# Group Velocity and Phase Velocity

1. Oct 14, 2006

Obtain the group velocity $$v_g$$ and phase velocity $$v_p$$ of 7.0 MeV protons/electrons. Write each answer as a multiple of the speed of light $$c$$.

My work:

1. Finding the group velocity:

$$v_g = \frac{\partial \omega}{\partial k} = \frac{\partial \left( E / \hbar \right)}{\partial \left( p / \hbar \right)} = \frac{\partial E}{\partial p}$$

$$v_g = \frac{\partial E}{\partial p} = \frac{\partial}{\partial p} \left( \sqrt{p^2c^2 + m^2c^4} - mc^2 \right) = \frac{pc^2}{\sqrt{p^2c^2 + m^2c^4}}$$

$$v_g = \frac{pc}{\sqrt{\left( pc \right) ^2 + \left( mc^2 \right) ^2}} \times c$$

If $$E = pc = 7.0 \times 10 ^6 \mbox{ eV}$$ and the mass of protons and electrons are known, it is possible to obtain $$v_g$$.

Assuming

$$m = 938.27 \mbox{ MeV}/c^2$$ for a proton
$$m = 0.51100 \mbox{ MeV}/c^2$$ for an electron

the values $$v_g \approx 0.007460 \times c$$ (for protons) and $$v_g \approx 0.9973 \times c$$ (for electrons) are obtained.

2. The phase velocity is simply

$$v_p = \frac{c^2}{v_g} = \frac{c}{v_g} \times c$$.

I believe there is a mistake in my approach; those numbers don't look right. Any help is highly appreciated.

Last edited: Oct 14, 2006
2. Sep 27, 2009

### Sasuke

Why the -mc2?