## What is Energy

The term** energy **is very 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 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.

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**, which is 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**.

## 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 are other forms 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 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**(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 kinetic energy,**, 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.”

*K*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 mv ^{2}**

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

**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 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/s^{2}.

**U = mgh**

**internal energy**(also called the

**thermal energy**) is defined as the energy associated with

**microscopic forms of energy**. It is an 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.

**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 = H _{2} – H_{1})** or work other than expansion work. For a variable-pressure process, the difference in enthalpy is not quite as obvious.

**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 (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 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).

**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.**

**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.

**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 = εσT ^{4}*

where** σ** is a fundamental physical constant called the **Stefan–Boltzmann constant**, which is equal to** 5.6697×10 ^{-8} W/m^{2}K^{4}**. 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}(T^{4}_{1} −T^{4}_{2}) [J/s]**

**q = εσ(T ^{4}_{1} −T^{4}_{2}) [J/m^{2}s]**

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 *n*th 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^{+} → X^{2+} + e^{−}

3rd ionization energy

X^{2+} → X^{3+} + 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:

**Reactor Physics and Thermal Hydraulics:**

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