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===== Defining A System ===== | ===== Defining A System ===== | ||
- | Choosing a system is one of the most important choices you make (either explicitly or implicitly) when solving | + | |
+ | Choosing a system is one of the most important choices you make (either explicitly or implicitly) when investigating | ||
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+ | ==== Lecture Video ==== | ||
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+ | {{youtube> | ||
==== The System ==== | ==== The System ==== | ||
- | The system is the object or the collection of objects that you would like to describe. 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 | ||
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+ | 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: | ||
When we refer to a quantity as " | When we refer to a quantity as " | ||
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+ | [{{ 184_notes: | ||
==== The Surroundings ==== | ==== The Surroundings ==== | ||
- | The surroundings contain everything that you are not interested in describing. Objects that are in the surroundings can certainly influence or interact with the system, but generally, you are not interested in describing the impact of the system on the surroundings. For example, if you drop a ball, you are typically very interested in describing how the ball's motion changes: it speeds up, gets closer to floor, it experiences a gravitational force from the Earth, etc. However, you do not talk about how the Earth' | ||
- | When we refer to a quantity as " | + | The surroundings contain everything that you are not interested in describing. Objects that are in the surroundings can certainly influence or interact with the system, but generally, you are not interested in describing the impact of the system on the surroundings. |
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+ | For example, if you drop a ball, you are typically very interested in describing how the ball's motion changes: it speeds up, gets closer to floor, it experiences a gravitational force from the Earth, etc. However, you do not talk about how the Earth' | ||
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+ | When we refer to a quantity as " | ||
==== Implications of Defining a System ==== | ==== Implications of Defining a System ==== | ||
- | Choosing your system has particularly | + | |
+ | Choosing your system has particularly | ||
* If you choose your system so that there are no external forces on the system, then momentum is conserved for that system. (This is particularly relevant for [[183_notes: | * If you choose your system so that there are no external forces on the system, then momentum is conserved for that system. (This is particularly relevant for [[183_notes: | ||
- | * If you choose your system so that there is no external work or heat transferred, | + | * If you choose your system so that there is no external work or heat transferred, |
- | * If you choose your system so that there is no external torque on the system, then angular momentum is conserved for that system. | + | * If you choose your system so that there is no external torque on the system, then angular momentum is conserved for that system. |