U7EnergyNotes - UNIT VII - ENERGY (WITH LESS WORK) Overview...

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(WITH LESS WORK) Overview The traditional approach to work and energy in a standard physics course often ends up being a rather imprecise, confusing mass of equations and definitions, such as "energy is the ability to do work" and W = Φ ξ χοσ θ . For example, the work-energy theorem is a point-particle model that is often inappropriately applied to situations that require the consideration of internal structure, and the 1st Law of Thermodynamics is rarely used to analyze mechanical systems, despite its universal applicability. This unit focuses on energy , defined as a conserved, substance-like quantity with the capability to produce change. Work is de-emphasized, and is more accurately called "working", indicating the nature of "work" as a process of transferring energy into or out of a system via external forces. The 1st Law of Thermodynamics is used as the primary means of analysis of mechanical systems because of its fundamental, universal nature. All energy interactions can be characterized as energy transfer mechanisms or energy storage modes , depending on how the system is defined. Energy storage modes are kinetic, potential and internal energies, designated as ∆E with corresponding subscripts (∆E k + ∆E el + ∆E g + ∆E int +∆E chem = ∆E). Energy transfer mechanisms are working (W), heating (Q), and radiating (R) . As awkward as it may be at first for the physics teacher to refer to W as “working”, the gerund is deliberately chosen to emphasize the process of energy transfer. The relationship between energy storage and transfer is shown by the 1st Law of Thermodynamics, ∆E= W (+ Q + R). This is shown by the system schema below: ÆE Q W R It shows that energy transferring into and out of the system affects the nature of the energy storage in the system. The 1st Law of Thermodynamics and the Law of Conservation of Energy state that the algebraic sum of these energy changes and transfers must add up to zero, accounting for all changes relative to the system. This crucial concept is incorporated into the pie chart and bar graph representational tools used in this unit. The power of using the 1st Law of Thermodynamics for analysis is that it makes it possible to take into account the internal structure of the system, since energy dissipated by frictional forces (E int ) can be accounted for as energy stored internally in the kinetic and interaction energies of the particles that make up the objects in the system. In its expanded form, the 1st Law of Thermodynamics is W + Q + R = ∆E, where ∆E = ∆E k + ∆E g + ∆E el + ∆E chem + ∆E int So for mechanics (in this unit), neglecting Q and R) W = ∆E k + ∆E g + ∆E el + ∆E chem +∆E int Notice that when the internal structure of the system can be ignored, the work-energy theorem appears naturally, from the 1st Law: ∆E k = W assuming no other storage modes are involved. (∆E
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This note was uploaded on 12/22/2011 for the course PHYSICS 204 taught by Professor Dr.lairdkramer during the Fall '11 term at FIU.

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U7EnergyNotes - UNIT VII - ENERGY (WITH LESS WORK) Overview...

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