Mass and Energy equivalence

1. Particles
Mass and Energy
Rest Mass
Rest Energy
Thursday, 08 December 2011

2. Mass and Energy
In the fission of a nucleus of Uranium-235 the nucleus splits into
two lighter nuclei releasing energy. The mass at the end of the
reaction is less than the mass at the beginning and the missing
mass has been converted into energy.
Incoming
neutron
Kr-90
U-235 nucleus
nucleus
Ba-144
nucleus

3. Mass and Energy
A high energy photon like a -ray can vanish to form a pair particle –
anti-particle. This is the opposite of annihilation and we call it PAIR
PRODUCTION.
e+ e-
In what way is this reaction similar to the fission of Uranium-235 and in
what way is it different?

4. Mass and Energy
In both reactions energy and mass are involved.
In the fission reaction mass is converted into energy.
In the pair production energy is converted into mass.
So, mass and energy are an equivalent form of each
other.
N.B. Their are not the same but they can be turned
into each other in certain reactions.

5. Rest Mass
Every particle has a rest mass. This is the value of the
mass when the particle is stationary.
Does this mean that moving particles have a different
mass?
Yes, because they have kinetic energy, which is an
equivalent form of mass.

6. Rest Energy
What is rest energy then?
It is the energy equivalent of the mass of the particle
when it is stationary.
And in the case of the pair production the photon has
to have an energy at least equal to the sum of the
rest energy of the electron and the positron for the
reaction to happen.