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The kinetic energy of an electron moving with a speed of 0.6c and rest mass energy of 0.511 MeV, is:
1. 0.224 MeV
2. 0.128 MeV
3. 0.64 MeV
4. 0.540 MeV

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Correct Answer - Option 2 : 0.128 MeV

Concept:

Variation of Mass with Velocity

According to classical physics, the inertial mass of a body is independent of the velocity of light. It is regarding as a constant. However special theory of relativity leads us to the concept of variation of mass with velocity. It follows from special theory of relativity that the mass m of a body moving with relativistic velocity v relative to an observer is larger than its m0 when it is at rest.

According to Einstein, the mass of the body in motion is different from the mass of the body at rest.

\(m = \frac{{{m_o}}}{{\sqrt {1 - \frac{{{v^2}}}{{{c^2}}}} }}\)

This is the relative formula for variation of mass with velocity where m0 is the rest mass and m is the relativistic mass of the body.

Total energy = Rest energy + Kinetic Energy

Since E = mc2

mc2 = moc2 + KE

K E = (m – mo) c2

This can be written as:

\(KE = \left( { \frac{{{m_o}}}{{\sqrt {1 - \frac{{{v^2}}}{{{c^2}}}} }}- {m_0}} \right){c^2}\)

\(KE =m_0c^2 \left( { \frac{{{1}}}{{\sqrt {1 - \frac{{{v^2}}}{{{c^2}}}} }}- {1}} \right)\)

Calculation:

Putting on the respective values, we get:

\(= 0.511\;\left( {\frac{1}{{\sqrt {1 - 0.36} }} - 1} \right)\)

\(= \frac{{0.51}}{4}\)

= 0.128 MeV

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