Internal Energy Change by Heat and Work

In thermodynamics and engineering analyses, the exact definition of a system is very important. By doing this, we need to define the system boundaries and make the general thermodynamical analyses and other analyses according to this system. For example in energy analyses of the systems, the strşct calculations of outgoing and incoming energies are very important. Here we summarize how to make energy calculations from system boundaries. 

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Energy Calculations for System Boundaries

First of all, we need to state that the energy transfer from system boundaries takes place in two forms; heat and work. We need to understand how the heat and work energy transfer takes place for a system. 

For example, in the analysis of an internal combustion engine piston, there are two forms of energy transfer with the surroundings; mechanical energy which takes place with the moving action of the piston, and the heat energy that takes place between the inside of the piston and the surroundings. The total energy transfer between the surroundings and the piston is calculated by considering these incidents. 

Energy Transfer by Heat

One of the forms of energy transfer is heat. Heat energy transfer only takes place with the temperature difference between the system and the surroundings. If there is no temperature difference between the system and the surroundings, there is no energy transfer by heat. 

The outgoing heat energy from the system to the surroundings is called negative heat energy transfer. If the heat transfer takes place from the surroundings to the system, it is a positive heat transfer. 

As you understand that, if the total temperature of the system is higher than the surroundings, the heat transfer takes place is negative. But if the temperature of the surroundings is higher than the system, the heat transfer is positive. 

Adiabatic Systems

There is a special ideal condition that the system is completely isolated to heat transfer from surroundings. This situation is called an adiabatic system. Regardless of the temperature difference between the system and the surroundings, there is no heat transfer between the surroundings and the system for adiabatic systems. 

The unit of the total heat transfer between the surroundings and the system is kJ and the total heat transfer for unit time is called W. In English units, the total heat transfer is called BTU. 

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Heat Transfer Mechanisms

There are three heat transfer mechanisms between a system and the surroundings; conduction heat transfer, convection heat transfer, and radiation heat transfer. 

In the conduction heat transfer mechanism, particles that are in the higher energy state give heat energy to the particles that are in a lower energy state. 

In the convection heat transfer mechanism, the heat of the solid surface is transferred to liquid molecules which carry to other places surroundings with the movement of these liquid molecules. 

In the radiation heat transfer is taking place with the emission of the electromagnetic particles. 

Energy Transfer of Work

If there is no heat transfer thus no temperature difference between the system and the surroundings and if there is an energy transfer, this energy transfer is probably because of the work transfer between them. Like heat, the positive work is the work-energy income to the system, and the negative work is the work energy that outgoes the system. 

In the sign conventions of the energy transfers, all the energy transfers are thought according to the system. The system is important for us. 

The units of the work energy and heat energy are the same kJ or BTU. The work is done in a unit time is also called power and the unit is W. 

There are different forms of work; mechanical work and electrical work. 

Electrical Work

Electrical work can be done to a system by electrical wiring where the electrical current comes from the outside. The total electrical work done is calculated by; 

W = VIt

Here, V is the voltage of the electrical current, I is the total electrical current in amperes, and t is the total time in seconds that electrical current takes place. 

Mechanical Work

In general, there are different forms of mechanical work that come to the system or go from the system to its surroundings. 

The first one is the shaft work where total work comes from a shaft from the system boundaries. 

The second one is that spring work can act through the moving boundaries of a system. The total work done by spring action to the system or from the system is related to the elasticity of the spring. Also, there are other forms of elasticity of shapes for example elastic solid bars. 

For liquids, there is a surface tension on the surfaces of the liquids. Any work that is related to the surface tension is called the surface tension work which is also classified as mechanical work. 

Change in potential energy and the kinetic energy of a system is also associated with the mechanical work that is done to a system. 

Apart from the mechanical works that the system is exposed to, there are other non-mechanical work forms which are magnetic work and electrical depolarization works. 


These are the general considerations in the energy analysis of a system that is selected to be analyzed in terms of thermodynamics, heat transfer, or fluid dynamics. 

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