Thermodynamics is that branch of physics that studies the inter-relationships and interconversions of the various forms of energy. There are three laws of thermodynamics and all events in the physical world conform to and are determined by these laws, writes William Reville
The First Law states that energy can neither be created nor destroyed. Therefore when any physical change occurs, the total energy of the universe must remain the same. The familiar forms of energy are heat, light, mechanical, chemical (battery) and electrical.
Different forms of energy are interconvertible. For example, heat can be converted into mechanical energy in a steam engine; mechanical energy can be converted into electrical energy in a hydroelectric power station; electrical energy is converted into chemical energy when you charge up your mobile phone; green plants convert light energy into chemical energy. The First Law insists that when such interconversions occur the total energy of the universe remains constant.
If a salesperson offers you a machine that will do work without the necessity to power (add energy to) the machine, tell him/her to go away as the machine promises to violate the First Law of Thermodynamics.
Mechanical energy is expressed in units called ergs, heat and chemical energy in units called calories, and electrical energy in units called watts. The First Law implies that there is a quantitative correspondence between the different forms of energy.
The First Law tells us that in any physical change energy is conserved, but it says nothing about the direction in which spontaneous physical change proceeds. This is where the Second Law comes in and it basically tells us that spontaneous change in a closed system always proceeds in a direction that increases randomness or disorder. This is probably the most fundamental law in physics.
Consider the following situation. You have two blocks of copper, one at a higher and the other at a lower temperature. Place the two blocks in contact with each other and observe what happens. Heat flows from the hotter to the cooler block. The hotter block cools and the cooler block warms. Heat stops flowing when both blocks arrive at a temperature intermediate between the two initial temperatures - the equilibrium position.
Continue to observe the two copper blocks. They will both remain at the same intermediate temperature. You will never observe one of the blocks spontaneously warming up again at the expense of the other. You have observed the Second Law in action. The initial situation in which heat was unevenly distributed between the two blocks is more orderly than the final equilibrium situation where the heat is randomised between the two blocks. Having reached equilibrium radomisation the heat will never again spontaneously unrandomise itself. A spontaneous resegregation of the heat would not violate the First Law, because the total energy of the system would remain unchanged, but it would violate the Second Law.
The Second Law has profound consequences. The universe is a closed system and therefore is continuously increasing in disorder and is doomed to eventually run itself down into total disorder, often referred to as entropic doom.
Although the overall disorder in the universe must continually increase until it eventually reaches a maximum trillion of years in the future, this does not preclude local transient decreases occurring. Life is the ultimate demonstration of this. However, the complex order of life is bought at the expense of much energy and, life is transient, and its complex order eventually becomes randomised into the universe.
Consider a refrigerator. It cools its interior, thereby reducing entropy, but it can only do this by expending electrical energy which is used to remove heat from inside the refrigerator and dump it out into the room where it randomises.
Now consider a closed room containing a refrigerator that is switched off. Measure the entropy in the room. Now switch on the refrigerator and leave it run for several hours. As it cools its internal space, the entropy of that space decreases. The heat is dumped into the rest of the room where entropy increases. After several hours measure the total entropy of the room again. The entropy of the rest of the room has increased more than the entropy inside the fridge has decreased.
Temperature measures the degree of heat in a body. The absolute scale of temperature ranges from absolute zero upwards in degree units, each equal in size to a degree Celsius. At absolute zero, a body contains no heat and its constituent atoms are motionless.
The amount of effort (work) required to lower the temperature of a body by a given amount increases as temperature decreases. As you approach absolute zero the work required increases exponentially and the Third Law states that it is impossible to ever reach absolute zero temperature (which would require an infinite amount of work), although it can be approached very closely.
It is said that the novelist and physicist CP Snow devised three clever phrases to help one remember the three laws of thermodynamics:
First Law - You can't win.
Second Law - You can't even break even.
Third Law - You can't get out of the game.