# L02 - Chapter 2 Energy Principles Motivation 2.1,2 Energy...

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Chapter 2: Energy Principles – Motivation • Energy is a physical quantity defined as the ability to do work – a measure of the exchange of energy between interacting objects • Albeit conceptually the same, energy and work can be specified in different manners, depending on the context: mechanical , electrical , thermodynamic, nuclear , etc. with their own formalism and preferred units: Ex : energy units: Joule (J), kilowatt-hour (kWh), British thermal unit (Btu), electronvolt (eV). .. • In this chapter, we’ll learn about how we can quantify energy with an emphasis on mechanical energy , since typically the conversion of energy involves motion (later 2.1,2 on, we’ll refer to other forms of energy as well) • In order to apply these principles to the energy issues in the society, it is also important to understand how we can describe the rate of exchange of energy (power) and the amount of useful energy in conversion processes (efficiency) • Physics explains the mechanisms of the nature using scientific theories presented in the form of mathematical models : always perfectible tools consistent with our thinking patterns • Consequently, we have to introduce some physical theories and try to employ the respective concepts in constructing models for how to deal with energy issues

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Rates: a useful mathematical tool • In Physics we are especially interested in studying the change in certain physical quantities, such as the position of a moving object or the amount of substance • However, note that the change itself is irrelevant if it is not referred to a certain frame, such as the time elapsed: 2.3 Def : The change c in a certain quantity divided by the finite time t necessary for the respective change is called average rate : r c t = • The average rate can be seen as a modeling tool to make (sometimes wrong) predictions about the future value of a certain quantity based on the history of the respective quantity • Obviously, rates may change throughout certain time intervals, and the level of usefulness of an average rate calculated over the respective interval depends on the level of accuracy of the estimation • The relationship can be used for instance to calculate the amount of change when the rate is known (and constant) or the time necessary for a certain change. However, pay attention to how you handle units…
2.3 Ex 2 : Due to the sometimes unpredictable social influences when dealing with energy issues, Rates: Examples Ex 1 : A window looses heat at a rate r = 500 Btu/day. Say that the house has 5 windows and the lost heat is replenished by burning natural gas which releases E g = 1000 Btu/cubic feet of gas volume (cfg). The price rate of gas is p = \$2.3/100 cfg. How much is to be paid monthly (30 days) for the heat lost through the windows? Energy lost per month is:

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## This note was uploaded on 03/19/2012 for the course PHYSICS phy 121 taught by Professor Mario during the Fall '11 term at Miami University.

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L02 - Chapter 2 Energy Principles Motivation 2.1,2 Energy...

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