What is Conservation of Energy?

In physics, the rule of the conservation of energy states that energy doesn’t disappear from the atmosphere; instead, it changes its form from one type to another. Energy is one of the most important physical quantities in nature, and it cannot be created or destroyed — only transformed.

Different Forms of Energy Conservation

Energy is always conserved in the atmosphere, but it can be converted into various different types. We will discuss some of these important types of energy below.

• Potential energy
• Kinetic energy
• Heat energy
• Work energy
• Friction energy
• Enthalpy
• Entropy
• Elastic energy
• Stress energy

Potential Energy

Potential energy is the energy that is stored inside the body. The amount of potential energy of a body depends on its mass and position. If an object is observed at rest in a particular position, the energy inside the object is known as potential energy.

It is denoted by the equation P.E. = mgz

m – mass

g – acceleration due to gravity

z – datum height

Kinetic Energy

This energy is obtained by an object or body when it is in motion. If an object starts to move from its rest position, its energy is converted from potential energy to kinetic energy. Kinetic energy is also known as constant velocity energy.

It is denoted by the equation K.E. = ½ mv²

m – mass

v – velocity

Heat Energy

Heat energy is energy is created due to temperature difference. If the temperature is increased in a body from a lower temperature range to a higher temperature range, gravitational potential energy is changing into heat energy.

It is denoted by the equation Q = m x c x ⊗T

m – mass

c – specific energy

⊗T – temperature difference

Work

Heat energy is mutually convertible into work; based on thermodynamics, if we pass heat energy into a turbine in a power plant, it will convert heat energy into work. Work is calculated by multiplying pressure and the amount of volume displaced due to pressure.

It is denoted by the equation W = ∫PdV

P – pressure

dV – displacement volume

Enthalpy

Enthalpy is the maximum energy that a system can possess or the maximum energy obtained from the system. It is denoted by H and measured in the unit kJ/kg.

Entropy

Entropy is the unavailable energy or lowest energy, which we cannot obtain from the system. It is denoted by S and measured in the unit kJ/kg-K.

Systems in Thermodynamics

While talking about energy or conservation of energy, we often use the word "system." If energy wants to travel from one place to another, it needs a medium to travel. That medium is known as the system. A system is a small area chosen in space, which has a definite or indefinite boundary. This is a real or imaginary boundary, with surroundings outside the boundary and the universe outside the surrounding.

There are Three Systems of Transfer:

• Open system, or energy and mass transfer system
• Closed system, or energy transfer system
• Isolated system, or no energy or mass transfer system

Open System

If both energy and mass can enter the system and exit the system, it is termed an open system.

For instance, imagine you are pumping your bicycle wheel with a manual hand pump. While pumping, air from the atmosphere goes into the hand pump and gets compressed in the pump. From there, it moves to the bicycle. This is known as mass transfer. While you are pumping the lever of the pump, your potential energy is getting changed into mechanical energy. This means an energy transfer has also happened. Since mass and energy both are entering and exiting the system, the compressor is an open system.

Closed System

If only energy can be transferred from one place to another, it is termed a closed system.

For instance, imagine the automatic doors in a bus, which work under the hydraulic law. A mass of hydraulic fluid is placed inside the hydraulic piston. The mass is not going to change. When we want to get off the bus at the station, the driver will press a button, applying mechanical energy. This energy gets converted into hydraulic energy, and the piston arm extends and opens the door. So, in this system, mass is conserved and energy is transferred from one place to another, making it a closed system.

Isolated System

The universe is assumed to be in an isolated system. If a system has no mass transfer, meaning mass is constant, and no energy transfer, meaning energy is constant, it is termed an isolated system.

For instance, in the universe, no energy or mass escapes. Everything is present inside the universe only. Since we are yet to understand the total knowledge of the universe, we are assuming it as an isolated system.

A thermal flask is an example of an isolated system. The water inside the flask has no motion, hence mass is not going to change. Similarly, there is no motion of energy transfer; hence, it is an isolated system.

Energy Conservation Principle in Open System

Every system will obey the conservation law of physics. Consider an open system, in which both mass and energy transfer occur, for applying the law of conservation. It can be solved using thermodynamics' second law by the steady flow energy equation (SFEE). The law resembles Bernoulli's law.

Imagine water is pouring in an open system, as shown in the image. We will calculate energy transfer at entry and exit of the system.

At Entry

Potential energy + kinetic energy + enthalpy + heat = 0

Neglecting the friction energy of the entry fluid and assuming only these four energies happen in the system:

P.E + K.E + h + Q = 0

At Exit

Potential energy + kinetic energy + enthalpy + work = 0

Neglecting the friction energy of the exit fluid:

P.E + K.E + h + W= 0

Entry = Exit

MgZ_e + ½ MV_e² + h_e + Q_e = MgZ_i + ½ MV_i² + h_i + W_i

i= inlet

e = exit

According to the energy conservation principle, the amount of energy entering the system is equal to the amount of energy exiting the system.

The heat supplied at the entry changes into work energy as it exits. This denotes the physics law “No human can create energy or destroy energy from the universe; hence energy will transfer from one form to another form.” In the above system, no energy is created; the gravitational potential energy, kinetic energy, and enthalpy is transported from entry to exit. All the mechanical energy is transferred from one place to another. The heat is converted to work, so no energy is destroyed; instead, it changes from heat energy form to work energy form. Hence, the conservation law is proved.

Context and Applications  

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for:

• B.S. in Physics  
• M.S. in Physics