Calculate the kinetic energy, in electronvolts, of an electron that...

Total energy of a relativistic particle is given by $E = mc^2$.

Given that the mass of the electron is twice its rest mass, $m = 2m_0$.

Total energy $E = 2m_0c^2$.

Kinetic energy $K$ is the difference between the total energy and the rest mass energy:

$K = E - m_0c^2$

$K = 2m_0c^2 - m_0c^2 = m_0c^2$

The rest mass energy of an electron is $m_0c^2 \approx 0.511 \text{ MeV} = 511 \times 10^3 \text{ eV}$.

Therefore, the kinetic energy of the electron is $511 \times 10^3 \text{ eV}$.