The Physics
Hypertextbook
Opus in profectus

# Energy

## Summary

• Definition
• A system possesses energy if it has the ability to do work.
• Energy…
• is an abstract scalar quantity
• is a quantity that an object or system of objects is said to possess
• is not a thing that can be measured
• is a quantity that acquires meaning only through computation
• Forms
• Potential energy
• Definitions
• energy due to arrangement
• energy due to position (of a quantity in a field)
• Types
• gravitational potential energy
• electromagnetic potential energy
• electric potential energy
• magnetic potential energy
• chemical potential energy
• elastic potential energy
• strong nuclear potential energy
• weak nuclear potential energy
• Kinetic energy
• Definition
• energy due to motion
• Types
• mechanical energy: macroscopic motion
• thermal energy (heat, internal energy): random motion of microscopic particles
• electric energy: bulk flow of charged particles
• propagating disturbance of electric and magnetic fields (classical physics)
• motion of photons (modern physics)
• Work-energy theorem
• Work is done when the energy of an object changes form (energy transduction).
• Work is done when energy is transfered from one object to another (energy transfer).
• Work is done when energy changes.
 W = ∆E
 F∆s cos θ = ∆E
calculus version
 ⌠⌡ F · ds = ∆E

• Units
• The SI unit of work and energy is the joule, named after the English physicist and brewer James Joule (1818–1889).

[J = N m = kg m2/s2]

• The electronvolt is an acceptable non SI unit of energy. It is used for some applications in electromagnetism; solid state, atomic, nuclear, and particle physics; and related sciences like biophysics, chemistry, and astronomy. An electron volt is the work done a particle with one elementary charge when it is moved between two points with a potential difference of one volt. It is described in more detail in the section on electric potential (a.k.a. voltage).

1 eV = (1.6 × 10−16 C)(1 V) = 1.6 × 10−16 J