Enthalpy of Sublimation

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 photovoltaic effect is the generation of voltage and electric current in a material upon exposure to light. It is a physical and chemical phenomenon. A common application is the solar cell, which uses this effect to convert sunlight directly into electricity. The effect is based on photons of light exciting electrons into a higher state of energy.

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.

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 Clausius-Clapeyron relation describes the relationship between pressure and temperature for a substance at a phase transition, such as liquid and vapor. For water vapor, it shows that saturation vapor pressure increases exponentially with temperature. The approximate form is \(\frac{dp}{dT} = \frac{L}{T(V_v - V_l)} \approx \frac{L p}{R_v T^2}\), where L is latent heat.