First Law Of Thermodynamics
The first law of thermodynamics was announced by Julius Robert von Mayer in 1841. It is the principle of conservation of energy.
Definition of the first law of thermodynamics: The total energy of an isolated system is neither created nor destroyed, it remains constant. Energy only transforms from one type to another. When one energy class disappears, an equivalent quantity of another class must be produced.
A body can have a certain speed with what has kinetic energy. If it loses speed, this kinetic energy that it loses becomes another type of energy, whether it is potential energy (if it acquires height), heat energy (if there is some kind of friction that causes it to heat up), etc.
The same principle applies to photovoltaic solar energy and solar thermal energy. The atoms of the particles that form the Sun contain energy, through a nucellar reaction this energy is transformed into radiation. The solar radiation that reaches the Earth is captured by photovoltaic solar panels or thermal collectors, which transform this energy into electrical energy (photovoltaic) or heat (thermal) energy.
Now, let's see how such conclusions were reached.
The first steam engines or thermal machines were developed for the first time in Roman times. The Romans built the first device that used steam to function. This steam engine consisted of a hollow balloon supported by a pivot so that it could rotate around a pair of stumps, one of them hollow. By said stump, steam could be injected, which escaped from the balloon to the outside by two tubes bent and tangentially oriented in opposite directions and placed at the ends of the diameter perpendicular to the axis of the balloon. When steam was expelled, the balloon reacted to this force and rotated around its axis.
From that moment, a large number of steam engines were built and used for various purposes. One of the uses of steam engines was to use a water pump to raise water to homes and distribute it through their rooms, or to lift weights through a cylinder and a piston. Little by little steam engines were used for a greater number of uses as their efficiency increased.
The development and refinement continued until the steam engine became the usual machine for marine navigation and land transportation (locomotives), achieving very high vapor pressures and considerable piston speeds. Technologically, steam engines improved a lot, although at the moment, scientifically, there was no complete explanation of their physical functioning.
The main idea of steam engines is to convert the maximum amount of heat energy into another type of energy: mechanical work. At the moment there was no scientific explanation, but entering the nineteenth century, through experimentation, begins to understand its meaning.
Rumford, in 1798 performs an experiment that consisted of a bronze cylinder used to a sharp steel drill. This drill was forced against the bottom of the cylinder and the cylinder was rotated on its axis by means of a drilling machine operated with horses. The cylinder and the drill were placed in an airtight box filled with water at room temperature. After operating the device for some time, the cylinder and water were heated, this heating continued until the water boiled.
This implied that the water was heated without using fire, only through work (of the cylinder).
The decisive studies that led to establishing the equivalence between mechanical work and heat were made in 1840 by James Joule in Great Britain. These studies were inspired by Rumford's work.
James Joule proposed a device that consisted of a rotating shaft with a series of vanes rotating between four sets of stationary vanes. The purpose of these paddles was to stir the liquid that was placed in the free space between them. The shaft was connected by a system of pulleys and very thin ropes to a pair of masses of known weight.
The experiment consisted of winding the rope by holding the masses on the pulleys until they were placed at a certain height on the ground. By dropping the masses, the shaft rotated which in turn generated a rotation of the revolving arms by stirring the liquid contained in the container. This process was repeated twenty times and the final temperature of the stirred liquid was measured. The walls of the container were airtight and made of a very thick wood to simulate an adiabatic wall.
After a careful repetition Joule concluded that the amount of heat produced by the friction between the bodies, whether liquid or solid is always proportional to the amount of mechanical work supplied.
Their experiments were repeated in different substances, tabulating the obtained values of mechanical force (represented by the fall of a mass by a certain distance), to raise the temperature of a known volume of substance.
First law of thermodynamics
The results obtained by Joule show that for systems isolated from their exterior, and to which the same amount of mechanical energy is supplied in different ways, the change observed in the system is the same. In this experiment the change is recorded by the variation of the system temperature.
It is important to note that in these experiments the system does not move, its kinetic energy is zero, it does not move with respect to the ground level, its potential energy remains constant and yet the system has absorbed a certain amount of energy. We call this energy the internal energy of the system. These experiences serve to extend this observation to any thermodynamic system and postulate that: if to any isolated system, we supply a certain amount of mechanical energy W, this only causes an increase in the internal energy of the system U, by the quantity U in a way what:
This equality that points out that energy is applied to the isolated system, constitutes the definition of the internal energy U. The existence of this quantity for any system is the postulate known as the first law of thermodynamics.
If Joule's or similar experiments on other systems were carried out without taking the precaution of isolating the system from its surroundings, we would note that:
The simplest example is the one that happens when heating the same amount of substance used by Joule, but putting it directly on the fire until obtaining the same variation in temperature. Taking precautions so that no other properties change, we conclude that the same energy supplied by W in the Joule experiments, it was now supplied by fire, that is, an amount of heat energy Q. It is clear that the energy missing in the above equation is due to the heat losses caused by the heat flux of the system to the outside, by virtue of its temperature differences.
Then we can write:
This is: the energy is conserved in any process if the heat is taken into account, understanding by process the mechanism by means of which a system changes its variables or thermodynamic properties.
In summary, we can say that the mathematical formulation of the first law of thermodynamics, the previous equation, contains three related ideas:
- The existence of an internal energy function.
- The principle of conservation of energy,
- The definition of heat as energy in transit
- Universidad Nadional Abierta y a Distáncia - Postulados fundamentales de la termodinámica
- Primera Ley de la Termodinámica | Fisicalab
Last review: September 25, 2016