SEM314 Topic 2 - 1st Law of Thermodynamics

SEM314 Topic 2 - 1st Law of Thermodynamics - Topic 2 Energy...

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Deakin University 2003 Topic 2 Energy conservation: The First Law of Thermodynamics Throughout this topic you will need to refer to sections 1.6–1.8 in your textbook. Conservation of energy The First Law of Thermodynamics simply introduces the concept of ‘conservation of energy’. In other words, the (amount of) energy cannot be created or destroyed , that is, energy never dies, it just changes form. This law cannot be proved mathematically. It is based on experimental observations and nothing has yet been found that violates this law. A nuclear reaction in which mass is converted into energy is not considered a violation of the law; rather mass is treated as yet another form of energy. In its basic form, the First Law of Thermodynamics can be stated as follows: When a system undergoes a thermodynamic cycle then the net heat supplied to the system from its surroundings is equal to the net work done by the system on its surroundings, or, mathematically, Σ dQ = Σ dW (2.1) where Σ represents the sum for a complete cycle. Equation 2.1 is NOT meant to imply that if a certain amount of work is done on a system, then it is all converted into heat or, conversely, if a certain amount of heat is supplied to a system, then it is all converted into work. Equation 2.1 simply means that if some work is converted into heat or some heat is converted into work then the relationship between the heat and work so converted will be of the form W = Q. Actually, it is possible to convert work completely into heat by friction, for example. The reverse process of converting heat completely into work (through a cycle) is impossible and the reason will be explained in topic 5. Now please read through section 1.6 and example 1.3 in your textbook. ±
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2-2 Principles of Thermodynamics Deakin University 2003 The non-flow energy equation In a non-flow process, an extension of equation 2.1 may be used: W + Q = Δ U (2.2) This equation is known as the closed system energy equation . This equation takes into account that not all the energy needs to be of a heat– work or work–heat conversion form. Some energy may be concerned with an internal energy change, Δ U, as equation 2.2 shows. Now read section 1.7 in your textbook and make sure you understand the two examples given (examples 1.4 and 1.5). Here, I will expand the discussion of the First Law of Thermodynamics and give you some more solved examples. Note: Equation 2.1 is talking about the net heat and work involved in a complete cycle, that is, the system goes back to its original state, while equation 2.2 gives the relationship between heat work and internal energy change in a non-flow process (reversible or irreversible). To facilitate understanding of the first law, consider the following analogy.
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SEM314 Topic 2 - 1st Law of Thermodynamics - Topic 2 Energy...

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