The First Law of Thermodynamics; Definition and Explanation

Thermodynamics is a very important area in the natural sciences. Lots of engineering systems depend on thermodynamics science. Also, there are fundamental laws of thermodynamics. The first law of thermodynamics is one of the most important ones. In this article, we explain the general principles of the first law of thermodynamics.

What is the First Law of Thermodynamics?

First of all, the first law of thermodynamics is about the energy conservation between systems. According to the first law of thermodynamics, you can not create energy from scratch or can not destroy energy in your absence. However, you can only convert energy between different energy types.

Think about a flowing flow from a high hill to a hydroelectrical energy power plant turbine. There is potential energy in the fluid or water at high elevations. This potential energy is converted into kinetic energy. This kinetic energy is related to the velocity of the fluid that comes to the turbine. This kinetic energy of water rotates the turbine. So, potential energy is converted to kinetic energy and kinetic energy is converted into mechanical energy. 

As you understand above, the energy conversion took place but no energy creation take place from scratch in the hydroelectric power plant.

Adiabatic Processes

In adiabatic processes, we use the first law of thermodynamics in calculations. As you know that in the adiabatic processes, there is no energy transfer in the heat form. So, the change in the total energy of the two systems is equal to total work. In this case, energy transfer will take place with mechanical energy transfer.

The First Law of Thermodynamics in Heat Transfer

In some systems, energy transfer takes place only as heat. There are no work interactions or mass transfer. The systems transfer energy in heat form between themselves. For example, we are baking potatoes in an oven. In this case, there will be a temperature difference between the oven cavity and the potatoes. So, there will be a heat transfer between the potatoes and the oven atmosphere. And heat transfer will stop when the temperatures of the oven cavity and the potatoes are equal.

So, there is an energy transfer between the oven and the potatoes because of the temperature difference. But here, there are no mechanical means of work or mass transfer. This situation

Conservation of Energy

People and scientists also call the first law of thermodynamics as conservation of energy principle. Conservation of energy states better the purpose of this law. As its name implies, energy is conserved in every type of system. You can not create or destroy any kind of energy.

Conservation of Energy for Electrical Work

Electrical energy is also a very important energy form. The first law of thermodynamics is also valid for electrical energy. Think about a room where there is an electrical heater inside it. When the electrical heat starts to work, the ambient temperature of the room increase. So, we use the electrical energy to heat the room by heat transfer. This can be a very good example of electrical energy use.

Boundary Work Phenomenon

If you compress the air or gasses, the temperature and the pressure of them will increase. So, there is an energy transfer to the gas. You can compress the air if you squeeze it through a boundary or pressure. Scientists and engineers call this phenomenon boundary work.

The first law of thermodynamics is also valid for the energy transfers due to the boundary work phenomenon.

So, there can be various heat and work interactions between the different systems. The conservation of energy principle is valid for all these types of energy types.

Energy Balance in the First Law of Thermodynamics

In most engineering calculations, engineers use the system approach to calculate the required parameters. One of these parameters is energy. In a system, the total energy situation is very important. The efficiency and performance of an engineering system depend on the total energy.

We can calculate the total energy of the system. If we subtract the total energy leaving the system from the total energy entering the system, we can find the total energy change. Engineers call this phenomenon as energy balance. Energy balance is related to the first law of thermodynamics.

Calculation of the Total Energy Change of a System

We calculate the total energy change of the system according to the conservation of energy principle in thermodynamics. So, we calculate the total energy change of the system by adding the;

• Energy change in internal energy of the system,
• Change in the kinetic energy of the system,
• Change in the potential energy of the system.

Engineers calculate most of the systems with these calculations of energies, with the use of the first law of thermodynamics.

In general, the kinetic and the potential energies of the stationary systems are not considered. So the total energy change of these systems is equal to the total internal energy change of the system.

Energy Transfer Mechanisms in the First Law of Thermodynamics

In general, energy transfer takes place in three forms; heat, work and mass flow. According to the first law of thermodynamics, all the energy that enters and leaves the system is in these three forms.

Heat Transfer

First of the energy transfer mechanism is heat transfer. Heat transfer is a very important subject that engineers use in calculations. Heat energy can be transferred between the systems. So, if heat leaves the system, the total energy decrease. And if the heat enters the system, the total energy of the system increase. The conservation of energy principle is valid for heat transfer.

Work Transfer

There are different forms of work. Pistons in the engine, rotating shaft of mechanisms, and electrical wire that crosses over the system are the general work transfer examples.

If a system makes work, the total energy of the system decreases according to the conservation of energy principle. For example, an engine piston goes down because of the burning fuel inside it. The chemical energy of the fuel is transferred to the crankshaft as mechanical work.

Also, if we make work to a system, the total energy of the system increases. For instance, the rising piston in an engine. If the piston rises, the air-fuel mixture is squeezed and the pressure and temperature increase inside the piston. So, the total energy of the piston system increases according to the first law of thermodynamics.

Different engineering systems work in this way. Pumps, compressors, and internal combustion engines consume work.

Mass Flow

If we think about a boundary of a system, there can be a mass flow through the boundaries. Because of this mass flow, the total energy of the system change. Mass includes energy content inside it. So with the entering or leaving mass from a boundary of a system, total energy content changes according to the first law of thermodynamics.

Think about hot water added to a heat exchanger system. If we consider the system boundaries of the heat exchanger, the total energy will increase according to the conservation of energy principle.

Rate Form of Energy Change

The unit of the total energy change of a system is BTU or Joule in SI units. If we divide this unit with time, we can find the rate form of energy change. Rate form of energy change has the unit of ft⋅lbf/s or Watts in SI units.

In addition, we can also find the total energy change per unit mass of a system. If we divide all the energy changes by the mass of the system, we can find the total energy change per unit mass.

Also, the first law of thermodynamics is valid for rate forms and unit mass forms of energies.

Closed Energy Cycles

Above all, engineers are using the energy cycles in the design of thermodynamical systems. In an energy cycle, systems are generally closed systems. And in closed systems, there is no mass flow through the boundaries. So, the only energy interaction between the system and surroundings is the heat and work.

In a closed system cycle, the total energy input and output are the same. So, the total energy change is zero for each cycle according to the first law of thermodynamics.

So, the total input work energy equals to total output heat energy. And total output work is equal to the total input heat energy according to the conservation of energy principle.

Conclusion for the First Law of Thermodynamics

So, we tried to talk about the first law of thermodynamics which also engineers and scientists call it conservation of energy principle. The first law of thermodynamics is a very important principle for most engineering calculations. As you understand, this law is valid for most engineering calculations and engineering systems.

Also, in most engineering systems, there are three types of energy transfer. And the conservation of energy principle works around these types of energy transfers. So according to the types of energy transfers that an engineering system has, different classes of systems that we have. They can be cycles, adiabatic processes, etc. For all the systems, the first law of thermodynamics is valid.

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