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Mass and Energy E=mc2 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 (m0) 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 particle Rest mass (m0/kg) Rest Energy (J) electron 9.11x10-31 8.20x10-14 proton 1.67 x 10-27 1.50 x10-10 Units of Energy ---------------------------------------------- Potential difference of 1 volt A negatively charged free 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 x10-19J The mass equivalent of the electron is 8.20x10-14J Which is the same as 8.20x10-14 ÷ 1.60x 10-1 9 = 511099eV particle Rest mass (m0/kg) Rest Energy (J) Rest Energy (MeV) electron 9.11x10-31 8.20x10-14 0.511 proton 1.67 x 10-27 1.50 x10-10 938 These more sensible numbers are often used directly in calculations. The unified atomic mass unit • The unified atomic mass unit (u) is defined 1 as of the mass of a 12C atom. 12 6 • On this scale the normal hydrogen atom has a mass of 1.008. • The oxygen-16 atom is 15.995 etc • 1u = 1.661 x 10-27kg = 931.5MeV