Transcript Mass and Energy - Beverley High School

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