What is Energy – Physics – Definition

Joule (unit: J). Joule  is a derived unit of energy. It is equal to the energy transferred to an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre.

• • 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 in 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.

Calorie (unit: cal). Calorie is a traditional unit of heat. It is part of the  International System of Units (SI). It is defined to be the amount of heat that must be absorbed by 1 gram of water to raise its temperature by 1 °C. Its counterpart in the British Imperial system of units is the BTU, which is defined as the amount of heat required to raise the temperature of one pound of water by one degree thermodynamics/thermodynamic-properties/what-is-temperature-physics/fahrenheit-scale-fahrenheit-temperature/”>Fahrenheit. But we have to distinguish between small calorie and large calorie. The large calorie (unit: Cal) is defined in terms of the kilogram rather than the gram. It is equal to 1000 small calories and it is  i.e. 1 kilocalorie (unit: kcal).  It is used by nutritionists to characterize the energy-producing potential in food.

• • 1 calorie = 4.184 J
  • 1 calorie = 0.00397 BTU
  • 1 calorie = 1.16 x 10^-6 kWh

British Thermal Unit (unit: BTU).  British Thermal Unit is a traditional unit of heat. It is part of the British Imperial system of units. It is defined to be the amount of heat that must be absorbed by a 1 one pound of water to raise its temperature by 1 °F at the temperature that water has its greatest density (approximately 39 degrees thermodynamics/thermodynamic-properties/what-is-temperature-physics/fahrenheit-scale-fahrenheit-temperature/”>Fahrenheit). Its counterpart in the International System of Units (SI) is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree thermodynamics/thermodynamic-properties/what-is-temperature-physics/celsius-scale-celsius-temperature/”>Celsius.

• • 1 British Thermal Unit (BTU) = 1055 J
  • 1 British Thermal Unit (BTU) = 252 calories
  • 1 British Thermal Unit (BTU) = 0.000293 kWh

Foot-pound force (unit: ft.lbf). Foot-pound force is a derived unit of work and energy. It is equal to the energy transferred to an object when a force of one pound-force (lbf) acts on that object in the direction of its motion through a distance of one foot. The corresponding SI unit is the joule. The foot-pound is often used in ballistics, especially in the United States. Typically muzzle energies of bullets are given in foot-pound force.

• • 1 foot-pound force = 1.356 J
  • 1 foot-pound force = 0.324 cal
  • 1 foot-pound force = 0.00129 BTU

Kilowatt-hour (unit: kWh). Kilowatt-hour is a derived unit of energy. It is used to measure energy, especially electrical energy in commercial applications. One kilowatt-hour is equal to one kilowatt of power produced or consumed over a period of one hour (kilowatts multiplied by the time in hours). The kilowatt hour is commonly used by electric utilities as a billing unit for energy delivered to consumers. 1kW . h = 1kW . 3600s = 3600kWs = 3600kJ = 3600000J. One kilowatt-hour corresponds to the heat required to evaporate of 1.58 kg of liquid water at 100°C. A radio rated at 100 watts operating for 10 hours continuously uses one kilowatt hour.

• • 1 kWh = 3.6 x 10⁶ J
  • 1 kWh = 8.6 x 10⁵ cal
  • 1 kWh = 3412 BTU

Megawatt-day (unit: MWd).  Megawatt-day is a derived unit of energy. It is used to measure energy produced, especially in power engineering. One megawatt-day is equal to one megawatt of power produced by power plant over a period of one day (megawatts multiplied by the time in days). 1 MWd = 24,000 kWh. At nuclear power plants there are also gigawatt-days, because it approximately corresponds to energy produced by power plant over a period of one day. This unit (MWd) was also used to derive unit of fuel burnup. The most commonly used measure of fuel burnup is the fission energy release per unit mass of fuel. Therefore fuel burnup of nuclear fuel is normally have units of megawatt-days per metric tonne (MWd/MTU), where tonne refers to a metric ton of uranium metal (sometimes MWd/tU HM as Heavy Metal). In this field, the megawatt-day refers to the thermal power of the reactor, not the fraction which is converted to electricity. For example, for a typical nuclear reactor with a thermal power of 3000 MWth, about ~1000MWe of electrical power is generated in the generator. For example, a reactor with 100 000 kg of fuel operating at 3000MWth power level for 1000 days would have a burnup increase of 30,000 MWd/MTU. In words of fissions,  fissioning of about 1 g of U-235 produces about 1 MWd of thermal energy (see: Energy Release per Fission).

• • 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. Electronvolt is equal to energy gained by a single electron when it is 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 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 vacuum (from E = mc²). For example, it can be said the proton has 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)

• • 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 temperature 290K (17 °C or 62 °F). Most probable energy at 17°C (62°F) for Maxwellian distribution is 0.025 eV (~2 km/s).
• 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, has a total width of only 0.027 eV, and the mean life of this state is 2.4×10^-14s.
• 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, which is one of nuclides in the ²³²U decay chain, emits gamma rays of 2.6 MeV which are very energetic and highly penetrating.
• 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 the energy of antineutrinos that are radiated away.
• Cosmic ray can have energies of 1 MeV – 1000 TeV.

In physics, thermodynamics/what-is-energy-physics/what-is-mechanical-energy/”>mechanical energy (E_mech) is the energy associated with the motion and position of an object usually in some force field (e.g. gravitational field). Mechanical energy (and also the thermal energy) can be separated into two categories, transient and stored. Transient energy is energy in motion, that is, energy being transferred from one place to another. Stored energy is the energy contained within a substance or object. Transient mechanical energy is commonly referred to as work. Stored mechanical energy exists in one of two forms: kinetic or potential.

The thermodynamics/what-is-energy-physics/what-is-kinetic-energy/”>kinetic energy, K, is defined as the energy stored in an object because of its motion. An object in motion has the ability to do work and thus can be said to have energy. It is called kinetic energy, from the Greek word kinetikos, meaning “motion.”

The kinetic energy depends on the speed of an object and is the ability of a moving object to do work on other objects when it collides with them. On the other hand, the kinetic energy of an object represents the amount of energy required to increase the velocity of the object from rest (v = 0) to its final velocity. The kinetic energy also depends linearly on the mass, which is a numerical measure of object’s inertia and the measure of an object’s resistance to acceleration when a force is applied.

We define the quantity:

K = ½ mv²

to be the translational kinetic energy of the object. It must be added, it is called the “translational” kinetic energy to distinguish it from rotational kinetic energy

thermodynamics/what-is-energy-physics/what-is-potential-energy/”>Potential energy, U, is defined as the energy stored in an object subjected to a conservative force. Common types include the gravitational potential energy, the elastic potential energy of an extended spring, and the electric potential energy of an electric charge in an electric field and so on.

In classical mechanics, the thermodynamics/what-is-energy-physics/what-is-potential-energy/gravitational-potential-energy/”>gravitational potential energy (U) is energy an object possesses because of its position in a gravitational field. Gravitational potential (V; the gravitational energy per unit mass) at a location is equal to the work (energy transferred) per unit mass that would be needed to move the object from a fixed reference location to the location of the object. The most common use of gravitational potential energy is for an object near the surface of the Earth where the gravitational acceleration can be assumed to be constant at about 9.8 m/s².

U = mgh

In thermodynamics,thermodynamics/what-is-energy-physics/internal-energy-thermal-energy/”> internal energy (also called the thermal energy) is defined as the energy associated with microscopic forms of energy. It is an thermodynamics/extensive-and-intensive-properties/”>extensive quantity, it depends on the size of the system, or on the amount of substance it contains. The SI unit of internal energy is the joule (J). It is the energy contained within the system, excluding the kinetic energy of motion of the system as a whole and the potential energy of the system. Microscopic forms of energy include those due to the rotation, vibration, translation, and interactions among the molecules of a substance. None of these forms of energy can be measured or evaluated directly, but techniques have been developed to evaluate the change in the total sum of all these microscopic forms of energy.

In addition, energy is can be stored in the chemical bonds between the atoms that make up the molecules. This energy storage on the atomic level includes energy associated with electron orbital states, nuclear spin, and binding forces in the nucleus.

In thermodynamics, the thermodynamics/what-is-energy-physics/what-is-enthalpy/”>enthalpy is a measurement of energy in a thermodynamic system. It is the thermodynamic quantity equivalent to the total heat content of a system. The enthalpy is defined to be the sum of the internal energy E plus the product of the pressure p and volume V. In many thermodynamic analyses the sum of the internal energy U and the product of pressure p and volume V appears, therefore it is convenient to give the combination a name, enthalpy, and a distinct symbol, H.

The enthalpy is the preferred expression of system energy changes in many chemical, biological, and physical measurements at constant pressure. It is so useful that it is tabulated in the thermodynamics/steam-tables/”>steam tables along with thermodynamics/thermodynamic-properties/what-is-specific-volume/”>specific volume and thermodynamics/what-is-energy-physics/internal-energy-thermal-energy/specific-internal-energy/”>specific internal energy. It is due to the fact, it simplifies the description of energy transfer. At constant pressure, the enthalpy change equals the energy transferred from the environment through heating (Q = H₂ – H₁) or work other than expansion work. For a variable-pressure process, the difference in enthalpy is not quite as obvious.

In thermodynamics and statistical physics, thermodynamics/what-is-energy-physics/what-is-entropy/”>entropy is a quantitative measure of disorder, or of the energy in a system to do work.

In statistical physics, entropy is a measure of the disorder of a system. What disorder refers to is really the number of microscopic configurations, W, that a thermodynamic system can have when in a state as specified by certain macroscopic variables (thermodynamics/thermodynamic-properties/what-is-volume-physics/”>volume, thermodynamics/what-is-energy-physics/”>energy, thermodynamics/thermodynamic-properties/what-is-pressure-physics/”>pressure, and thermodynamics/thermodynamic-properties/what-is-temperature-physics/”>temperature). By “microscopic states”, we mean the exact states of all the molecules making up the system.

Mathematically, the exact definition is:

Entropy = (Boltzmann’s constant k) x logarithm of number of possible states

S = k_B logW

This equation, which relates the microscopic details, or microstates, of the system (via W) to its macroscopic state (via the entropy S), is the key idea of statistical mechanics. In a closed system, entropy never decreases, so in the Universe entropy is irreversibly increasing. In an open system (for example, a growing tree), entropy can decrease and order can increase, but only at the expense of an increase in entropy somewhere else (e.g. in the Sun).

In thermodynamics, the thermodynamics/what-is-energy-physics/gibbs-free-energy/”>Gibbs free energy is a thermodynamic potential that is defined as the thermodynamics/what-is-energy-physics/what-is-enthalpy/”>enthalpy of the system minus the product of the temperature times the thermodynamics/what-is-energy-physics/what-is-entropy/”>entropy of the system. Since the enthalpy is defined to be the sum of the internal energy E plus the product of the pressure p and volume V.

Electrical energy is the energy derived from electric potential energy or kinetic energy. A typical nuclear power plant has an electric-generating capacity of 1000 MWe. It produces 1 000 000 000 joules of electrical energy per second. The heat source in the nuclear power plant is a nuclear reactor. As is typical in all conventional thermal power stations the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity. The turbines are heat engines and are subject to the efficiency limitations imposed by the second law of thermodynamics. In modern nuclear power plants the overall thermodynamic efficiency is about one-third (33%), so 3000 MWth of thermal power from the fission reaction is needed to generate 1000 MWe of electrical power.

Since voltage is electric potential energy per unit charge, the Kirchhoff’s voltage law can be seen to be a consequence of conservation of electrical energy. Kirchhoff’s voltage law states:

The algebraic sum of the voltages (drops or rises) encountered in traversing any loop of a circuit in a specified direction must be zero.

The algebraic sum of the voltages (drops or rises) encountered in traversing any loop of a circuit in a specified direction must be zero.

Simply, the voltage changes around any closed loop must sum to zero. The sum of the voltage rises is equal to the sum of the voltage drops in a loop. No matter what path you take through an electric circuit, if you return to your starting point you must measure the same voltage, constraining the net change around the loop to be zero. This rule is equivalent to saying that each point on a mountain has only one elevation above sea level. If you start from any point and return to it after walking around the mountain, the algebraic sum of the changes in elevation that you encounter must be zero.

In physics, radiant energy is the energy of electromagnetic and gravitational radiation. The term “radiant energy” is most commonly used in the fields of radiometry, solar energy, heating and lighting. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calculated by integrating radiant flux with respect to time. Radiant heat transfer is very important in power industry, because it is one of the most important ways , how thermal energy can be transferred. It does not need a medium, such as air or metal, to take place. Any material that has a temperature above absolute zero gives off some radiant energy. Most energy of this type is in the infra-red region of the electromagnetic spectrum although some of it is in the visible region.

Radiant heat transfer rate from a body (e.g. a black body) to its surroundings is proportional to the fourth power of the absolute temperature and can be expressed by the following equation:

q =  εσT⁴

where σ is a fundamental physical constant called the Stefan–Boltzmann constant, which is equal to 5.6697×10^-8 W/m²K⁴. This relationship is called the Stefan–Boltzmann law. The emissivity, ε, of the surface of a material is its effectiveness in emitting energy as thermal radiation and varies between 0.0 and 1.0. By definition, a black body in thermal equilibrium has an emissivity of ε = 1.0. It can be seen, radiation heat transfer is important at very high temperatures and in a vacuum.

Two bodies that radiate toward each other have a net heat flux between them. The net flow rate of heat between them is given by:

Q = εσA_1-2(T⁴₁ −T⁴₂)  [J/s]

q =  εσ(T⁴₁ −T⁴₂) [J/m²s]

The area factor A_1-2, is the area viewed by body 2 of body 1, and can become fairly difficult to calculate.

Ionization energy, also called ionization potential, is the energy necessary to remove an electron from the neutral atom.

X + energy → X⁺ + e⁻

where X is any atom or molecule capable of being ionized, X⁺ is that atom or molecule with an electron removed (positive ion), and e⁻ is the removed electron.

There is an ionization energy for each successive electron removed. The electrons that circle the nucleus move in fairly well-defined orbits. Some of these electrons are more tightly bound in the atom than others. For example, only 7.38 eV is required to remove the outermost electron from a lead atom, while 88,000 eV is required to remove the innermost electron.

• Ionization energy is lowest for the alkali metals which have a single electron outside a closed shell.
• Ionization energy increases across a row on the periodic maximum for the noble gases which have closed shells.

For example, sodium requires only 496 kJ/mol or 5.14 eV/atom to ionize it. On the other hand neon, the noble gas, immediately preceding it in the periodic table, requires 2081 kJ/mol or 21.56 eV/atom.

The ionization energy associated with removal of the first electron is most commonly used. The nth ionization energy refers to the amount of energy required to remove an electron from the species with a charge of (n-1).

1st ionization energy

X → X⁺ + e⁻

2nd ionization energy

X⁺ → X²⁺ + e⁻

3rd ionization energy

X²⁺ → X³⁺ + e⁻

For example, only 7.38 eV is required to remove the outermost electron from a lead atom, while 88,000 eV is required to remove the innermost electron.

Nuclear energy comes either from spontaneous nuclei conversions or induced nuclei conversions. Among these conversions (nuclear reactions) belong for example nuclear fission, nuclear decay and nuclear fusion. Conversions are associated with mass and energy changes. One of the striking results of Einstein’s theory of relativity is that mass and energy are equivalent and convertible, one into the other. Equivalence of the mass and energy is described by Einstein’s famous formula: