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183_notes:heat [2014/10/28 19:57] – created caballero | 183_notes:heat [2021/04/15 17:09] – [Q can be positive or negative] stumptyl | ||
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- | ===== Heat: Energy Transfer due to a Temperature Difference ===== | + | Section 7.5 in Matter and Interactions (4th edition) |
+ | ===== Heat Exchange: Energy Transfer due to a Temperature Difference ===== | ||
+ | Earlier, you read about [[183_notes: | ||
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+ | ==== Lecture Video ==== | ||
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+ | {{youtube> | ||
+ | ==== Two Blocks in Contact ==== | ||
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+ | [{{ 183_notes: | ||
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+ | Two blocks are placed in contact (figure to right). One block is hot (red) and the other is cold (blue). Your experience tells you that in time, the hot block will become cooler and the cold block will become warmer. Eventually, the two blocks will [[183_notes: | ||
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+ | How does this happen? | ||
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+ | The average kinetic energy of atoms in the high-temperature block is higher than the low-temperature block. Some of this [[183_notes: | ||
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+ | At the interface, faster moving atoms in the high-temperature block collide with slower-moving atoms in the low-temperature block transferring energy across the boundary. These atoms collide with their neighbors within their own block and the energy is propagated across the rest of the block. You can think about this as work is done by motion of atoms, but it's not [[183_notes: | ||
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+ | Because each atom has a different kinetic energy, it is possible that a fast-moving atom from the low-temperature block will transfer kinetic energy to a slow-moving atom in the high-temperature block. However, this is less likely than the reverse because there are more atoms on average with high kinetic energies in the high-temperature block. | ||
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+ | The net result is an energy transfer across the boundary of the two blocks from the hot block to the cold block. This is also referred to as the //heat exchanged//, | ||
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+ | === The Energy Principle === | ||
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+ | You now have a complete description of the energy principle. **The change in energy of a system is due both to macroscopic and microscopic work.** | ||
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+ | $$\mathrm{Energy\: | ||
+ | $$\Delta E_{sys} = W_{surr} + Q $$ | ||
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+ | ==== Q Can Be Positive(+) or Negative(-) ==== | ||
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+ | The microscopic work ($Q$) can be positive or negative, just as [[183_notes: | ||
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+ | For now, consider a system that just exchanges energy through $Q$ (i.e., there is no macroscopic work done). | ||
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+ | $$\Delta E_{sys} = Q$$ | ||
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+ | If the system under consideration is warmer than its surroundings, | ||
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+ | If, instead, the system under consideration is cooler than its surroundings, | ||
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+ | {{ 183_notes: | ||
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+ | ==== Examples ==== | ||
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+ | * [[183_notes: | ||
+ | * [[: |