Mass, Energy and the Unified Atomic Mass Unit
Mass and energy are one quantity written in two units, tied together by E = mc squared. On the nuclear scale that makes one atomic mass unit worth about 931 MeV, which is why nuclear energies dwarf chemical ones.
What a learner can do afterwards
- Converts a mass in kilograms or atomic mass units into an energy in joules and in MeV
- Compares the energy per particle from a nuclear change with that from a chemical one
- Explains why the change in mass goes unnoticed in everyday energy transfers
1 · Read
Mass and energy are one quantity in two units, tied by E equals m c squared, which is why a power station can run a city on barely any fuel. The c squared factor is huge: with c at 3.0e8 m/s, c squared is 9.0e16. Losing just 0.004 kg in a reaction releases 0.004 times 9.0e16, which is 3.6e14 J.
On the nuclear scale one atomic mass unit is worth about 931 MeV. A chemical change shuffles outer electrons and pays only a few eV per particle, like burning coal to heat a home. A nuclear change rebuilds the nucleus and pays millions of eV, roughly a million times more for each particle than coal.
The conversion runs both ways. Building a particle of mass 2.0e-28 kg from pure energy costs 2.0e-28 times 9.0e16, which is 1.8e-11 J. The formula only gives the exchange rate, not the machine that performs the swap.
Do not expect the scales to notice everyday transfers. Heating water adds energy, so its mass grows by E over c squared, but dividing by that enormous c squared leaves an unweighable speck. The change is real yet far too small for any balance.
Lost mass times c squared equals released energy, which is why nuclear changes dwarf chemical ones.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.