Mass and Energy E=mc 2 and all that

MASS and REST MASS In 1905 Einstein showed that the mass of a moving object, as measured by a stationary observer, increases as it approaches the speed of light. This effect is important for fundamental particles which can be accelerated to very high velocities.Therefore we use the term rest mass (m 0 ) of the particle to mean the mass of the particle when it is stationary ?

The energy equivalence of mass We can talk abou the rest mass and the rest energy of a particle interchangeably. However the units are awkward because they are so small It is convenient to have another unit for energy in particle physics particleRest mass (m 0 /kg) Rest Energy (J) electron 9.11x x proton 1.67 x x10 -10

Potential difference of 1 volt Units of Energy A negatively charged electron is accelerated from a negative electrode towards a positive electrode and in doing so gains kinetic energy. The amount of kinetic energy it acquires,just before it strikes the anode is called 1 electron volt. An electron volt is a tiny quantity of energy 1eV=1.60 x J

particleRest mass (m 0 /kg) Rest Energy (J) Rest Energy (MeV) electron 9.11x x proton 1.67 x x The mass equivalent of the electron is 8.20x J Which is the same as 8.20x ÷ 1.60x = eV These more sensible numbers are often used directly in calculations.

The unified atomic mass unit The unified atomic mass unit (u) is defined as of the mass of aatom. On this scale the normal hydrogen atom has a mass of The oxygen-16 atom is etc 1u = x kg = 931.5MeV