Energy is generally defined as the potential to do work or produce heat. This definition causes the SI unit for energy to be the same as the unit of work – the joule (J). Joule is a derived unit of energy, and it is named in honor of James Prescott Joule and his experiments on the mechanical equivalent of heat. In more fundamental terms, 1 joule is equal to:
1 J = 1 kg.m2/s2
Since energy is a fundamental physical quantity and is used in various physical and engineering branches, there are many energy units in physics and engineering. These units are summarized in the following points:
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- 1 joule = 0.239 Calories
- 1 joule = 9.48 x 10-4 BTU
- 1 joule = 2.778 x 10-7 kWh
Examples of Energy of 1 Joule:
One joule in everyday life and science corresponds to approximately:
- The kinetic energy of an object with mass 1 kg moving at √2 ≈ 1.4 m/s.
- The kinetic energy of a 50 kg object (e.g.,, human) moving very slowly – approximately 0.72 km/h.
- The energy required to lift a medium-size apple (100 g) 1 meter vertically from the surface of the Earth.
- The heat required to raise the temperature of 1 g of water by 0.24 °C.
- The heat required to evaporate of 0.00044 g of liquid water at 100°C.
- The amount of electricity required to light a 1 watt LED for 1 s.
- Is released by approximately 3.1⋅1010 fissions in a nuclear reactor.
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- 1 calorie = 4.184 J
- 1 calorie = 0.00397 BTU
- 1 calorie = 1.16 x 10-6 kWh
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- 1 British Thermal Unit (BTU) = 1055 J
- 1 British Thermal Unit (BTU) = 252 calories
- 1 British Thermal Unit (BTU) = 0.000293 kWh
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- 1 foot-pound force = 1.356 J
- 1 foot-pound force = 0.324 cal
- 1 foot-pound force = 0.00129 BTU
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- 1 kWh = 3.6 x 106 J
- 1 kWh = 8.6 x 105 cal
- 1 kWh = 3412 BTU
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- 1 MWd = 8.64 x 1010 J
- 1 MWd = 2.06 x 1010 cal
- 1 MWd = 8.19 x 107 BTU
Electronvolt (unit: eV). Electronvolts are a traditional unit of energy, particularly in atomic and nuclear physics. An electronvolt is equal to the energy gained by a single electron when accelerated through 1 volt of electric potential difference. The work done on the charge is given by the charge times the voltage difference, therefore the work W on electron is: W = qV = (1.6 x 10-19 C) x (1 J/C) = 1.6 x 10-19 J. Since this is a very small unit, it is more convenient to use multiples of electronvolts: kilo-electronvolts (keV), mega-electronvolts (MeV), giga-electronvolts (GeV), and so on. Since Albert Einstein showed that mass and energy are equivalent and convertible one into the other, the electronvolt is also a unit of mass. It is common in particle physics, where units of mass and energy are often interchanged, to express mass in units of eV/c2, where c is the speed of light in a vacuum (from E = mc2). For example, it can be said the proton has a mass of 938.3 MeV, although strictly speaking, it should be 938.3 MeV/c2. For another example, an electron-positron annihilation occurs when a negatively charged electron and a positively charged positron (each with a mass of 0.511 MeV/c2) collide. When an electron and a positron collide, they annihilate, resulting in the complete conversion of their rest mass to pure energy (according to the E=mc2 formula) in the form of two oppositely directed 0.511 MeV gamma rays (photons).
e− + e+ → γ + γ (2x 0.511 MeV)
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- 1 eV = 1.603 x 10-19 J
- 1 eV = 3.83 x 10-20 cal
- 1 eV = 1.52 x 10-22 BTU
Example of Energies in Electronvolts
- Thermal neutrons are neutrons in thermal equilibrium with a surrounding medium of the temperature of 290K (17 °C or 62 °F). Most probable energy at 17°C (62°F) for Maxwellian distribution is 0.025 eV (~2 km/s).
- The thermal energy of a molecule is at room temperature, about 0.04 eV.
- Approximately 1 eV corresponds to an infrared photon of wavelength 1240 nm.
- Visible light photons have energies in range 1.65 eV (red) – 3.26 eV (violet).
- The first resonance in n + 238U reaction is at 6.67 eV (energy of incident neutron), which corresponds to the first virtual level in 239U, which has a total width of only 0.027 eV mean life of this state is 2.4×10-14s.
- The ionization energy of atomic hydrogen is 13.6 eV.
- Carbon-14 decays into nitrogen-14 through beta decay (pure beta decay). The emitted beta particles have a maximum energy of 156 keV, while their weighted mean energy is 49 keV.
- High energy diagnostic medical x-ray photons have kinetic energies of about 200 keV.
- Thallium 208, one of the nuclides in the 232U decay chain, emits gamma rays of 2.6 MeV, which are very energetic and highly penetrating.
- The typical kinetic energy of alpha particle from radioactive decay is about 5 MeV. It is caused by the mechanism of their production.
- The total energy released in a reactor is about 210 MeV per 235U fission, distributed as shown in the table. In a reactor, the average recoverable energy per fission is about 200 MeV, being the total energy minus the energy of antineutrinos that are radiated away.
- Cosmic rays can have energies of 1 MeV – 1000 TeV.
Examples of Energy of 1 Joule
One joule in everyday life and science corresponds to approximately:
- The kinetic energy of an object with mass 1 kg moving at √2 ≈ 1.4 m/s.
- The kinetic energy of a 50 kg object (e.g.,, human) moving very slowly – approximately 0.72 km/h.
- The energy required to lift a medium-size apple (100 g) 1 meter vertically from the surface of the Earth.
- The heat required to raise the temperature of 1 g of water by 0.24 °C.
- The heat required to evaporate of 0.00044 g of liquid water at 100°C.
- The amount of electricity required to light a 1 watt LED for 1 s.
- Is released by approximately 3.1⋅1010 fissions in a nuclear reactor.
- J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
- J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
- W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
- Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
- Todreas Neil E., Kazimi Mujid S. Nuclear Systems Volume I: Thermal Hydraulic Fundamentals, Second Edition. CRC Press; 2 edition, 2012, ISBN: 978-0415802871
- Zohuri B., McDaniel P. Thermodynamics in Nuclear Power Plant Systems. Springer; 2015, ISBN: 978-3-319-13419-2
- Moran Michal J., Shapiro Howard N. Fundamentals of Engineering Thermodynamics, Fifth Edition, John Wiley & Sons, 2006, ISBN: 978-0-470-03037-0
- Kleinstreuer C. Modern Fluid Dynamics. Springer, 2010, ISBN 978-1-4020-8670-0.
- U.S. Department of Energy, THERMODYNAMICS, HEAT TRANSFER, AND FLUID FLOW. DOE Fundamentals Handbook, Volume 1, 2, and 3. June 1992.