The Discovery of the Law of Energy Conservation

At least 12 scientists (Sadi Carnot, Marc Séguin, Karl Holtzmann, Gustave-Adolphe Hirn, William Grove, Michael Faraday, Ludwig Colding, Karl Friedrich Mohr, Justus von Liebig, James Prescott Joule, Julius Robert von Mayer, and Hermann von Helmholtz) state, with varying degrees of perfection, the law of conservation of energy between 1839 and 1850. Heinrich Helmholtz probably gives the most achieved version of this law in 1847. It is a little surprising to note that so many scientists were able to discover the law almost simultaneously but it must be said that the conditions necessary for the discovery of the energy conservation were met at the beginning of the 19 th century. Thus, several researchers succeeded in assembling the pieces of the puzzle leading to the discovery of the law.

What were these favourable conditions?

New Transformations

At the beginning of the 19 th century, several new phenomena linked to energy transformations can be studied. Many of these phenomena were discovered through the study of steam engines and the use of the electric battery.

In order to improve their efficiency, the study of steam engines took a paramount importance at the beginning of the 19 th century. This study implies that several transformation processes must be examined such as:

1) Combustion generates heat. 2) Heat can generate work. 3) The compression of a gas generates heat. 4) Compressed gas can do work. 5) Friction generates heat.

The electric battery, invented in 1800, allows the study of several new transformation processes related to electricity such as:

1) An electric current generates heat and sometimes even light. 2) A current can be created by a chemical reaction (battery). 3) An electric current can separate compounds (electrolysis). 4) A magnetic field exerts a force on currents (which led to the invention of the electric motor). 5) A moving loop in a magnetic field generates a current. 6) Heat can generate a current (Seebeck effect).

There are also some other transformation phenomena studied such as:

1) Light can generate chemical transformations (photograph 1827). 2) Respiration is related to body heat and to work in humans and animals.

The study of these transformations required a lot of work since it was necessary to first define and measure a whole series of quantities. The examples of the measure of the efficiency of machines and heat shows this difficulty. Savery proposed, as early as 1702, to measure the efficiency of steam engines with the work of F∆scos θ and this way of measuring did not impose itself until the end of the 18 th century after a struggle against many other ways of measuring that had been proposed. (Besides, it is the use of the work F∆scos θ by the engineers which brings its introduction in mechanics in the 1820s.) As for heat, it is distinguished from temperature by Joseph Black in 1761 and several more years were needed before everyone agrees on how to measure it and elucidates how it makes a steam engine work.

The study of these phenomena leads directly to the laws of energy transformations because Newton’s laws can hardly be applied in these cases. To explain a phenomenon with Newton’s laws, the details of each step of the transformation must be known, and this can be almost impossible in some cases. With energy conservation, it is enough to know only the initial and final configurations without having to know all the details of the transformation. This simplicity means that the laws of conservation of energy are often found first.

A Conviction That the Principle of Conservation Must Exist

There is also a deep conviction among some physicists of the day that there must be some unity in physics and that this connection implies some law of conservation.

The idea was already present with Descartes (conservation of mv ) and Leibniz (conservation of mv ²). For the living force mv ², several scholars of the time firmly believed that it should always be conserved, even if it was observed that it was conserved only in a collision with well-bouncing balls. To preserve the principle of conservation, it was said that the lost living force must be found in another form. In the case of collisions, it was suggested that the living force could be transformed in the deformation or in the internal energy of the object’s atoms (an idea already formulated by Gassendi in 1647 to justify the apparent loss of Descartes’ mv in collisions). Note that no one thought at the time that kinetic energy could be lost as heat in a collision.

The idea of a certain unity between physical phenomena is also present in a fairly popular philosophy in Germany at the beginning of the 19 th century (the Naturphilosophy ). It was this belief in certain connections between physical phenomena that had motivated Œrsted when he discovered the link between electricity and magnetism in 1820. At least 7 discoverers of the principle of conservation (Colding, Helmholtz, Liebig, Mayer, Mohr, Hirn and Séguin) have been influenced, to varying degrees, by this philosophy.

For several centuries, it was also believed that perpetual motion was impossible. According

to this principle, it is impossible get something out of nothing. It also involves a certain idea of transforming one form of energy into another and the existence of conversion factors. For example, Volta claimed in the early 19th century that the electric currents supplied by a battery come from simple contact between metals. Many physicists, including Faraday, argue that without chemical transformations, a current would be obtained out of thin air and that this could result in perpetual motion. Transformations in a loop were also an important clue to the existence of the principle of conservation of energy. We have transformation in a loop if the system returns to its initial state after a series of transformations. For example, there is such a loop when one battery is used to make a current and then this current is used to charge another battery. To prevent a perpetual motion, the charge received by the second battery should not be greater than that given by the first battery. This implies a certain equivalence between the current generated and the amount of chemicals transformed in the reaction.

How Energy Conservation Was Discovered

Beginning from very different starting points, the 12 discoverers will all come to the same conclusion. Some, like Colding and Mohr, start from this conviction that there must be a law of conservation and others (most of them) start from the study of a specific conversion process (in fact, they all start, except Mayer and Helmholtz, from the study of steam engines or electric motors). In all cases, their work then leads them to incorporate the results of the work of those who study other transformations, in order to arrive at an increasingly global vision of physics and, finally, to formulate the principle of conservation of energy.

This line of events can be illustrated by following the career of James Prescott Joule. It all began in 1838 when he was studying the efficiency of electric motors. Wanting to quantify this efficiency, he discovered the work of those who studied the efficiency of steam engines. When he became interested in the batteries that power these motors, he discovered Faraday’s work in chemistry. Joule then discovered in 1841 that part of the energy was lost as heat when the current passed through the wires of the electric motor. In wanting to quantify this link between heat and work, in 1842 he developed a famous experiment in which descending masses turn blades in the water. http://www.youtube.com/watch?v=bZbTZN6V7YI The stirring of the water caused by the motion of the blades then causes an increase in temperature. Joule can then measure the conversion factor between the work done by the descending masses and the heat generated. Having observed so many connections between different phenomena, Joule came to formulate his first version of the principle of conservation of energy in 1843.

One would have thought that the conservation of energy was discovered from the conservation of mechanical energy, but that was not really the case. The conservation of mechanical energy has actually played only a very minor role. Carnot made the link between work and kinetic energy quite early, but it was not until the 1820s before this link was clearly established in mechanics. Once this link had been made, we still have to wait

until the 1840s before two of the discoverers (Helmholtz and Mayer) mention it (by rediscovering it themselves). The discovery of the law of conservation of energy was made by studying transformations that did not really involve a change in kinetic energy by researchers primarily interested in steam engines and electric motors. The links that made it possible to discover the conservation of energy were made from the concept of work used for a century by engineers and not by generalizing the principle of conservation of mechanical energy.