Second Law of Thermodynamics

A fundamental principle stating that the total energy of an isolated system remains constant over time. Energy can neither be created nor destroyed, only transformed from one form to another, such as from potential to kinetic energy. In classical mechanics, for systems with only conservative forces, the total mechanical energy \(E = T + V\) is conserved.

Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials, like a rubber band, strain when stretched and quickly return to their original state once the stress is removed. Viscoelastic materials have elements of both.

The first commercially successful rechargeable battery. It uses a lead (Pb) anode, a lead dioxide (PbO₂) cathode, and a sulfuric acid (H₂SO₄) electrolyte. During discharge, both electrodes are converted into lead sulfate (PbSO₄), and the sulfuric acid is consumed. This process is chemically reversible by applying an external current, making it a practical and robust energy storage system.

For a Newtonian fluid, viscosity is a function of temperature and pressure but not shear rate. In liquids, viscosity decreases significantly as temperature increases because higher thermal energy allows molecules to overcome cohesive intermolecular forces more easily. Conversely, in gases, viscosity increases with temperature as more frequent molecular collisions at higher speeds lead to greater momentum transfer.

The enthalpy of sublimation, \(\Delta H_{\text{sub}}\), is the heat required to convert one mole of a substance from solid to gas at a given temperature and pressure. According to Hess's Law, since enthalpy is a state function, this energy change is the sum of the enthalpy of fusion (\(\Delta H_{\text{fus}}\)) and the enthalpy of vaporization (\(\Delta H_{\text{vap}}\)).