Energy: A short note

Energy

[1] Energy is the quantitative property that must be transferred to an object in order to perform work on, or to heat, the object.

[2] The total energy of a system can be subdivided and classified into potential energy, kinetic energy, or combinations of the two in various ways. Kinetic energy is determined by the movement of an object – or the composite motion of the components of an object – and potential energy reflects the potential of an object to have motion, and generally is a function of the position of an object within a field or may be stored in the field itself.

[3] Energy is a conserved quantity; Energy is transferred from potential energy [Ep] to kinetic energy [Ek] and then back to potential energy constantly. This is referred to as the conservation of energy. In this closed system, energy cannot be created or destroyed; therefore, the initial energy and the final energy will be equal to each other. This can be demonstrated by the following:

Ep + Ek = Etotal

The first law of thermodynamics, states that a closed system's energy is constant unless energy is transferred in or out by work or heat and that no energy is lost in the transfer. The total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the system.

Whenever one measures (or calculates) the total energy of a system of particles whose interactions do not depend explicitly on time, it is found that the total energy of the system always remains constant.

[4] While heat can always be fully converted into work in a reversible isothermal expansion of an ideal gas, for cyclic processes of practical interest in heat engines the second law of thermodynamics states that the system doing work always loses some energy as waste heat. This creates a limit to the amount of heat energy that can do work in a cyclic process, a limit called the available energy.

[5] Mechanical and other forms of energy can be transformed in the other direction into thermal energy without such limitations. The total energy of a system can be calculated by adding up all forms of energy in the system.

Conservation of energy and mass in the transformation

[6] Energy gives rise to weight when it is trapped in a system with zero momentum, where it can be weighed. It is also equivalent to mass, and this mass is always associated with it. Mass is also equivalent to a certain amount of energy, and likewise always appears associated with it, as described in mass-energy equivalence. The formula E = mc2, derived by Albert Einstein

Conversion of energy into different forms

[7] Energy may be transformed between different forms at various efficiencies. Items that transform between these forms are called transducers. Examples of transducers include a battery, from chemical energy to electric energy; a dam: gravitational potential energy to kinetic energy of moving water (and the blades of a turbine) and ultimately to electric energy through an electric generator; or a heat engine, from heat to work.

Examples of energy transformation include generating electric energy from heat energy via a steam turbine or lifting an object against gravity using electrical energy driving a crane motor.

Reversible and non-reversible transformations

[8] Thermodynamics divides energy transformation into two kinds: reversible processes and irreversible processes. An irreversible process is one in which energy is dissipated (spread) into empty energy states available in a volume, from which it cannot be recovered into more concentrated forms without degradation of even more energy. A reversible process is one in which this sort of dissipation does not happen. For example, the conversion of energy from one type of potential field to another is reversible.

Energy transfer

[9] Closed systems

Energy transfer can be considered for the special case of systems that are closed to transfers of matter. The portion of the energy which is transferred by conservative forces over a distance is measured as the work the source system does on the receiving system. The portion of the energy which does not do work during the transfer is called heat

?E=W+Q, where E is the amount of energy transferred, W represents the work done on the system Q represents the heat flow into the system.

 [10] Open systems

Beyond the constraints of closed systems, open systems can gain or lose energy in association with matter

?E=W+Q+E. 

Credit: Google