Some Key Definitions And Concepts

Before we dive into the Laws of Thermodynamics, there are some key definitions and concepts that we must understand. 

These are given below. 

• System and Surroundings

In order to avoid confusion, scientists discuss thermodynamic values in reference to a system and its surroundings. Everything that is not a part of the system constitutes its surroundings. The system and surroundings are separated by a boundary. If matter is not able to pass across the boundary, then the system is said to be closed; otherwise, it is open.

For example, if the system is one mole of a gas in a container, then the boundary is simply the inner wall of the container itself. Everything outside of the boundary is considered the surroundings, which would include the container itself.

• Thermodynamic States: 

The application of thermodynamic principles begins by defining a system that is in some sense distinct from its surroundings. 

For example, the system could be a sample of gas inside a cylinder with a movable piston, an entire steam engine, a marathon runner, the planet Earth, a neutron star, a black hole, or even the entire universe. 

A system’s condition at any given time is called its thermodynamic state. 

For a gas in a cylinder with a movable piston, the state of the system is identified by the temperature, pressure, and volume of the gas. These properties are characteristic parameters that have definite values at each state and are independent of the way in which the system arrived at that state. In other words, any change in value of a property depends only on the initial and final states of the system, not on the path followed by the system from one state to another. Such properties are called state functions. 

In contrast, the work done as the piston moves and the gas expands and the heat the gas absorbs from its surroundings depend on the detailed way in which the expansion occurs.

• Thermodynamic Equilibrium

It is observed that some property of an object, like the pressure in a volume of gas, the length of a metal rod, or the electrical conductivity of a wire, can change when the object is heated or cooled. If two of these objects are brought into physical contact there is initially a change in the property of both objects. But, eventually, the change in property stops and the objects are said to be in thermal, or thermodynamic equilibrium.

• Work and Energy

Energy is a measurement of the ability of something to do work. It is not a material substance. Energy can be stored and measured in many forms.

Energy is never really destroyed. It is just transferred from one form to another, doing work in the process.

• Total Internal Energy

The sum of all the microscopic forms of energy is called the internal energy of a system and is denoted by U.

The internal energy of a gas, depends only on the state of the gas and not on any process. It is a state variable.

• Kinetic Energy

The energy that a system possesses as a result of its motion relative to some reference frame is called kinetic energy (KE).

• Potential Energy

The energy that a system possesses as a result of its elevation in a gravitational field is called potential energy (PE).

• Heat 

Heat is defined as the form of energy that is transferred between two systems (or a system and its surroundings) by virtue of a temperature difference (Q).

The amount of heat transferred into, or from a gas also depends on the initial and final states and the process which produces the final state

• Work

Energy can cross the boundary of a closed system in the form of heat or work. Therefore, if the energy crossing the boundary of a closed system is not heat, it must be work. Work is the energy transfer associated with a force acting through a distance (W).

In the observations of the work done on, or by a gas, it is found that the amount of work depends not only on the initial and final states of the gas but also on the process, or path which produces the final state.