Homogeneity (from ὁμός homόs "equal" and γένεσις genesis "creation, birth", thus roughly: same quality) denotes the equality of a physical property over the entire extent of a system or the similarity of elements of a system. The term has a wide scope and can contain different meanings in detail. A measure or method with which a material or system is made homogeneous or its homogeneity is increased is called homogenization.
Opposites to homogeneity
What is not homogeneous is called inhomogeneous or heterogeneous .
A distinction is usually made between these two terms, the use of the word varies somewhat.
- A body made of a uniform material but with a density that varies from place to place is, for example, referred to as inhomogeneous.
- Heterogeneous (two or more phases), on the other hand, is a body made of macroscopically different components, such as a concrete slab with steel reinforcement.
In the figure, the differences in homogeneity, heterogeneity and inhomogeneity are illustrated from left to right.
In physics , matter , viewed atomically, is fundamentally not homogeneous, since the building blocks of matter do not have a uniform spatial filling . Even in the atom itself, the mass and charge distribution is not homogeneous, since it is distributed unevenly between the atomic nucleus and the atomic shell . If the atoms or molecules are distributed approximately evenly (not necessarily with the regularity of a crystal lattice , but without macroscopic fluctuations from place to place), the matter is homogeneous from a practical point of view .
The term is also applied to fields . A field, e.g. B. a magnetic field is called homogeneous if the field strength is the same at every location, otherwise inhomogeneous. Homogeneous fields are characterized by straight, parallel and evenly distributed field lines . When it comes to gradient fields, the equipotential surfaces are parallel planes which are penetrated at right angles by the field lines. While dipoles are aligned and attracted by inhomogeneous fields, homogeneous fields exert aligning moments on dipoles, but not attractive forces. Examples of approximately homogeneous fields are:
- The electric field in a plate capacitor .
- The magnetic field in a long coil .
- The gravitational field on the earth's surface, provided the dimensions of the experimental setup are very small compared to the earth.
Finally, in theoretical physics , one speaks of the homogeneity of space when one wants to express that physical laws are invariant to translation . From this it follows, according to Noether's theorem , that the momentum is a conserved quantity .
Dependence on the size scale
An example of matter that is heterogeneous on a microscopic level but appears homogeneous on a macroscopic level is milk . A microscopic distinction can be made between areas in milk that contain fat and those that contain water . And although the two cannot mix, both areas are so small that, viewed macroscopically, they appear homogeneously distributed. Nevertheless, it can happen in such mixtures that their components separate over time and, in the case of milk, it no longer appears macroscopically homogeneous, since its watery areas are clearly different from their high-fat areas (cream). To prevent this segregation or separation, you can z. B. with the help of homogenization for an even distribution of fat and water even after a long time.
Meaning of homogeneous substances
The extraction of sufficiently homogeneous starting materials and / or intermediate products for industry, e.g. B. in the manufacture of the various semiconductor components of the modern electronics and computer industry, is one of the key problems of scientific and technical development. It often requires a great deal of effort (especially when extracting pure substances and / or reducing their error tolerances).
Consequences of chemical homogeneity
Homogeneous matter has the same density and composition everywhere . When in a large container with a homogeneous substance, e.g. B. with a gas , a subset V 1 is considered at one point , it contains the same amount of substance as a subset with the same volume V 1 at another point. If you divide the total amount of substance into two volumes of equal size, they each contain the same amount of substance (in this case half of the original). It follows:
The volume of homogeneous substances is proportional to the amount of substance at constant pressure p and constant temperature T.
For T = const and p = const the following applies:
These laws apply to all homogeneous substances as long as temperature and pressure remain unchanged, including ideal gases for which the thermal equation of state of ideal gases applies. The quotient is called the molar volume , the quotient is the concentration . The aforementioned relationships are also the basis of volumetry .
The relationships also apply to homogeneous substances
- Brockhaus Encyclopedia, 19th edition, Mannheim 1988