Group Velocity and Phase Velocity

[DivGradCurl]

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.

 

[Sasuke]

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.

Why the -mc²?