Laws of Thermodynamics

The study of thermal energy and its transformations led to the formulation of various laws of thermodynamics. These are

• Zeroth Law
  If two objects are each separately in thermal equilibrium with a third object then they are also in thermal equilibrium with each other. The Zeroth Law is used everytime we use a thermometer.
• First Law
  The change in internal energy of an object is equal to the heat input minus the work done by the object. Thus the First Law of Thermodynamics is just our familiar Law of Conservation of Energy, which states that energy, of which heat is one form, cannot be created or destroyed but only transformed from one form to another. Heat energy if not converted into mechanical work, increases the objects internal potential energy. The conservation of energy disallows the existence of ``perpertual motion machines of the first kind": Machines that can function perpetually in a cyclical fashion without any input of energy. So, for example, you cannot run your car-engine without fuel. (If a car running on a smooth road has its engine turned off, it can still continue some distance until losses due to friction with the road and air-resistance consume its energy and it comes to a stop: Without friction or resistance the most the car can do without any energy input is to continue in a straight line.) However not all conversions which are allowed in principle by the First law are possible in practise! For example we know from experience that it is impossible to construct an engine that would work cyclically by only taking in heat and transforming it completely into mechanical work. If such a thing were possible we, in our equatorial land, would have been able to design such an engine to extract heat from the hot air around us, cooling our surroundings in the process, while using the converted energy to surf the Internet -- paradise! The impossibility mentioned above disallows ``perpetual machines of the second kind'', and is encoded in the Second Law of Thermodynamics, of which one version is :
• Kelvin's Statement of the Second Law
  It is impossible to construct a cyclical engine that solely transforms heat completely into work. Kelvin's statement can be shown to be equivalent to the following:
• Clausius Statement of the Second Law
  It is impossible for an engine to solely take heat from a colder reservoir and deliver it completely to a hotter reservoir. (That is, heat will not flow spontaneously from a cold body to a hot one).