# What is Energy – Physics

## Article Summary & FAQs

### What is energy?

Energy is a scalar physical quantity. Energy is generally defined as the potential to do work or produce heat. Sometimes it is like the “currency” for performing work. You must have the energy to accomplish work. To do 1 kilojoule of work, you must expend 1 kilojoule of energy. The total energy cannot be created or destroyed.

### Key Facts

• One of the most wonderful properties of the Universe is that energy can be transformed from one type to another and transferred from one object to another.
• The SI unit for energy is the same as the unit of work – the joule (J).
• 1 British Thermal Unit (BTU) = 1055 J
• One joule in everyday life and science corresponds to approximately the energy required to lift a medium-size apple (100 g) 1 meter vertically from the surface of the Earth.
• Energy exists in many forms. Common energy forms include mechanical energy, kinetic energy, or thermal energy.
• A fundamental aspect of the energy concept is that energy is conserved. This principle is known as the first law of thermodynamics.
• One of the striking results of Einstein’s theory of relativity is that mass and energy are equivalent and convertible, one into the other.
• The IEA estimates that, in 2013, the total primary energy supply (TPES) was 157.5 petawatt hours or 1.575×1017 Wh (157.5 thousand TWh; 5.67×1020 J; 13.54 billion toes).
What are primary energy sources?
What are primary energy sources?

Primary energy (PE) is an energy resource found in nature that has not been subjected to any conversion or transformation process. It is the energy contained in raw fuels and other forms of energy received as input to a system. Primary energy sources take many forms, including nuclear energy, fossil energy — like oil, coal, and natural gas — and renewable sources like wind, solar, geothermal, and hydropower.

What is thermodynamics used for?
What is thermodynamics used for?

Despite the fact it is a very broad subject that affects most fields of science, including biology and microelectronics, we will concern mostly with large-scale observations. Knowledge of thermodynamics is essential to engineers who deal with power plants. Our goal here will be to introduce thermodynamics as the energy conversion science and introduce some of the fundamental concepts and definitions used in the study of engineering thermodynamics. These fundamental concepts and definitions will be further applied to energy systems and finally to thermal or nuclear power plants.

What is nuclear energy?
What is nuclear energy?

Nuclear energy comes either from spontaneous nuclei conversions or induced nuclei conversions. It is associated with nuclear binding energy. If nuclear energy is generated (splitting atoms, nuclear fission), a small amount of mass (saved in the nuclear binding energy) transforms into pure energy (such as kinetic energy, thermal energy, or radiant energy).

The term energy is very broad, and it has many definitions. Technically, energy is a scalar physical quantity that is associated with the state of one or more objects. Energy is generally defined as the potential to do work or produce heat. Sometimes it is like the “currency” for performing work. You must have the energy to accomplish work. To do 1 kilojoule of work, you must expend 1 kilojoule of energy. It must be added. This interpretation can be misleading because energy is not necessarily available to do work. The Sun generates its energy by nuclear fusion of hydrogen nuclei into helium. At its core, the Sun fuses 620 million metric tons of hydrogen each second.Source: hyperphysics.phy-astr.gsu.edu

One of the most wonderful properties of the Universe is that energy can be transformed from one type to another and transferred from one object to another. Moreover, when transformed from one type to another and transferred from one object to another, the total amount of energy is always the same. It is one of the elementary properties of the Universe.

For example, burning gasoline to power cars is an energy conversion process we rely on. The chemical energy in gasoline is converted to thermal energy, then converted to mechanical energy that makes the car move. The mechanical energy has been converted to kinetic energy. When we use the brakes to stop a car, that kinetic energy is converted by friction back to heat or thermal energy.

## Energy Units

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 used in various branches of physics and engineering, there are many units in physics and engineering. These units are summarized in the following points:

Joule (unit: J)
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 meter.

• 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 3.11010 fissions in a nuclear reactor.
Calorie (unit: cal)
Calorie (unit: cal). A calorie is a traditional unit of heat. It is part of the  International System of Units (SI). It is defined as 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 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).  Nutritionists use it 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 (unit: BTU).  British Thermal Unit is a traditional unit of heat. It is part of the British Imperial system of units. It is defined as the amount of heat that must be absorbed by 1 pound of water to raise its temperature by 1 °F at the temperature that water has its greatest density (approximately 39 degrees Fahrenheit). Its counterpart in the International System of Units (SI) is the calorie, defined as the amount of heat required to raise the temperature of one gram of water by one degree 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 (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 (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). Electric utilities commonly use the kilowatt-hour 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 1.58 kg of liquid water at 100°C. A radio rated at 100 watts continuously operating for 10 hours 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 (unit: MWd).  Megawatt-day is a derived unit of energy. It is used to measure the energy produced, especially in power engineering. One megawatt-day is equal to one megawatt of power produced by a 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 the 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 normally has 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 reactor’s thermal power, not the fraction converted to electricity. For example, for a typical nuclear reactor with 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, the 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) 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.

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)

• 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 3.11010 fissions in a nuclear reactor.

## Forms of Energy

Energy exists in many forms. Common energy forms include mechanical energy that is classically divided into kinetic and potential energy. The kinetic energy is related to the velocity of a moving object. The potential energy is related to an object’s position in a force field (gravitational, electric, or magnetic). Tension in a spring or surface film tension is another form of potential mechanical energy (elastic energy). There are many other forms of energy, including electrical, magnetical, chemical, and nuclear energy.

In thermodynamics, the concept of energy is broadened to account for other observed changes. Thermodynamics deals with another type of energy called “thermal energy” or “internal energy”. The only ways the energy of a closed system can be changed are through the transfer of energy by work or by heat. Further, based on the experiments of Joule and others, a fundamental aspect of the energy concept is that energy is conserved. This principle is known as the first law of thermodynamics. In general, energy is a fundamental concept of thermodynamics and one of the most significant aspects of engineering analysis.

Mechanical energy
In physics, mechanical energy (Emech) is the energy associated with the motion and position of an object, usually in some force field (e.g.,, gravitational field). Mechanical energy (and 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.
Kinetic energy
The kinetic energy, K, is defined as the energy stored in an object because of its motion. An object in motion can do work and thus can be said to have energy. It is called kinetic energy, from the Greek word kinetics, 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 an 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, and it is called the “translational” kinetic energy to distinguish it from rotational kinetic energy

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.
Gravitational energy

In classical mechanics, the gravitational potential energy (U) is the 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

Internal energy
In thermodynamics, internal energy (also called thermal energy) is defined as the energy associated with microscopic forms of energy. It is an extensive quantity, and 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 system’s potential energy. 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. Still, techniques have been developed to evaluate the change in the total sum of all these microscopic forms of energy.

In addition, energy 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.

Enthalpy
In thermodynamics, the 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 steam tables along with specific volume and 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.

Entropy
In thermodynamics and statistical physics, entropy is a quantitative measure of disorder or 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 the number of microscopic configurations, W, that a thermodynamic system can have when in a state as specified by certain macroscopic variables (volume, energy, pressure, and 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 the number of possible states

S = kB 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 the order can increase, but only at the expense of increasing entropy somewhere else (e.g.,, in the Sun).

Gibbs Free Energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that is defined as the enthalpy of the system minus the product of the temperature times the 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
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 that produces electricity. The turbines are heat engines 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, Kirchhoff’s voltage law can be seen to be a consequence of the 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 elevation changes 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 concerning time. Radiant heat transfer is very important in the power industry because it is one of the most important ways to transfer thermal energy. 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.

The 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. It 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 and 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 = εσA1-2(T41 −T42)  [J/s]

q =  εσ(T41 −T42) [J/m2s]

The area factor A1-2 is the area viewed by body 2 of body 1 and can become fairly difficult to calculate.

Ionization Energy
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 ionizing, X+ is that atom or molecule with an electron removed (positive ion), and e is the removed electron.

There is 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 the removal of the first electron is most commonly used. The nth ionization energy refers to the 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
Nuclear energy comes either from spontaneous nuclei conversions or induced nuclei conversions. Among these conversions (nuclear reactions) are 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: ## Mass-energy Equivalence

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: where M is the small amount of mass and C is the speed of light.

What does that mean? If nuclear energy is generated (splitting atoms, nuclear fusion), a small amount of mass (saved in the nuclear binding energy) transforms into pure energy (such as kinetic energy, thermal energy, or radiant energy).

The energy equivalent of one gram (1/1000 of a kilogram) of mass is equivalent to:

• 89.9 terajoules
• 25.0 million kilowatt-hours (≈ 25 GW·h)
• 21.5 billion kilocalories (≈ 21 Tcal)
• 85.2 billion BTUs

or to the energy released by combustion of the following:

• 21.5 kilotons of TNT-equivalent energy (≈ 21 kt)
• 568,000 US gallons of automotive gasoline

Any time energy is generated, the process can be evaluated from an E = mc2 perspective.

## Principle of Conservation of Energy

One of the most wonderful properties of the Universe is that energy can be transformed from one type to another and transferred from one object to another. Moreover, when transformed from one type to another and transferred from one object to another, the total amount of energy is always the same. It is one of the elementary properties of the Universe.

In thermodynamics, the concept of energy is broadened to account for other observed changes. The principle of conservation of energy is extended to include a wide variety of ways systems interact with their surroundings. The only ways the energy of a closed system can be changed are through the transfer of energy by work or by heat. Further, based on the experiments of Joule and others, a fundamental aspect of the energy concept is that energy is conserved. This principle is known as the first law of thermodynamics. The first law of thermodynamics can be written in various forms:

In words:  The physical layout of the four main devices used in the Rankine cycle and basic energy transfers.

Equation form:

∆Eint = Q – W

where Eint represents the internal energy of the material, which depends only on the material’s state (temperature, pressure, and volume),is the net heat added to the system, and W is the net work done by the system. We must be careful and consistent in following the sign conventions for Q and W. Because W in the equation is the work done by the system, then if work is done on the system, W will be negative, and Eint will increase.

Similarly, Q is positive for heat added to the system, so if heat leaves the system, Q is negative. This tells us the following: The internal energy of a system tends to increase if the system absorbs heat or if positive work is done on the system. Conversely, the internal energy tends to decrease if heat is lost by the system or if negative work is done on the system. It must be added Q and W are path-dependent, while Eint is path-independent.

Differential form:

dEint = dQ  – dW

The internal energy Eint of a system tends to increase if energy is added as heat Q and tends to decrease if energy is lost as work W is done by the system.

## Energy Sources Total Primary Energy Supply by resource 1993, 2011 and prediction for 2020.Source: World Energy Resources – 2013 SurveyUsed by permission of the World Energy Council

Energy sources have always played a very important role in the development of human society. Since the industrial revolution, energy has been a driving force for modern civilization development. Technological development and consumption of primary energy and the increase of the world population are interdependent. In the past 20 years, the world around us has changed significantly. Technology has become one of the main drivers of economic and social development. The rapid advancement of Information Technology (IT) worldwide has transformed not only the way we think but also the way we act. It must be noted that practically all technologies run on electrical energy. Therefore, the share of electricity is increasing rapidly, faster than Total Primary Energy Supply (TPES –  the sum of production and imports subtracting exports and storage changes.).

At present, fossil fuel is still the world’s predominant energy source. Its extraction, production, and use are not considered efficient regardless of the new technologies available to improve its use and extraction. When studying energy resources, we have to distinguish the primary energy sources and secondary energy sources.

### Primary Energy Sources Key indicators for 1993, 2011, and prediction for 2020.Source: World Energy Resources – 2013 SurveyUsed by permission of the World Energy Council

Primary energy (PE) is an energy resource found in nature that has not been subjected to any conversion or transformation process. It is the energy contained in raw fuels and other forms of energy received as input to a system. Primary energy sources take many forms, including nuclear energy, fossil energy — like oil, coal, and natural gas — and renewable sources like wind, solar, geothermal, and hydropower. These primary sources can be converted to secondary energy source, so-called energy carriers. Primary energy sources can be divided into:

• Non-renewable sources
• Fossil fuels
• Oil
• Coal
• Natural gas
• Mineral fuels
• Natural Uranium
• Natural Thorium
• Renewable sources
• Solar energy
• Wind energy
• Hydro and tidal energy
• Geothermal energy
• Biomass energy (if sustainably exploited)

### Secondary Energy Sources – Energy Carriers

Secondary energy sources, also called energy carriers, are derived from the transformation of primary energy sources. They are called energy carriers because they move energy in a useable form from one place to another. The well-known energy carriers are:

• Electricity
• Petrol
• Hydrogen

Electricity and hydrogen are made from primary energy sources such as coal, natural gas, nuclear energy, petroleum, and renewable energy sources. Electricity is particularly useful since it has low entropy (is highly ordered) and can be converted into other forms of energy very efficiently. Simply, we cannot say that hydrogen has the potential to offset fossil fuels.

Secondary energy sources are used because their use is easier than using a primary energy source. For example, using electricity for lighting is safer than using petroleum in candles or kerosene lamps.

On the other hand, any conversion of primary energy to energy carrier is associated with some inefficiency. Therefore when dealing with the secondary energy source, we always have to consider the way, how the carrier was made.

References:
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2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
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Thermodynamics