Static vs Dynamic Equilibrium
Equilibrium is a concept used in a variety of disciplines, to express a balance between two opposing forces in a considered system.
In this case, static equilibrium and dynamic equilibrium are two states of a physical system where two or more properties are in balance. These cases are specifically investigated in mechanics, and also in physical chemistry.
What is Static Equilibrium?
As a generic sense, static equilibrium is defined as a state in which both the macroscopic and microscopic properties of a system remain unchanged with time.
In mechanics, a system with no resultant force acting on it can be considered in an equilibrium state. It is sufficient to say that if,
• The vector sum of all external forces is zero; ∑ →FEXT = 0
• The sum of the moments of all external forces about any line is zero, ∑ →GEXT = 0
then the system is in equilibrium. Additionally, if the velocity of the system is also zero (i.e. →V = 0), then the system is in static equilibrium.
For example, consider an object lying on a table inside a room. The external forces on the object, or the gravitational pull (i.e. Weight), is countered by the reaction on the object by the table. Also, the reaction and weight are on the same line, so no moments are produced. Also, the table is on the ground in a room, and not moving. Therefore, we can deduce that the book is in static equilibrium.
What is Dynamic Equilibrium?
The dynamic equilibrium can be generically defined as a state of a system where the macroscopic properties remain unchanged while the microscopic properties are changing.
In mechanics, it can be specifically defined as a state of a system where the system is in equilibrium, but the velocity is not zero (i.e. The system is moving at a constant velocity). Therefore,
• ∑→FEXT = 0
• ∑→GEXT = 0
• →V ≠ 0
Again consider the table and the object, but instead of a room, it is placed inside a cabin of a train moving at a constant velocity.
In the context of thermodynamics, if the temperature of a system remains unchanged (i.e. the energy of the system is unchanged) while the heat and the work transfer are occurring. The necessary condition is that the sum of the work input and the heat input must be equal to the sum of the work output and the heat output.
In a chemical system, dynamic equilibrium occurs when the forward reaction and the backward reaction are occurring at the same rate in a reversible reaction. The concentration of the reactants and the products remain unchanged, but still some of the reactants are converted into products and products are converted into reactants. But these two opposing processes are occurring at the same rate.
For example, consider the NO2 and N2O4 system. When NO2 gas is compressed in a container, the increment in the pressure causes the system to be biased forward, and N2O4 is produced to reduce the number of molecules and eventually reduce the pressure. But at a certain point, the forward reaction seems to stop and the N2O4 production seems to stop. The concentrations (or partial pressure) of the system remain unchanged. But at the molecular level the NO2 is converted to N2O4 and vice versa.
What is the difference between Static and Dynamic Equilibrium?
• In static equilibrium, both microscopic and macroscopic properties remain unchanged whereas, in dynamic equilibrium, the microscopic properties change while the macroscopic properties remain unchanged.
• In mechanics, a system with no unbalanced external forces and external moments can be considered to be in equilibrium. Additionally, if the system is stationary, it is in static equilibrium, and if moving at a constant velocity, it is in dynamic equilibrium.
• In a thermodynamic system, if the temperature is constant and the heat and mass transfer input and output are at equal rates, the system is in (dynamic/ thermodynamic) equilibrium.
• In a chemical system, if the rate of the forward reaction and the backward reaction are the same, the system is said to be in dynamic equilibrium.