Joule’s experiments

Having met Joule for the first time at the 1847 meeting of the British Association for the Advancement of Science in Oxford, Thomson initially accepted that Joule s experiments had shown that work converted into heat. Committed to Carnot s theory of the production of work from a fall of heat, however, he could not accept the converse proposition that work had been converted into heat could simply be recovered as useful work. Therefore, he could not agree to Joule s claim for mutual convertibility. By 1848 he had appropriated from the lectures of the late Thomas Young (reprinted in the mid-1840s) the term energy as a synonym for vis viva (the term in use at the time, traditionally measured as mtc) and its equivalent terms such as work, but as yet the term appeared only in a footnote. 

An explanation of potential energy involves an explanation of force both terms are simply another way of saying that we know nothing about the thing to be explained. A distinct advance is made when a force can be explained in terms of the kinetic energy of a system in motion, an illustration of which is afforded by the kinetic theory of gases, which replaced the supposed forces of repulsion between the molecules of gases (the existence of which is disproved by Joule s experiment, 73) by molecular impacts. 

Figure 3.10 Schematic of Joule s experiment for irreversible expansion of a gas into an evacuated chamber in a water bath.

In fact, Joule s experiment is not sufficiently accurate to detect the actual (small) value of (dU/dV)T for real gases at near-ambient conditions. However, it will later be proven (Sidebar 5.6) that the vanishing of (dU/dV)T becomes exact in the ideal gas limit,... 

Joule s experiments on the free expansion of an ideal gas showed that the internal energy of such a system is a function of temperature alone. For a real gas, this is only approximately true. For condensed phases, which are effectively incompressible, the volume dependence on the change in internal energy is negligible. As a result, the internal energies of liquids and solids are also considered a function of temperature alone. For this reason, the internal energy of a system may loosely be referred to as the thermal energy . 

Work is readily transformed into other forms of energy for example, into potential energy by elevation of a weight, into kinetic energy by acceleration of a mass, into electrical energy by operation of a generator. These processes can be made to approach a conversion efficiency of 100 percent by elimination of friction, a dissipative process that transforms work into heat. Indeed, work is readily transformed completely into heat, as demonstrated by Joule s experiments. 

On a much smaller scale, in Joule s experiment. Figure 3.1, the energy of the stirred liquid is increased pro rata to the work put irreversibly into stirring, as mentioned earlier. 

A prime lesson in irreversible process theory is based on Figure 3.1, illustrating Joule s experiment. In that experiment, shaft power was dissipated irreversibly by a rotating paddle, to become energy in a tank of near ambient temperature water. The chaotically interactive translation, vibration and rotation of fluid molecules is energy.Energy is accessible to generate power, only by cyclic processes (heat cycles) as defined by Carnot. (Carnot cycle theory is outlined in Chapter 1.)... 

Figure 3.1 Joule s experiment, irreversible transformation, power > energy...

No heat is taken in to do internal work, and therefore there must be no internal work to do This is a thermodynamic proof of the conclusion to which we have already come on kinetic considerations, viz that there are no cohesive forces existing between the molecules of a perfect gas, and therefore there can be no internal work done on expanding (cf Joule s experiment (p 20)) Further, since for a perfect gas—... 

In the boring experiment, work is done by the surroundings on the system (the brass cannon), the energy of the system rises and heat is also released to the surroundings (water bath). The First Law of Thermodynamics and the mechanical equivalent of heat (1 calorie = 4.184 joule) were established in 1843 by James Prescott Joule (1818-89). In order to raise the temperature of 1 gram of water by 1 °C (1 calorie), 4.184 joule of mechanical work, such as spinning paddles in water (Joule s experiment), is required. 

For a readable discussion of Joule s experiments, see T. W. Chalmers, Historical Researches, Dawsons of Pall Mall, London, 1968. 

In some of Joule s experiments, work was done on water held in an adiabatic calorimeter. The work was done by a rotating paddle, driven by falling weights. Assume the volume of the water remains constant during these experiments. 

In this example we consider a variation of Joule s experiment a thermally insulated vessel contains 10 kg of water. The liquid is stirred by an impeller driven by a 1 kW motor. If the motor runs for 1 min and all of the work it produces is transferred to the liquid, analyze the experiment on the basis of the first law and report the relevant amounts of heat, work, and internal energy. 

Although Schonbein preferred his own tendency theory to the contact theory it was, he said, far from complete and the nature of electricity and its relations to chemical action (Chemismus) would have to be infinitely better known and more deeply investigated than they then were. His paper was never answered by the contactists but simply ignored as Ostwald said, the opposing parties had now talked themselves out and no further progress could be made until the meaning of Joule s experiments was imderstood. 

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