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| 183_notes:energy_dissipation [2018/05/29 21:45] – hallstein | 183_notes:energy_dissipation [2021/05/31 16:43] – [Where does the energy go?] stumptyl |
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| ===== Dissipation of Energy ===== | ===== Dissipation of Energy ===== |
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| You have read that [[183_notes:energy_cons|energy is always conserved]]. This is a true observable fact of the universe. Energy cannot be created or destroyed, it simply changes forms. However, sometimes those forms are less useful to us. For example, the increased thermal energy of a box due to the frictional interaction with the surface it is pushed across is unlikely to be useful. This type of transformation of energy is often referred to as "dissipating" energy. In these notes, you will read about one form of energy dissipation that is due to collisions with air molecules: air resistance. | You have read that [[183_notes:energy_cons|energy is always conserved]]. This is a true observable fact of the universe. Energy cannot be created or destroyed, it simply changes forms. However, sometimes those forms are less useful to us. For example, the increased thermal energy of a box due to the frictional interaction with the surface it is pushed across is unlikely to be useful. This type of transformation of energy is often referred to as "dissipating" energy. **In these notes, you will read about one form of energy dissipation that is due to collisions with air molecules: air resistance. |
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| ==== Air Resistance ==== | ==== Air Resistance ==== |
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| $$ mg = cv_{terminal}^2 \rightarrow v_{terminal} = \sqrt{\dfrac{mg}{c}}$$ | $$ mg = cv_{terminal}^2 \rightarrow v_{terminal} = \sqrt{\dfrac{mg}{c}}$$ |
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| where $c$ is a constant that contains all the constants in the [[183_notes:drag|turbulent drag force formula]]. As a result the work done by the surroundings is equal to the change in gravitational potential energy. | where $c$ is a constant that contains all the constants in the [[183_notes:drag|turbulent drag force formula]]. As a result, the work done by the surroundings is equal to the change in gravitational potential energy. |
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| $$W_{surr} = \Delta U _{grav} $$ | $$W_{surr} = \Delta U _{grav} $$ |
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| What can do work on the coffee filter if the Earth is in the system? The air molecules that collide with the coffee filter as it falls down. These molecules are struck by the filter on the way down and have their kinetic energy increased. The energy of the Earth-filter system is dissipated by the air. The air molecules gain kinetic energy, which cannot really be used for anything useful. | //__What can do work on the coffee filter if the Earth is in the system? The air molecules collide with the coffee filter as it falls down. These molecules are struck by the filter on the way down and have their kinetic energy increased.__// The energy of the Earth-filter system is dissipated by the air. The air molecules gain kinetic energy, which cannot really be used for anything useful. |
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