# Electrostatic Force Concept Question

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1. May 2, 2015

### whitejac

1. The problem statement, all variables and given/known data
Consider an electron-proton pair and compare its electrostatic force with that of the gravitational force [G=6.67x10^11 Nm^2/kg^2].

2. Relevant equations
F = k * |q1 * q2| / r^2
k = 1/4π∈
3. The attempt at a solution
I calculated the force by using the charge of an electron squared (abs value) with an arbitrary r^2. I don't understand what the question is asking? Compare it to what? My textbook's answer says Fe/Fg is the answer. That's nice. I'm more interested in the why this is important part and which gravitational force we're dealing with.

2. May 2, 2015

### Student100

The question is asking you to look at the ratio of the two forces. Basically, it wants to know which one is larger, since the answer in the book is stating Fe/Fg.

Last edited: May 2, 2015
3. May 2, 2015

### Student100

You're also looking at the gravitational force between the electron-proton pair, using the universal gravitational constant. Does the question give you any more information? Did you find the required r, Mp, and Me for the calculation? Does the question state the typical distances found in orbit around a hydrogen atom or any other element?

Basically you clean glean a few things from this. Which should be fairly obvious when you're done.

Last edited: May 2, 2015
4. May 2, 2015

### whitejac

Thank you for that insight on how to approach this problem. No, it did not give me any information other than what I presented. I found the equations myself and chose (appropriately I hope) my values for the charge. How does one get the gravitational force when r is only given generally? No masses were given either.

5. May 2, 2015

### whitejac

For context, I just picked this problem from the back of my book while studying. I am not well versed in how physicist go about things and figured it'd make more sense to others. To me it is incomplete - aside from what you said.

6. May 2, 2015

### Student100

Have you set up Fe/Fg? You'll discover that ratio is independent of the distance between the photon and electron, so you don't actually need a defined r. To get just Fg or Fe, it seems the question has generally left the distance between the pairs up to your choice. So you can play around with different values, for the standard distances in the orbit of a hydrogen atom you can use the value r = 0.53 x10^(-10) m. The mass of an electron is Me = 9.1x10^(-31) kg and Mp is 1.7x10^(-27)kg. In order to get the r value from the equations you would need to given a force in either equation.

Fg = GMpMe/r^2

While Fe is the equation you have listed.. ke^2/r^2

Fg will give the gravitational force between the two pairs. While Fe will give the electric force.

You can also play around with Fg by calculating the gravitational force between something like the earth and the sun.