# Homework Help: Change of measurement units

1. Oct 27, 2005

### asdf1

how do you change the measurement unit for an electron mass from kg to MeV/c^2?

2. Oct 27, 2005

### andrevdh

use Einstein's famous equation
$$E=mc^2$$
and use the conversion for joules per electron volt.

3. Oct 27, 2005

### asdf1

thanks!!! :)

4. Oct 28, 2005

### andrevdh

It is a pleasure.

5. Oct 30, 2005

### asdf1

@@a i'm still having trouble doing the calculations~
could you write out the whole equation to transfer the mass of an electron from kg to Mev?

6. Oct 30, 2005

### big man

You know the mass of the electron and you know the speed of light. Just plug those into $$E=mc^2$$. You will have an energy value in terms of joules. Since you also know that one electronvolt is equivalent to $$1.602*10^-^1^9$$ joules, you can find the energy value in terms of electronvolts instead. Rearrange the original expression and you can reexpress the mass in terms of $$MeV/c^2$$.

Last edited: Oct 31, 2005
7. Oct 31, 2005

### andrevdh

It is amazing how much confusion such a deceptively simple formula can generate. First calculate the rest mass of an electron:
$$\begin{split*} E_e = m_ec^2 \\ \ = 9.11 \times 10^{-31} \times (3.0 \times 10^8)^2 \\ \ = 8.2 \times 10^{-14} \ joule \\ \ = 512 \ keV \\ \ = 0.512 \ MeV \\ \ m_ec^2=0.512 \ MeV\end{split*}$$
Now get the mass in the "strange" new units:
$$m_e=0.512 \ MeV/{c^2}$$

Last edited: Nov 1, 2005
8. Oct 31, 2005

### asdf1

thank you very much!!! :)