Velocity required to achieve a momentum of 10mc:
To find the velocity at which an electron must be accelerated to have a momentum of 10mc, we can use the equation for momentum: p = mv, where p is the momentum, m is the mass, and v is the velocity.

Given that the momentum is 10mc, where m is the rest mass of the electron, we can substitute these values into the equation: p = 10mc.

Since the rest mass of the electron is m, we can rewrite the equation as: 10mc = m * v.

Now, we can solve for v: v = 10c.

Therefore, the velocity required to achieve a momentum of 10mc is 10 times the speed of light, denoted as 10c.

Energy of the electron at this speed:
To calculate the energy of the electron at a velocity of 10c, we can use the relativistic energy equation: E = γmc², where E is the energy, m is the rest mass, c is the speed of light, and γ is the Lorentz factor.

The Lorentz factor is given by the equation: γ = 1 / √(1 - (v²/c²)), where v is the velocity.

Substituting the velocity v = 10c into the Lorentz factor equation, we get: γ = 1 / √(1 - (10c)²/c²).

Simplifying this expression, we find: γ = 1 / √(1 - 100) = 1 / √(-99).

Since the Lorentz factor is imaginary due to the square root of a negative number, we conclude that it is not physically possible for an electron to reach a velocity of 10c.

Therefore, the energy of an electron at this speed cannot be determined, as it is not a valid scenario in accordance with the laws of physics.