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Energy and Forms of Energy


The ability of an object or a system to do something (do work or supply heat).

Gravitational Potential Energy

The energy of an object due to its position (height) in a gravitational field. Near the surface of the earth PE=mgh. Sometimes the letter “U” is used instead of PE.

Elastic Potential Energy

The energy of an object due to the stretching and compression of a spring. PE=0.5*k*x2. Sometimes the letter “U” is used instead of PE.

Kinetic Energy

The energy of an object or system due to its motion (speed). For non-relativistic-speeds KE=0.5*m*v2.

Mechanical Energy

The sum of elastic potential, gravitational potential, and kinetic energy. ME=PE+KE

Internal Energy

The sum of all types of energy of the particles inside a system, excluding the kinetic energy due to the motion of the system itself and the potential energy due to the position of the system. Sometimes the letter “U” is used, but be careful to not confuse it with potential energy, which might be referred to with “U”.

Thermal Energy

The kinetic energy within a system due to the random motion of its particles. Sometimes the term is also used for internal energy (if there is no chemical reaction, no change in state, and no change in pressure) or for heat (which can be set equal to the change in thermal energy under the restrictions above). ΔEth=mcΔT.

Chemical Energy

The energy stored in the bonds of molecules.

Nuclear Energy

The energy released in nuclear reactions due to conversion of mass. E=mc2

Binding Energy

The energy needed to form a nucleus from separate protons and neutrons.

Forms of Energy Transfer

(mechanical) Work

The transfer of energy due to external forces acting on an object/system.


The transfer of thermal energy due to conduction, convection, or radiation.


The transfer of internal kinetic energy from particles of one system to particles of another system through direct contact (collision).


The transfer of internal kinetic energy due to particles travelling between two systems.


The transfer of energy due to electromagnetic waves.

Flow of Matter/Charge

Transfer of energy due to matter/charge leaving or entering a system. 

Related Concepts


The rate of energy transfer (Energy/time). P=ΔE/Δt, SI unit: watt


The sum of internal energy plus the product of pressure and volume of a system. Usually referred to as “H“.
SI unit: joule.


A measurement of chaos in a system. Usually referred to as “S”. SI unit: joule/kelvin


A property of matter that can be measured with a thermometer. SI unit: kelvin.

Units of Energy and Energy transfer


The SI unit for all forms of energy and energy transfer


“calorie” (lower case c). The historic unit defined as the energy needed to increase the temperature of one gram of water by one degree celsius. 1 calorie = 4181 joules


“Calorie” (capital “C”) = 1000 calories = 1 kcal


Energy unit used by utility companies like Hydro-Québec. 1 kWh=3.6*106 joules.


Law of Conservation of Energy

Various forms exist. In the videos, we used. Ef=Ei+Win+Qin, with W the work done ON the system and Q the heat ADDED to the system.

Work-Energy theorem

The law of conservation of Energy without heat transfer. Ef=Ei+Win

First law of thermodynamics

The law of conservation of Energy, only considering internal Energy for E. Often written with the work done BY the system instead of the work done ON the system. Ef=Ei-Wout+Qin

Bernoulli’s Equation

The Law of Conservation of Energy (with no change in temperature) PER VOLUME, useful for fluid dynamics and fluid statics.

Kirchoff’s Loop rule

A practical application of conservation of energy for electric circuits. The sum of all voltages around a loop is zero.

Electric potential or Voltage

Electric potential energy per charge. Sometimes, especially in Europe, written using the letter “U”. (Risk of confusion with potential energy and internal energy)

Hess’s law

The enthalpy change is path-independent (cf. Kirchoff’s Loop rule).