# A How to calculate the electrical field of planets?

1. Dec 9, 2016

### enchantinggust

All,

I am not sure if this is the right forum for this but I believe it is the closest to the question I would like an answer to.

I would like to calculate the electrical force between Mars and the Earth. I believe I understand that in order to do this, I must apply Coulomb's law:

E= Q*q/(4*pi*e0*r^2)

where Q equals the point charge of the Earth, q equals the point charge of Mars, e0 equals the permittivity of vacuum constant, and r^2 equals the distance between the two.

Assuming I know the distance between the Earth and Mars, how would I go about calculating the point charges of the Earth and Mars? Just thinking about it makes my head hurt!

I figured I'd swallow my pride and ask people who know a whole lot more than I do. I believe I have the right equation as well.

Thank you,

Enchantinggust

2. Dec 9, 2016

### ZapperZ

Staff Emeritus
Go back a bit. How do you know that both the earth and mars have net charges? In other words, how do you know that Q and q are not roughly zero?

This is the crucial assumption and your starting point. If you cannot justify that these values are non-zero, then everything else is moot.

Zz.

3. Dec 9, 2016

### anorlunda

Focus on what @ZapperZ said, "net charge" , which is not the same as charge. The Q in your formula is net charge, the part left over after all plus and minus charges cancel each other.

4. Dec 9, 2016

### enchantinggust

Wouldn't Coulomb's law be similar to Kepler's gravitational law (Fg = m1*m2/r^2)? In that case, I would have 2 separate charges from different planets. I could assume that since the earth's magnetosphere deflects solar radiation and has a north and south pole, the overall charge of the earth electrical field could be positive or negative. Am I correct in assuming this?

5. Dec 9, 2016

### enchantinggust

Or was it how I typed it that was confusing?

6. Dec 9, 2016

### ZapperZ

Staff Emeritus
You are still not getting it. "magnetosphere" means that it has a magnetic field. We know this because our compasses work! But what does this have anything to do with Earth and Mars each having a net charge?

Again, you need to establish clearly the validity of your starting point, and in this case, that the Earth and Mars have a substantial charge. Till you can do this, there's no reason to invoke Coulomb's Law.

Zz.

7. Dec 9, 2016

### Staff: Mentor

You don't calculate it; it's something that has to be observed/measured so that you have some input data to put into your Coulomb's law calculation.

Here's an analogous situation. Someone asks you to calculate the water pressure at the deepest point in the ocean. You know the physical laws that govern water pressure have a formula that tells you how to calculate the pressure at a given depth. However, you won't be able to use it until you know the depth of the deepest point in the ocean and for that you need to ask an oceanographer who has observed and measured the ocean depths.

So for this problem, your first step is to find out what is already known about the charges of the Earth and Mars. (And if either charge happens to be zero, the calculation is going to be easy).

8. Dec 9, 2016

### enchantinggust

Ok, so someone would have had to have literally measured one of them before I could calculate the electric fields (makes more sense now). I didn't know if there was another formula that was used to calculate the point charge beforehand.

Enchantinggust

9. Dec 9, 2016

### ZapperZ

Staff Emeritus
Not exactly sure what you mean by "calculate the point charge"... There are ways to do that... IF... you have the electric field or if you know the Coulomb force.

But in your case, you have to first established that the body has charged. You don't just go and calculate the point charge of something theoretically without first establishing (usually via experiment), that that body HAS a net charge. Otherwise, you're simply doing something that does not apply. It is like asking "When did you stop beating your wife?" It has several levels of assumptions that have not been established to be valid.

Zz.