1. What Is a System?
A system is the part of the world we choose to study. It can be a gas in a cylinder, boiling water in a pot, or even the air in a room. Everything outside the system is called the surroundings.
1.1. Types of Systems
Based on how energy and matter can cross the boundary, systems are classified as:
1.2. Open System
An open system allows both energy and matter to enter or leave.
Example: A boiling pot without a lid allows steam and heat to escape.
1.3. Closed System
A closed system allows energy to cross its boundary but not matter.
Example: A sealed container on a stove lets heat in but prevents mass from entering or leaving.
1.4. Isolated System
An isolated system does not exchange energy or matter with the surroundings.
Example: A thermos flask (approximately isolated) keeps heat and matter inside.
2. Surroundings and Boundary
The surroundings include everything outside the system that can interact with it. The boundary is the surface separating the system and the surroundings. It may be real or imaginary, fixed or movable.
2.1. Nature of Boundary
A boundary can be:
- Fixed – like the walls of a rigid container
- Movable – like the piston head in a cylinder
- Imaginary – used to define a certain volume of gas
3. State of a System
The state of a system describes its condition at a particular moment. It is defined by variables such as pressure, volume, temperature, and internal energy.
3.1. State Variables (Thermodynamic Variables)
These are measurable properties that determine the state of a system:
- Pressure (P)
- Volume (V)
- Temperature (T)
- Internal energy (U)
Knowing these variables tells us everything about the system’s macroscopic condition.
4. Thermodynamic Process
A process is any change that takes a system from one state to another. During a process, one or more state variables change.
4.1. Examples of Processes
- Heating a gas (temperature increases)
- Compressing air in a piston (volume decreases)
- Expanding gas (pressure decreases)
5. Equilibrium
A system is in equilibrium when its macroscopic properties do not change with time. Once equilibrium is reached, no net flows of heat, matter, or energy occur within the system.
5.1. Types of Equilibrium
A system must satisfy all of these to be in complete thermodynamic equilibrium:
5.2. Thermal Equilibrium
There is no temperature difference within the system or between the system and surroundings.
5.3. Mechanical Equilibrium
No unbalanced forces act on any part of the system. Pressure remains uniform.
5.4. Chemical Equilibrium
No chemical reactions or compositional changes occur within the system.
5.5. Thermodynamic Equilibrium
A system is in full equilibrium only when it is simultaneously in thermal, mechanical, and chemical equilibrium.