# Electrostatic Coulomb Constant

#### Lagreeni

Hi,

I'm sorry if this ends up being a duplicate, but I don't think the post I submited yesterday actually went through.

I'm having an insane amount of difficulty figuring out how the electrostatic coulomb constant:

a<coul>=3/5*ke^2/r<0>

can equal around 0.8MeV.

With the constants:
k=9E9 [Nm^2/c^2]
e=1.6E-19 [c^2]
r<0>=1.07E-15 [m]

I don't get a number anywhere near 0.8MeV. I don't understand what is going on. I do get a value for a<coul>, 0.807MeV, IF I don't square the electric charge term. I don't get why this is happening. Can someone please help?

#### topsquark

Forum Staff
Hi,

I'm sorry if this ends up being a duplicate, but I don't think the post I submited yesterday actually went through.

I'm having an insane amount of difficulty figuring out how the electrostatic coulomb constant:

a<coul>=3/5*ke^2/r<0>

can equal around 0.8MeV.

With the constants:
k=9E9 [Nm^2/c^2]
e=1.6E-19 [c^2]
r<0>=1.07E-15 [m]

I don't get a number anywhere near 0.8MeV. I don't understand what is going on. I do get a value for a<coul>, 0.807MeV, IF I don't square the electric charge term. I don't get why this is happening. Can someone please help?
This post was in the moderation queue as well. You should be able to seeboth posts now.

It might help if we knew what a<coul> is, physically.

-Dan

#### studiot

Hi,

I don't get a number anywhere near 0.8MeV.
I'm not suprised.

0.8 MeV is a unit of energy.

The units in your expression work out as

$$\displaystyle \frac{{N{m^2}}}{{{c^2}}}*\frac{{{{\left( {{c^2}} \right)}^2}}}{m}$$

I don't know what these are, since the symbols are unconventional, but they are not units of energy.

Where did this come from?

#### Lagreeni

I actually figured it out yesterday and it was definitely a face palm mistake. So the units come out as Nm, which are joules (studiot, you accidentally squared coulombs twice). So I never converted the full thing to eV (which is 1.6E-19J=eV). That's why one of the electric charges becomes "obsolete". It's ridiculous how many hours I spent head scratching over this, but it ended up being a pretty trivial mistake.

Thank you for your response studiot, you pointed out exactly the right thing.

1 person

#### studiot

I actually figured it out yesterday and it was definitely a face palm mistake. So the units come out as Nm, which are joules (studiot, you accidentally squared coulombs twice). So I never converted the full thing to eV (which is 1.6E-19J=eV). That's why one of the electric charges becomes "obsolete". It's ridiculous how many hours I spent head scratching over this, but it ended up being a pretty trivial mistake.

Thank you for your response studiot, you pointed out exactly the right thing.

Firstly thanks for coming back with the end of the story.
Not many do that.

But there was nothing accidental about what I wrote.

e^2

and you also said

e= c^2

I know it is not easy to write formulae on this forum unless you know TEx or MathML but please try and use plenty of brackets in future.
It was not totally clear what your expressions actually meant.

But I've saved the best bit till last.

You actually figured it out for yourself, which is always the best way.

Well done