Mass–Energy Equivalence
E = mc² — mass in amu → J, MeV, kWh, megatons TNT. Presets for electron → ²³⁵U.
Key Notes
Einstein's mass-energy equivalence (special relativity, 1905): E = mc².
1 kg of mass corresponds to 9 × 10¹⁶ J of energy — vast, but only nuclear reactions release a measurable fraction.
Atomic mass unit: 1 u = 1.66054 × 10⁻²⁷ kg. Energy equivalent: 1 u · c² = 931.494 MeV.
Mass defect in any bound system = (sum of constituent masses) − (system mass). Always positive for bound states.
Energy released = (mass defect) × c². For nuclear reactions, mass defect is detectable (~MeV scale).
Chemical reactions also have mass defects — but ~10⁶× smaller (eV scale) ⇒ undetectable with chemical scales.
Particle-antiparticle annihilation: m + m → 2γ. Each photon carries mc² of energy. Pair production: γ → e⁺ + e⁻ requires hf ≥ 2m_e·c² = 1.022 MeV.
Formulas
Mass-energy equivalence
Rest energy of a particle of mass m.
Useful unit conversion
Converts mass defect in atomic units to energy in MeV.
Q-value of a reaction
Energy released if Q > 0 (exothermic), absorbed if Q < 0.
Pair production threshold
Minimum photon energy to produce an electron-positron pair.
Annihilation
Each photon's energy if particles were at rest.
Important Points
Mass and energy are interconvertible — they are TWO ASPECTS of the same thing.
Even chemical reactions have mass defects, but they are too small to detect (Δm/m ~ 10⁻⁹).
Nuclear reactions have detectable mass defects (Δm/m ~ 10⁻³).
1 u corresponds to 931.5 MeV. Memorise this: it's the bridge between nuclear masses and energies.
E = mc² is the rest energy. Moving particles have additional kinetic energy ⇒ total E = mc²/√(1−v²/c²).
Mass-energy CONSERVATION (not separate mass conservation) holds in all physics.
Mass–Energy Equivalence notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.