The Joule effect

As more experimental data have been obtained, empirical correlations have been suggested for determining the values of the cross coefficients, such as ai, from the properties of the mixtures rather than from those of the pure components. 

The Joule effect is discussed in Section 2.8 in conjunction with the definition of an ideal gas. When Equations (2.40) and (4.64) are combined, the expression for the Joule coefficient becomes. 

Equation (7.11) may be used to evaluate the numerator of the right-hand term, so 

We conclude from Equation (7.41) that the Joule coefficient is a function of both the temperature and the volume. Moreover, the value of the coefficient goes to zero as the molar volume approaches infinity. 

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The procedure may start with the reference experiment, which, in the case under analysis, involved a solution of ferrocene in cyclohexane (ferrocene is a nonphotoreactive substance that converts all the absorbed 366 nm radiation into heat). With the shutter closed, the calorimeter was calibrated using the Joule effect, as described in chapter 8, yielding the calibration constant s. The same solution was then irradiated for a given period of time t (typically, 2-3 min), by opening the shutter. The heat released during this period (g0, determined from the temperature against time plot and from the calibration constant (see chapter 8), leads to the radiant power (radiant energy per second) absorbed by the solution, P = /t. ... 

The calibration of the calorimetric unit P, leading to the calibration constant s (see chapter 9), can be made by the Joule effect, with a resistor inserted into the photochemical reactor cell. As justified shortly (equation 10.16), no calibration is required for the photoinert cell in unit R. 

An electric current heats an electrical resistor sunk in a sleeve of thermoplastic and causes its melting by the Joule effect. 

The thermal device used to elevate the temperature consists of a burner fed with a gaseous combustible mixture or, alternatively, in atomic absorption, by a small electric oven that contains a graphite rod resistor heated by the Joule effect. In the former, an aqueous solution of the sample is nebulised into the flame where atomisation takes place. In the latter, the sample is deposited on the graphite rod. In both methods, the atomic gas generated is located in the optical path of the instrument. 

Figure 14.9—Thermoelectric atomisation device, a) Graphite furnace heated by the Joule effect b) example of a graphite rod c) temperature program as a function of time showing the absorption signal. The first two steps of this temperature program are conducted under an inert atmosphere (argon scan).

When ions flow from one side of the electrolyte to the other, there is ohmic loss and generated heat, due to the Joule effect. 

The sample to be analyzed, say C60 fullerene, is mixed with an appropriate amount of KBr in an agate mortar and then transferred into a press and compressed at 4,000 Kg into a pellet with a diameter of 1.2 cm and a thickness of 0.2 cm. The pellet was mounted into the sample holder of the Specac variable temperature cell and inserted into the cell. The cell was then evacuated with the aid of a pump to a vacuum of 0.1 torr and then heated gradually at 120°C in order to permit the humidity absorbed on the internal surfaces of the cell and in the KBr pellet to evaporate. The sample was then cooled to the desired temperature to record the infrared spectrum. In order to go below room temperature, use was made of liquid nitrogen, added cautiously and in small amount in the cavity present inside the cell. Such cavity is connected with the sample holder and permits to cool the sample to the desired temperature. The temperature of the sample was monitored with adequate thermocouples. The lowest temperature reached with this apparatus was -180°C (93K) while the highest temperature was +250°C. Heating is provided by the Joule effect and an external thermal control unit. 

The concept behind the defining equation (Eq. (2.37)) comes from an experiment known as the Joule experiment, which is illustrated in Figure 2.3. The result of this experiment is known as the Joule effect. In this experiment the gas is confined in one part of a closed container and the other part is evacuated. The gas itself is taken to be the substance composing the system. However, the boundary between the system and its surroundings is chosen to be the walls of the container. The volume of the system is the total volume of the container and is not the same as the volume of the gas when it is... 

Adiabatic calorimetry is particularly useful for the study of closed adsorption systems at low temperatures (where radiation losses are small) and for temperature scanning experiments. It is the preferred type of measurement for the determination of the heat capacity of adsorption systems, especially in the temperature range 4-300 K (Morrison et al., 1952 Dash, 1975). The temperature scan is obtained by means of the Joule effect applied to the sample container the sample heating coil shown in Figure 3.14 is used for this purpose. 

In the present work, we shall not discuss the exact nature of the failure, i.e. its microscopic mechanism. In the fuse problem, the mechanism of the failure is very well-known (it is merely the Joule effect), but in the dielectric problem the mechanism is much more complicated (O Dwyer 1973). The reason is that we intend to attack the problem from a point of view which is of tremendous importance for statistical analysis. If the sample is perfectly homogeneous the failure will take place in all the portions of the sample. In the first case the current density is uniform in the sample and in the second, the electric field is the same everywhere. If the threshold value is reached, the failure will be general and the sample will explode. In fact, this never happens. The failure always begins as a local event and progressively becomes general. This is because there are weak points in the system. The failure always begins at these weak points. The existence of weak points is due to the fact that solids are never homogeneous. This means that the... 

Suppose a current I flows through a sample of resistance Rq. Rq is defined as the resistance of the sample at zero current. Because of the Joule effect, the resistance will increase by AR = Rof3AT, where (3 is the temperature coefficient of the resistance. As above, one can write AT rsj RqP, and this approximation therefore gives AR Rol)", The voltage V across the sample is therefore given by... 

Because of the Joule effect, each individual resistance r will increase by an amount brj, depending on the current ij flowing through it. Equating the... 

At the present state, only some qualitative explanations are available. When the sample is prepared, the carbon particles get compressed to come in contact with others. During the flow of the current, the particles are heated because of the Joule effect and the heat is transmitted to the surrounding epoxy matrix. As a result of the matrix dilatation, the contacts between the carbon particles disappear and the sample becomes an insulator. The process being gradual, one observes the regular increase of R with time. 

Graphite ovens heated by the Joule effect where the whole sample is introduced at once leading to a variable, peak-shaped, signal. 

We next present the BTE and MD predictions of thermal conductivities of silicon thin films. The thermal properties of silicon thin films are of paramount importance to the transistor industry. Silicon-on-insulator (SOI) and strained silicon transistors are composed of silicon thin films. In both cases, the thin silicon film is deposited on top of poor thermally conducting materials, and the thermal energy generated by the Joule effect has to be removed along the silicon film plane. A thorough understanding of the thermal properties of thin silicon films is essential for the accurate prediction of the thermal response of these transistors. The dimensions of the silicon thin film in... 

The influence of borax buffer concentration was investigated in the range from 10 to 100 mM. The sharpest peaks were obtained in the use of 10-30 mM concentrations and the of ENX was almost constant, and an increase was observed in the use of borax concentration above 30 mM, but peak deformation also occurred due to the heat production by the Joule effect. In order to achieve optimization of the proposed analytical... 

Other examples of irreversible processes are the equalization of temperature in a system, mechanical friction, viscous flow in fluids, the Joule effect, and diffusion. The idealization of changes as reversible involves the suppression of all phenomena such as those just listed, and very often this is quite impracticable. 

In 1913, Coolidge [COO 13] imagined another kind of X-ray source. The cathode is comprised of a tungsten filament heated by the Joule effect. According to the Edison effect, this filament emits electrons that are accelerated by an electrical field and bombard the anticathode which then emits X-rays. The entire device is placed in a sealed tube inside which the pressure must be as low as possible. A schematic view of such a tube is shown in Figure 2.3. 

Roughly 98% of the electron beam s energy is consumed by the Joule effect and heats up the anticathode, which is why it is important to set up a system to evacuate heat. A cooling system by water circulation is used. 

The variable magnetic field that is created confines the ions and the electrons to an annular path. As the medium becomes more and more conducting, by appearance of an Eddy current, temperature increases considerably by the Joule effect. The device behaves similar to the secondary coil of a short-circuited transformer. The plasma is isolated from the torch wall by a gaseous sheath of non-ionized argon, which is injected by an external tube concentric to the previous one and is used to keep quartz wall cool. 

The principle of this technique is to consider the adsorbent as an electrical resistance, which is heated by the Joule effect when an alternative current is applied. This regeneration is particularly suitable for activated carbon cloths or felts. After saturation of the adsorbent, the filter is heated by the Joule effect and the desorbate is flushed out of the reactor with an inert gas. Then, the solvent is condensed in a cold trap before being recycled. 

Some operating conditions for an indiistrial system using adsorption of a mixture of ethanol and ethyl acetate (50/50) onto activated carbon cloths and desorption by the Joule effect... 

Heat compensation can be achieved by the Joule effect if the studied phenomenon is endothermic and by the Peltier effect if it is exothermic. This was used by Tian, in 1924, to compensate the major part of die heat (by means of a constant power, which is easy to measure), whereas the remainder was measured with a heat-flowmeter [41]. 

A simpler and more elegant way to compensate either exothermic or endothermic effects was used by Dzhigit et al. in 1962 [42], A Joide effect is produced in the system and this is controlled to ensure a constant temperature difference (say, 5 K) between the system S and the thermostat T and, hence, a constant heat-flow. If the phenomenon studied is endothermic, the Joule effect is automatically increased to ensure the constant heat flow required. If it is exothermic, the Joule effect is simply decreased. These changes in the Joule effect are recorded and provide a direct measurement of the heat involved in the phenomenon studied. This was also used by Hansen et aL in 1982 in their high temperature battery calorimeter [43]. 

The constraints associated with the making of vapor sensors from CPCs are somewhat different from those which are required to make self-regulating heating elements. The voltages applied to the transducer materials are of a few volts only, whereas those which are used for energy dissipation by the Joule effect are several hundred volts. The physical phenomena associated therewith are therefore quite different, although the principle of variation of resistance caused by the discormection of the conductor network remains the same. 

Figure 6.3 illustrates the heat losses that occur in a micro-hotplate when operating. The thermal energy, Q, generated by the Joule effect in the microheater, is given by ... 

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