Joule

The joule (symbol: J) ( or sometimes ) is a unit in the SI system named after English physicist James Prescott Joule.

Classical mechanics

1 J is defined as the amount of energy transferred to an object when a force of 1 newton (kg m s -2 ) is exerted on it over a distance of 1 metre (1 J = 1 N × 1 m), which is related to the W, a unit of power (P): One watt is defined as one joule per second of energy transfer. Algebraically, work done in classical mechanics (W) can be expressed as: W F • s

Electronics

1 J is defined as the work done for one ampere of electric current to pass through an electric circuit with external resistance equivalent to one ohm per second. It can be illustrated by the following equations:

E = C V — (1) V = I R — (2) P = V I — (3) E = P t — (4) Combining (2), (3) and (4), E = I 2 R t

where V is the voltage, E is the electrical energy, C is the charge, I is the current, R is the resistance, P is the power and t is the time of energy transfer.

Atomic physics

Arithmetically, 1 J ≈ 6.24 × 10 12 MeV due to mass-energy equivalence. One unified atomic mass unit (u, amu or Da) is approximatey 931 MeV of energy (M = 10 6 ).

Derivation 1 amu ≈ 1.66 × 10 -27 kg (¹⁄₁₂ mass of a carbon-12 isotope by definition). Speed of light (c) = 3.00 × 10 8 ms -1 . ΔE = Δmc 2 ΔE = (1.66 × 10 -27 )(3.00 × 10 8 ) 2 = 1.49 × 10 -10 J ∵ 1 eV ≈ 1.60 × 10 -19 J 1 MeV = 1.60 × 10 -19 × 10 6 J = 1.60 × 10 -13 J 1.49 × 10 -10 J = MeV = 931 MeV (3 s.f.)

∴ 1 amu ≈ 931 MeV