?

Go on... no need for us to be holding your hand *that* tightly...

dynamicsolo
Homework Helper
8.36^24 * 10 * 1.602^-19?
And the stadium crowd goes wild! Now you're ready to apply Coulomb's Law.

Thanks guys.