# Electron mass/energy do not conform to e=mc^2

1. Oct 31, 2008

### timallard

Using E=mc^2, if you use the standard Coloumb charge for an electron and the typical value in textbooks for its mass, the difference is 10^5 off, it's way too heavy, 9.11x10^-31 vs 1.78x10-36 kg.

What does this difference represent, shouldn't the mass conform to the equation for that energy value?

2. Oct 31, 2008

### mgb_phys

How are you relating it's charge to it's energy?
And why does that energy have anything to do with it's mass?

3. Nov 1, 2008

### timallard

Most renditions I've seen equate energy and charge so that's what I've been doing.

As far as I've studied, energy and mass are directly related and under certain circumstances can transform from one to the other.

Since I'm concerned about these things at relativistic velocities I use the extra term for the energy equation most of the time and don't often leave it out.

I was just "checking" the mass value and found the standard value given for rest mass does not work when you set velocity to zero for the energy of an electron volt.

I didn't expect that and don't know the answer to why the measured rest mass is so far from satisfying the equation, a factor of 10^5, that's a huge difference, my guess is along the lines of experimental procedures and so on, any clues appreciated.

4. Nov 1, 2008

### ZapperZ

Staff Emeritus
You really have not answered mgb's question. How did you get the "number" for the energy using the charge? Did it come to you in a dream, or did you use a particular "formula"? We know what it is using its mass.

Zz.

5. Nov 1, 2008

### Staff: Mentor

The mass of an electron is 511 keV/c², so when an electron and a positron anhillate you get two photons of 511 keV each. The m of the electron and positron are converted to the E of the photons.

If you didn't get something similar then you did a calculation wrong or used a formula incorrectly.

6. Nov 1, 2008

### timallard

As stated in the original post, using the standard charge for an electron, 1.6x10-19 Coloumb in most texts, for E being used in E=mc^2, is there another value or equation I should be using?

7. Nov 1, 2008

### timallard

Cool, I wasn't using the right energy value, thanks.

8. Nov 1, 2008

### Staff: Mentor

You're welcome.
FYI, in the equation E=mc² the c is the speed of light, not the charge.

9. Nov 1, 2008

### ZapperZ

Staff Emeritus
Good grief. If this is what truly happened, then this whole thread is moot.

Zz.

10. Nov 1, 2008

### timallard

Well, I still don't understand what the relationship of charge and energy at rest for an electron is, none of that has been explained.

After thinking about it more, without knowing that relationship, what I want to learn has not been answered.

Last edited: Nov 1, 2008
11. Nov 1, 2008

### timallard

FYI: I use upper case C, or q, for charge, lower case c for speed of light ... for this was using E = q ...

Last edited: Nov 1, 2008
12. Nov 1, 2008

### ZapperZ

Staff Emeritus
Then how in the world were you able to compare the energy of the two? I mean, you were the one who came up with this whole thing. Why are you asking me when both I and mgb are the ones asking for how you came up with such comparison?

This is making very little sense.

Zz.

13. Nov 1, 2008

### Staff: Mentor

But E does not equal q, the units aren't even right.

14. Nov 1, 2008

### timallard

Hmmm, I thought they were both Nm2/s2 ... the way this has been going I'd better check ...

15. Nov 1, 2008

### Staff: Mentor

In the SI system charge is in units of coulombs where 1 C = 1 A s

In the SI system energy is in units of joules where 1 J = 1 kg m²/s²

16. Nov 1, 2008

### timallard

If you balance an orbital with charge the C's cancel out from Coloumb's constant k, F=k(qq'/r2) is where I'm coming from.

I'm trying to take energy into account due to using the centrifugal force equation, mv2/r, with v = c - 1x10-18.

So, since this is happening simultaneously, I thought the values used had to satisfy the energy equation E=mc2/(1-v2/c2)1/2 at the same time satisfying k(qq'/r2) = mv2/r

Last edited: Nov 1, 2008
17. Nov 1, 2008

### Hootenanny

Staff Emeritus
I'm not quite sure what you mean. Could you show us explicitly the calculations that you are doing?

18. Nov 1, 2008

### Staff: Mentor

But there is no energy in that equation, so I still don't see how you get that charge equals energy.

I agree with Hootenanny, please show us exactly what you are doing that makes you believe that charge is energy. I think you simply have a confusion about units, but if we can see exactly where it is coming from we can probably help you understand better.

Last edited: Nov 1, 2008
19. Nov 1, 2008

### Hootenanny

Staff Emeritus
You still can't equate force with energy, they are not equivalent quantities.
Where did you pull this relationship from? What are the units associated with the number 1x10-18?

Last edited: Nov 1, 2008
20. Nov 1, 2008

### Hootenanny

Staff Emeritus
Please stop editing your posts after we have responded to them. If you want to add more detail please use a new post.

I still have no idea what you're actually doing. I'll say again, can you show me explicitly from start to finish what your doing?