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| 184_notes:defining_a_system [2017/06/13 22:01] – dmcpadden | 184_notes:defining_a_system [2020/08/24 19:30] (current) – dmcpadden |
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| ~~NOTOC~~ | ~~NOTOC~~ |
| | /*[[184_notes:three_principles|Next Page: 3 Fundamental Principles of Mechanics]] |
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| | [[184_notes:math_review|Previous Page: Math Review]]*/ |
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| ===== Defining A System ===== | ===== Defining A System ===== |
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| Choosing a system is one of the most important choices you make (either explicitly or implicitly) when investigating a physical situation. What we define as our physical system of interest tells us what we intend to focus on, which [[https://en.wikipedia.org/wiki/Conservation_law|quantities the we might expect to be conserved]], and which ones we expect to be influenced by external agents. For solving physics problems, what you define as "inside" and "outside" the system will influence the physical quantities that you are solving for, how you set up your equations, and how you model the situation. While you are free to define your system as you wish, there are often certain choices of system(s) that will make your calculations easier or more efficiently use the information you are given. In fact, [[183_notes:system_choice|some choices of system will make certain situations very difficult to model or some problems impossible to solve]]. Being explicit about your choice of system and how that influenced your choices and resulting solution also ensures that everyone understands what work you've done. | Choosing a system is one of the most important choices you make (either explicitly or implicitly) when investigating a physical situation. What we define as our physical system of interest tells us what we intend to focus on, which [[https://en.wikipedia.org/wiki/Conservation_law|quantities the we might expect to be conserved]], and which ones we expect to be influenced by external agents. For solving physics problems, what you define as "inside" and "outside" the system will influence the physical quantities that you are solving for, how you set up your equations, and how you model the situation. While you are free to define your system as you wish, there are often certain choices of system(s) that will make your calculations easier or more efficiently use the information you are given. In fact, [[183_notes:system_choice|some choices of system will make certain situations very difficult to model or some problems impossible to solve]]. Being explicit about your choice of system and how that influenced your choices and resulting solution also ensures that everyone understands what work you've done. |
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| | ==== Lecture Video ==== |
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| | {{youtube>SdATPEWh2B8?large}} |
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| ==== The System ==== | ==== The System ==== |
| The system is the object or the collection of objects that you are interested in (usually to describe its motion, explain why it follows some path, etc.). A system has various physical quantities associated with it: mass, energy, momentum, angular momentum, and entropy. These quantities can change for a system by interacting with the surroundings. | The system is the object or the collection of objects that you are interested in (usually to describe its motion, explain why it follows some path, etc.). A system has various physical quantities associated with it: mass, energy, momentum, angular momentum, and entropy. These quantities can change for a system by interacting with the surroundings. |
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| In mechanics, you often picked the object that is in motion as your system because you are primarily interested in describing that motion or how that motion is changing. Your job in mechanics was to predict or explain the motion of objects. In E&M, the choice of system can be a bit more nuanced. You are still interested in choosing a system that helps you predict or explain something about the system, but what that something is can be motion, energy, or something altogether new - like [[184_notes:pc_efield|a field]] or [[184_notes:circuits|circuit]]. | In mechanics, it was common to pick the object that is in motion as your system because you are primarily interested in describing that motion or how that motion is changing. Your job in mechanics was to predict or explain the motion of objects. In E&M, the choice of system can be a bit more nuanced. You are still interested in choosing a system that helps you predict or explain something about the system, but what that something is can be motion, energy, or something altogether new - like [[184_notes:pc_efield|a field]] or circuit. |
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| When we refer to a quantity as "internal," we are referring to something that is //inside// the system we chose. | When we refer to a quantity as "internal," we are referring to something that is //inside// the system we chose. |
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| {{ 184_notes:syssurr.png?250}} | [{{ 184_notes:syssurr.png?250|This figure shows a system and its physical quantities (charge (q), mass, momentum (p), etc.) as well as the ways the surroundings can influence the system (force, heat, torque, etc.).}}] |
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| ==== The Surroundings ==== | ==== The Surroundings ==== |