^Einstein’s mass-energy equivalence
is called relativistic energy or Einstein’s mass-energy equivalence as it reveals that mass & energy are interconvertible. Here the quantity m0 c2 is the energy associated with the rest mass of the body, K is the kinetic energy of the body & the sum of KE & rest mass energy is called the total energy (E) of the body.
Differentiating this relation we get
c2.2 m dm – m2.2v dv – v2.2m dm = 0
or c2 dm = mv.dv + v2 dm _ _ _ _ (1)
Also according to Newton’s second law of motion, force acting on a body is defined as the rate of change of its momentum i.e.
Now if this force F displaces the body by a distance dx, its energy increases by
Equating (1) & (2) we get
dK = dm.c2 _ _ _ _ (3)
If particle is accelerated from rest to a velocity v, let its mass m0 increase to m, then its total increase in KE can be obtained by integrating equation (3) , i.e.
Suppose a body of rest mass m0 moves with a velocity v << c. Its mass at speed v is
As mc2 is the total energy and m0 c2 is the rest mass energy. Therefore the term (mc2 – m0 c2) represents the kinetic energy, thus KE of the body is
This is the same result that we obtain by classical Newtonian mechanics.
1. Zero rest mass i.e. m0 = 0 means massless particle & for such particles, E = pc i.e. even a massless particle can possess energy.
2. An electron and photon have same de-broglie wavelength. Which has more total energy?
Ans. Same λ means same p (as, p = h/λ) & quantity having more m0 will have more E, i.e. Electron.
3. An electron and proton have same total energy. Which has more de-broglie wavelength?
Ans. In order to have same E the quantity having less m0 must have more p or less λ (as, p = h/λ), i.e. Electron.
4. Also KE of a particle is
5. If a photon and electron has same de Broglie wavelength, then the KE of the photon is times the kinetic energy of the electron.
Commonly used values
1. Charge of electrons = e = – 1.6 x 10-19 C
2. Rest mass of electrons = m0 = 9.1 x 10-31
3. Sp. charge of electron = e/m0 = 1.76 x 1011 C/ kg
4. hc = 1.989 x 10-25 » 2 x 10-25