This chapter illustrates how to apply calculus of variations ideas to disparate branches of science and engineering. Electrical engineers typically use current and voltage to describe circuits. Chemists use temperature, pressure, entropy, and volume when describing chemical reactions. Engineers and scientists in each discipline have their own favorite quantities. However, energy conversion is a common topic of study. Calculus of variations provides a unifying language. Scientists and engineers typically specialize, becoming experts in a particular area. However, open questions are more often found at the boundary between disciplines, where there is less expertise. Comparing ideas between different disciplines is useful because ideas from one discipline may answer questions in another, and challenges in one discipline may pose interesting research questions in another.
By studying the mass spring system of Sec. 11.4, the resulting equation of motion was Newton's second law. By studying the capacitor inductor system of Sec. 11.5, the resulting equation of motion was Kirchoff's voltage law. In this chapter we identify the equation of motion for multiple other systems. Through this procedure, we encounter some of the most fundamental laws of physics including including Gauss's laws, conservation of momentum, conservation of angular momentum, and the second law of thermodynamics.
The discussion in this chapter is necessarily limited. Entire texts have been written about each energy conversion processes discussed. Additionally, the idea of applying calculus of variations to these energy conversion processes is not novel. Other authors have compared electrical, mechanical, and other types of energy conversion processes too  .
Some rather drastic assumptions are made in this chapter. We assume energy is converted between one form and another with no other energy conversion process occurring. For example in a mass spring system, energy is converted between kinetic energy and spring potential energy while ignoring heating due to friction, energy conversion due to gravitational potential energy, and so on that might occur in a real system.