Effect Thomson

The numerical value of a- always increases with the temperature, but is not always proportional to the absolute temperature, as some authors have stated. 

Conversely, we must assume that wherever we have a fall of temperature along a wire, we must also have a fall of potential. It is, however, impossible to detect this fall of potential when the circuit is composed of one metal only, as the sum of the potential differences round the whole circuit is zero. If the 

If the circuit is made up of different metals, the phenomena are complicated by the processes which take place at the boundaries between the various conductors. We shall therefore discuss these first. 

The vapor pressure of water is built up to approx. 2.5 bar, which does not explain the result. In this case, the expansion of the liquid with temperature is important. At Ti = 400 K, the liquid density of water changes from 996.5 to 937.2 kg/m Referring to 900 kg water, its volume increases from 0.903 to 0.961 m This means that approx. 60% of the space for the vapor phase left at Ti = 300 K is consumed by the additional volume required by the liquid, giving a corresponding pressure increase, which is, on the other hand, mitigated by the increased solubility of nitrogen in water due to the elevated pressure. 

Nevertheless, a change in temperature can occur. Behind the orifice, vortices are formed, and two effects compete the flow involving friction of the vortices causes a temperature rise, and the pressure loss causes a lowering of the temperature, as energy is necessary to overcome the attractive forces between the molecules.  

The oule-Thomson coefficient 3T/dP)h determines which of these two effects dominates. It can be evaluated as follows. 

Nitrogen (Ti = 300 K, P] = 200 bar) is expanded to P2 = 1 bar by a throttle valve. Calculate the outlet temperature using 

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Joule-Thomson effect, Joule-Kelvin effect... 

In 1857, Thomson (Lord Kelvin) placed the whole field on firmer footing by using the newly developing field of thermodynamics (qv) to clarify the relationship between the Seebeck and the Peltier effects. He also discovered what is subsequently known as the Thomson effect, a much weaker thermoelectric phenomenon that causes the generation or absorption of heat, other than Joule heat, along a current-carrying conductor in a temperature gradient. 

The primary thermoelectric phenomena considered in practical devices are the reversible Seebeck, Peltier, and, to a lesser extent, Thomson effects, and the irreversible Eourier conduction and Joule heating. The Seebeck effect causes a voltage to appear between the ends of a conductor in a temperature gradient. The Seebeck coefficient, L, is given by... 

Kelvin effect The electrical potential gradient caused by a temperature gradient along a conducting wire. Also known as the Thomson Effect. 

Seebeck s outstanding scientific achievement was the discovei"y of one of the three classical thermoelectric effects, which are the Seebeck, the Peltier, and the Thomson effects. Seebeck s discovery was the first, dating from 1822—1823, followed by that of Jean-Charles-Athanase Peltier in 1832 and that of William Thomson in 1854. Seebeck obseiwed that an electric current in a closed circuit comprised different metallic components if he heated the junctions of the components to different temperatures. He noted that the effect increases linearly with the applied temperature difference and that it crucially depends on the choice of materials. Seebeck tested most of the available metallic materials for thermoelectricity. His studies were further systematized by the French physicist... 

Kohlrausch s theory leaves quite unexplained the fact that no thermoelectric current is set up in a homogeneous wire along which a current of heat is flowing, whilst the theory of Lord Kelvin is difficult to reconcile with the fact that thermoelectric currents cannot be set up in a circuit of liquid metals, although these show the Thomson effect. The latter seems, therefore, to be to a certain extent independent of the Peltier effect. Theories intended to escape these difficulties have been proposed by Planck (1889), and Duhem, in which the conception of the entropy of electricity is introduced. 

A characteristic of the non-ideal gas is that it has a finite Joule-Thomson effect. This relates to the amount of heat which must be added during an expansion of a gas from a pressure Pi to a pressure P2 in order to maintain isothermal conditions. Imagine a gas flowing from a cylinder, fitted with a piston at a pressure Pi to a second cylinder at a pressure Pi (Figure 2.2). 

For an ideal gas, under isothermal conditions, AU = 0 and /V 2 = Pp - Thus q = 0 and the ideal gas is said to have a zero Joule-Thomson effect. A non-ideal gas has a Joule-Thomson effect which may be either positive or negative. 

Many gases can be liquefied by making use of the Joule-Thomson effect, cooling... 

FIGURE 4.31 Cooling bv the Joule-Thomson effect can be visualized as a slowing of the molecules as they climb away from each other against the force of attraction between them. 

However, in this case the EMF measured will be distorted by another effect [i.e., the variation of electrostatic potential within a given conductor, which is caused by a temperature gradient in the conductor (the Thomson effect, 1856)]. Potential gradients will arise even at zero current, in both the electrolyte (between points and Aj)... 

The Thomson Effect The evolution of heat as an electric current traverses a temperature gradient in a material. 

Although this book is devoted to molecular fluorescence in condensed phases, it is worth mentioning the relevance of fluorescence spectroscopy in supersonic jets (Ito et al., 1988). A gas expanded through an orifice from a high-pressure region into a vacuum is cooled by the well-known Joule-Thomson effect. During expansion, collisions between the gas molecules lead to a dramatic decrease in their translational velocities. Translational temperatures of 1 K or less can be attained in this way. The supersonic jet technique is an alternative low-temperature approach to the solid-phase methods described in Section 3.5.2 all of them have a common aim of improving the spectral resolution. 

Joule-Thomson Coefficient. Knowing that a process is isenthalpic, we can formulate the Joule-Thomson effect quantitatively. 

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