Measurement via the Standard Hydrogen Electrode (SHE)

The Mach number (M) is a dimensionless quantity representing the ratio of flow velocity past a boundary to the local speed of sound: \(M = v/a\), where v is the flow velocity and a is the speed of sound. It is the primary indicator of compressibility effects. As Mach number approaches and exceeds 1, the density of the air changes significantly, altering aerodynamic forces.

In electrochemical cells like batteries and fuel cells, EMF is generated by chemical reactions. The separation of charge is driven by oxidation and reduction reactions occurring at two different electrodes. The maximum EMF of a cell is related to the change in Gibbs free energy (\(\Delta G\)) of the reaction by \(\mathcal{E} = -\frac{\Delta G}{nF}\), where \(n\) is moles of electrons and \(F\) is the Faraday constant.

The Reynolds-Averaged Navier-Stokes (RANS) equations are time-averaged equations of motion for turbulent fluid flow. This approach, called Reynolds decomposition, separates flow variables into a mean and a fluctuating component. The averaging process introduces an additional term, the Reynolds stress tensor, which represents the effect of turbulence and must be modeled to achieve closure, making simulations computationally tractable.

The Kutta-Joukowski theorem quantifies the lift force generated by an airfoil. It states that the lift per unit span (\(L'\)) is directly proportional to the fluid density (\(\rho\)), the free-stream velocity (\(V\)), and the circulation (\(\Gamma\)) around the body: \(L' = \rho V \Gamma\). This links the abstract concept of circulation to the physical force of lift.

The boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant. Introduced by Ludwig Prandtl, this concept simplifies fluid dynamics problems by dividing the flow into two regions: the thin boundary layer where viscosity dominates and the outer region where inviscid flow theory can be applied.

The von Mises yield criterion predicts that yielding of a ductile material begins when the second deviatoric stress invariant, \(J_2\), reaches a critical value. It is often expressed in terms of the von Mises stress, \(\sigma_v\), a scalar value that must be less than the material's yield strength, \(\sigma_y\). Yielding occurs when \(\sigma_v = \sigma_y\).

Shear-thickening, or dilatancy, is a non-Newtonian behavior where viscosity increases with the rate of shear stress. A classic example is a suspension of cornstarch in water (oobleck), which feels liquid when stirred slowly but becomes almost solid when struck or stirred rapidly. This phenomenon is caused by the jamming of suspended particles under high shear.

The Nernst equation relates the reduction potential of a half-cell (or the total voltage of an electrochemical cell) to the standard electrode potential, temperature, and the activities (often approximated by concentrations) of the chemical species undergoing redox. The equation is \(E = E^{\circ} - \frac{RT}{nF} \ln Q\), where Q is the reaction quotient.

Motional EMF is generated when a conductor moves through a magnetic field. The magnetic component of the Lorentz force, \(\mathbf{F} = q(\mathbf{v} \times \mathbf{B})\), acts on the charge carriers within the conductor, causing them to move and create a charge separation. This separation establishes an electric field and a potential difference. The resulting EMF is given by the line integral \(\mathcal{E} = \oint (\mathbf{v} \times \mathbf{B}) \cdot d\mathbf{l}\).

In a system at thermodynamic equilibrium, the electrochemical potential, \(\bar{\mu}_i\), for any given charged species `i` must be uniform throughout all phases. If a gradient exists (\(\nabla \bar{\mu}_i \neq 0\)), ions will spontaneously move from regions of higher potential to lower potential, creating a current or flux, until this gradient is eliminated and equilibrium is re-established.

Energy is not continuous but comes in discrete packets called quanta. The energy \(E\) of a single quantum of electromagnetic radiation (a photon) is directly proportional to its frequency \(\nu\). This relationship is defined by the Planck-Einstein relation, \(E = h\nu\), where \(h\) is the Planck constant. This concept fundamentally challenged classical physics.

A statistical ensemble is a conceptual tool consisting of a large number of virtual copies of a system, each representing a possible microstate. By averaging properties over all systems in the ensemble, one can calculate macroscopic observables. The main types are the microcanonical (isolated system with fixed N, V, E), canonical (closed system, fixed N, V, T), and grand canonical (open system, fixed µ, V, T).

The equivalence principle is a cornerstone of general relativity. It posits that the effects of a uniform gravitational field are indistinguishable from the effects of uniform acceleration. This means that an observer in a windowless, free-falling elevator experiences weightlessness, just like an observer in deep space, far from any gravitational source, forming the conceptual basis for gravity as a geometric phenomenon.

A key prediction of general relativity is that time passes at different rates in different gravitational potentials. A clock in a stronger gravitational field (closer to a massive object) will tick slower than a clock in a weaker field. This effect, known as gravitational time dilation, has been experimentally verified and is a crucial factor in modern technology like GPS.