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Yan Diderik van der WaalsDutch physicist
Date of Birth: 23.11.1837
Country:  Netherlands |
Content:
- Johannes Diderik van der Waals
- Academic Career
- Contributions to Physics
- The Van der Waals Equation
- Critical Phenomena and the Law of Corresponding States
- Later Research and Legacy
Johannes Diderik van der Waals
Childhood and EducationJohannes Diderik van der Waals was born in Leiden, Netherlands, on November 23, 1837. His father, Jacobus van der Waals, was a carpenter, and his mother was Elisabeth van der Waals (nee van den Burgh). After graduating from primary and secondary school in Leiden, van der Waals became a primary school teacher. From 1862 to 1865, he attended Leiden University as a non-degree student. In 1864, he obtained a teaching certificate in mathematics and physics and taught physics first in Deventer in 1864 and then in The Hague, where he became headmaster in 1866.
Academic Career
Soon after, van der Waals began pursuing graduate studies in physics and received his doctorate from Leiden in 1873. His doctoral dissertation, entitled "On the Continuity of the Gaseous and Liquid States," was met with great acclaim by James Clerk Maxwell, one of the greatest physicists of the 19th century, who said of van der Waals' work: "It has at once placed his name in the first rank of scientific investigators." Translated into German and French, this dissertation not only established van der Waals' reputation as a brilliant physicist but also defined the subject matter of his research for the rest of his scientific career. Four years after earning his doctorate, he became the first Professor of Physics at the newly founded University of Amsterdam, where he remained until his retirement in 1908, when he passed on the position to his son.
Contributions to Physics
Van der Waals' ideas were influenced by an 1857 paper by Rudolf Julius Emmanuel Clausius, a German physicist who had made major contributions to the kinetic theory of gases. According to this theory, gas molecules are in rapid motion in all directions. Their impacts on the walls of their container account for the pressure of the gas, and the average speed of the molecules (their kinetic energy) is directly related to the temperature. Clausius showed how this theory could be used to derive the law discovered experimentally in 1662 (before molecules were known about) by Robert Boyle, the Irish physicist and chemist. Boyle's Law states that for a given mass of gas at constant temperature, the product of pressure and volume is constant. For example, if the volume is decreased by pushing a piston into a cylinder, the pressure will increase by an amount that keeps the product constant.
Later in the 19th century, other scientists, such as the French physicists Jacques Alexandre César Charles and Joseph Louis Gay-Lussac, showed that at constant pressure, the ratio of volume to absolute temperature is constant. This law can also be derived directly from kinetic theory. These two laws can be combined into a single equation of state, which is valid for not too high densities: PV = RT, where P is pressure, V is volume, T is absolute temperature, 0 K (or -273 °C), and R is a constant for all gases if one gram-molecule of gas occupies the volume.
The Van der Waals Equation
It was well known that this equation was not exactly accurate, and this was the case to different extents for different gases and different conditions. Gases that obeyed this equation most closely were called ideal gases. In investigating possible sources of error, van der Waals noted that the equation was based on two assumptions: that molecules act as point masses (which is roughly true if they are far apart) and that molecules do not interact with each other (except by collisions). He introduced a finite volume for each molecule and an attractive force between molecules (not specifying its nature) that diminishes with increasing distance. (Other researchers later worked out the details, but the weak, non-chemical attraction between molecules is still often called the van der Waals force.)
Van der Waals then derived a modified equation of state for a real gas:
(P + α/V²) (V - b) = RT, where α expresses the mutual attraction of the molecules of the gas (divided by V² to account for the weakening of this force at larger volume, i.e., at larger average distance between the molecules), and b expresses the molecular volume. Both α and b take different values for different gases.
While van der Waals' equation did not perfectly agree with experimental data, it was a significant improvement over the simpler ideal gas law and had important consequences. The attraction between molecules leads to what van der Waals called an internal pressure that tends to hold the molecules together. As the volume is decreased by external pressure, the internal pressure increases much faster than the external pressure. If it becomes equal to or greater than the external pressure, the molecules will stick together and no longer require the pressure of their container. The gas will condense into a liquid. This illustrates van der Waals' conviction, expressed in his dissertation, that there is no essential difference between the gaseous and liquid states. The same forces and molecular volume effects are at play in both cases. The difference in properties between gases and liquids is due to differences in the magnitude, not the type, of forces and volume effects, as molecules are either closer together or farther apart.
Critical Phenomena and the Law of Corresponding States
Van der Waals' equation greatly clarified the previously discovered existence of a critical temperature, different for different gases, above which a gas could not be liquefied, no matter how much pressure was applied. The critical temperature is related to a critical volume and a critical pressure, which together define the critical point, a set of values of temperature, pressure, and volume at which there is no visible distinction between gas and liquid: under these conditions, the two states are essentially identical, and there is no sharp transition between them.
Van der Waals used the critical point to derive an equation in which the variables temperature, pressure, and volume are expressed in terms of their values at the critical point. This resulted in a universal relationship that applies to all gases and depends in each case only on the critical temperature, pressure, and volume, not on the nature of the gas. This formed the basis for the formulation in 1880 of his most important discovery, the law of corresponding states. According to this law, if the behavior of some gas and its liquid is known at all temperatures and pressures, then the behavior of any other gas or liquid can be calculated for any temperature and pressure provided its behavior is known at its critical temperature.
This law is not a perfect description of the extremely complex behavior of gases and liquids, but it is accurate enough to provide an approximate way to determine the conditions necessary to liquefy gases, based on existing experimental data. Guided by this law, in 1898 Scottish physicist James Dewar liquefied hydrogen, and in 1908 Heike Kamerlingh Onnes, van der Waals' Dutch colleague, liquefied helium.
Later Research and Legacy
In his later research, van der Waals tried to account for deviations from the equation of state for a real gas by introducing a variable molecular volume. He suggested that molecules could aggregate into clusters, which then behave as single molecules of larger size. Since a cluster could contain any number of single molecules, the gas could be composed of a complex mixture. Although van der Waals' original equation remained useful for a wide range of cases, its simplicity had to be largely sacrificed in order to more accurately describe gas behavior.
Van der Waals received the 1910 Nobel Prize in Physics "for his work on the equation of state for gases and liquids." According to Oscar Montelius, a member of the Royal Swedish Academy of Sciences, in his presentation speech to the laureate, "van der Waals' investigations are of very great importance, not merely for pure science. The modern construction of refrigerating apparatus, which has now become of such enormous consequence in our economics and industry, is based principally upon the theoretical investigations of the prize-winner."
Van der Waals married Anna Magdalena Smit in 1864. She died when their three daughters and son were still young, and he never remarried. A short man who lived modestly, van der Waals spent his leisure time playing billiards, reading, or playing solitaire. He died in Amsterdam on March 8, 1923.
