![]() ![]() Secondly, the equation ( ii) defining entropy change does not recognise that the system has to be at equilibrium for it to be valid. First the units of entropy are Joules per Kelvin but the degree of disorder has no units. This more modern approach has two disadvantages. However, it is more common today to find entropy explained in terms of the degree of disorder in the system and to define the entropy change, Δ S, as: Where Q is the quantity of heat and T the thermodynamic temperature. Many earlier textbooks took the approach of defining a change in entropy, Δ S, via the equation: Generations of students struggled with Carnot's cycle and various types of expansion of ideal and real gases, and never really understood why they were doing so. The concept of entropy emerged from the mid-19th century discussion of the efficiency of heat engines. Entropy is a thermodynamic property, like temperature, pressure and volume but, unlike them, it can not easily be visualised. Thermodynamic properties depend on the current state of the system but not on its previous history and are either extensive - their values depend on the amount of substance comprising the system, eg volume - or intensive - their values are independent of the amount of substance making up the system, eg temperature and pressure. ![]() Thermodynamics deals with the relation between that small part of the universe in which we are interested - the system - and the rest of the universe - the surroundings. Entropy is dynamic - something which static scenes don't reflect ![]()
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