The First Law of Thermodynamics


The First Law: the concepts

The basic concepts
A system is the part of the world in which we have a special interest. It may be a reaction vessel, an engine, an electrochemical cell, and so on. Around the system is its surroundings, where we make our observations. The two parts may be in contact and are separated by a boundary, and specifying the system and its surroundings amounts to careful specification of the boundary between them. When matter can transfer through the boundary between system and its surroundings the system is open; otherwise it is closed. An isolated system is closed system that has neither mechanical nor thermal contact with its surroundings.

  Work, heat, and energy
Work, heat, and energy are the basic concepts of thermodynamics, and of these the most fundamental is work. As we shall see, all measurements of heat and changes in energy amount to measurements of work.
Work is done during a process if that process could be used to move an object certain distance or to change its height.         W = F x d   = m g h   (J).
We shall say that work is done by the system   if a weight has been raised up in the surroundings and it is given a negative sign (-ve) and this work = - mgh (you may imagine your hand is the system which will raise certain weight). The work has been done on the system when a weight has been lowered down and it is given a positive sign (+ve) and this work = mgh. When we need to measure the amount of work we use its definition as force × distance.
Energy is the capacity of a system to do work. When the energy of a system changes as result of a temperature difference between it and its surroundings we say that energy has been transferred as heat. When a beaker of water (the system) stands on hot plate, the capacity of the system to do work increases, so its energy has increased; since the increase has occurred as result of a temperature difference, that energy has...