A

cation such as calcium that is adsorbed onto a surface, usually clay or humus and is capable of being easily replaced by another cation such as potassium. Exchangeable cations are readily available to plants.

Symbol and abbreviation of ampere.

(A) A thermodynamic property that can be used to predict whether a process will occur spontaneously at constant volume and temperature. Helmholtz free energy A is defined as A = U - TS where U, T and S are the internal energy , temperature , and entropy . Changes in A correspond to changes in free energy for processes occuring at constant temperature and volume. The sign of Delta A is negative for spontaneous processes and zero for processes at equilibrium .

 Say it (a) Compare with molar absorptivity and absorbance .The absorbance of a solution per unit of path length and per unit concentration; a = A/(bc) where a, A, b, and c are the absorptivity, absorbance, path length, and concentration, respectively. Absorptivity varies with wavelength of the incident light.

(a) An effective thermodynamic concentration used in thermodynamic calculations in place of the actual concentration to allow equations developed for ideal solutions to be used to treat real solutions.

 Say it (A) The SI unit of electric current , equal to flow of 1 coulomb of charge per second. An ampere is the amount of current necessary to produce a force of 0.2 micronewtons per meter between two arbitrarily long, arbitrarily thin wires, placed parallel in a vacuum and exactly 1 m apart. Named for 19th century physicist Andr&eacute Marie Amp&egravere.

(a) Prefix used in the SI system meaning "multiply by 10^-18". For example, 3 am means 3&times 10^-18 meters.

The first term in the van Deemter equation. See Eddy diffusion term and van Deemter equation.