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Description: The stored energy of a system, E, is analogous to the
total amount of water in a lake. Part of the water (stored energy) may be frozen in winter forming an ice layer (mechanical
energy), but no matter how cold it gets, the lake is never completely frozen and there is always liquid water (internal
energy) in the lake. The exact amount of water in the lake is unknown because the depth is difficult to measure. Likewise, the
exact amount of stored energy in a system is almost impossible to tell. A change in water level, however, can be used
to determine the change in the total amount of water in the lake. Likewise, a change in E is measurable through change in other properties.
The animation shows different ways in which water is added and removed. Just as water can be transferred to or from the lake through the pipes (or streams in the case of a real lake), energy can be transported whenever there is a bulk flow. The rain and evaporation (water in transit), on the other hand, are
analogous to heat and work transfer, which are energies in transit. Once water is transferred through pipe, rain, or condensation (reverse evaporation), it loses its identity. Energy transferred by mass, heat, or work also meet a similar fate as they become part of the stored energy. Looking at the lake water, it is impossible to tell where it is from (rain, stream, or condensations). Similarly,
the stored energy in a system is a property and does not care whether heat, work, or mass transfer is responsible
for its current value. Even the sign convention for heat and work can be remembered from this analogy. The positive values of
heat and work match the sense of mass addition by rain and mass loss by evaporation. |
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