Introduction

Thermodynamics involves the study of the interaction of heat and other forms of energy. To understand any concept in thermodynamics, we must first understand the basic laws of thermodynamics, like heat and work. 

Work is defined as a force used to transfer energy and generate heat in a system and its surroundings. Heat is the transfer of thermal energy between two bodies/systems. Both heat and work are required together for the transfer of energy in a system. Internal energy is a term used in thermodynamics that refers to the energy related to the molecules of a system (that includes the kinetic and potential energy). 

Whenever the system undergoes any interaction due to heat, work or internal energy, it then undergoes various conversions and transfers, but the total energy always remains constant. The first law of thermodynamics is sometimes also known as the law of conservation of energy.

Defining the first law of thermodynamics

According to the first law of thermodynamics, heat is a form of energy, and the law applies to the conservation of energy. Heat can neither be created nor destroyed. Nonetheless, it can be transferred from one system to another or converted into other forms of energy. 

Consider the example of a heat engine where thermal energy is converted into mechanical energy. It works in the basic principle of using different relationships between pressure, volume and heat of gas, which acts as a working fluid. Sometimes in the working, phase change is also involved. The gas changes into liquid and then again into gas.

First law of thermodynamics equation

The first law of thermodynamics equation can be articulated as:

ΔU = Q + W, 

Where 

ΔU is the change in the internal energy of a system

Q is the sum of heat transfer between the surroundings and the system

W is the work transfer of the system between the system and its surroundings.

Work is also equal to the product of negative external pressure and the change in volume,

i.e.,  W = -PΔV,

Where 

P = external pressure on the system

ΔV = change in volume

W = work

This work is called the ‘pressure-volume’ work.

The internal energy of the system can be decreased if the system does any work. This means that the internal energy of the system can be increased if the work is done on a system. So any work or heat that comes in or goes out of the system changes the internal energy of the system. Since energy can never be created and never be destroyed (according to the first law of thermodynamics), the change in the total energy of a system is always zero.

Therefore, if the energy is lost by a system, then it is absorbed by the external surroundings, and if the energy is absorbed by a system, it means the energy was lost by the surroundings.

ΔUsystem = – ΔUsurroundings

where  ΔUsystem = total energy in a system

and ΔUsurroundings = total energy in the surroundings.

The limitations of the first law of thermodynamics

• It does not define whether the process is possible or not.
• It can be observed that in an engine that the chemical energy of the fuel is converted to mechanical work, but it is impossible to convert this mechanical work into fuel again.
• It does not define whether the process will occur itself or not. For example, when an ice cube is kept on the table, it melts into water, but the water never gets converted into an ice cube by itself.
• It does not provide any conditions for which a process can take place. For example, the heat flow is always observed from a high-temperature body to a low-temperature body. But the reverse is impossible.
• A perpetual motion machine is a hypothetical machine that violates the first law of thermodynamics. As such, a system is not possible where heat energy is itself formed and used and reformed again and again.

The first law of thermodynamics for a closed system

For a closed system, the first law of thermodynamics is given by:

ΔU = Q – W,

Where 

Q = heat transferred to system

W = work done by the system

ΔU = change in internal energy

 Deriving each term by the mass of system ‘m’, we get the energy equation for the closed system as

Δu = q – w (kJ/Kg)

Work

In general, three modes of work are considered. The three forms of work involved are:

• Boundary work (Wb): This is work done due to the expansion or compression of a system in a piston-cylinder device.
• Shaft work (Ws): Work done due to the use of a paddle wheel.

Applications of the first law of thermodynamics

• Isothermal process: In the isothermal process, the temperature remains constant. 

So, dQ = dU + dW

dQ = dW

This defines that the heat is used to do work on the system.

• Melting process: When a solid is melted into liquid, its internal energy increases.

Let m be the mass of the liquid

L be the latent heat of the solid

Then, the amount of heat absorbed by the system is denoted by dQ = mL

When a small amount of expansion occurs,

ΔV = 0 

dW = PΔV = 0

So, do = dU + dW 

where dU = mL

Thus, the internal energy is increased when the solid is melted.        

Conclusion

The first law of thermodynamics is applied to stationary systems. The energy that is transferred between the system and the surroundings is in the form of heat, work or internal energy. Various conversions and transfers of energy take place in the system, but in the end, the total energy of the system is always conserved. Internal energy can be defined as the molecular activity that is associated with the change of the state or temperature of the system. Whenever there is a transfer of heat in a system, it occurs due to the difference in temperature between the surroundings and the system. The transfer of the work can be done in three ways: boundary work, shaft work and electrical work.