iPad 20:59 21% 4 of 8 Edit Moments 4. (30 points Limit your responses for each p
ID: 1769200 • Letter: I
Question
iPad 20:59 21% 4 of 8 Edit Moments 4. (30 points Limit your responses for each part below to no more than one half of a page in your blue book. (a) For the two state paramagnet, explain why the entropy at high temperature approaches Nk ln 2. (b) What is negative temperature, and how does one create a negative temperature in the two state paramagnet? (c) In the Rankine cycle, why is it a good approximation to assume that entropy remains constant in the Turbine step? (d) What are the key differences between an engine based on the Otto Cycle and an engine based on the Diesel Cycle. (e) Describe the Carnot cycle and its significance. (f Why does a rubber band become warmer when it is stretched?
Explanation / Answer
b) In colloquial usage, "negative temperature" may refer to temperatures that are expressed as negative numbers on the more familiar Celsius or Fahrenheit scales, with values that are colder than the zero points of those scales but still warmer than absolute zero.
Negative temperatures can only exist in a system where there are a limited number of energy states (see below). As the temperature is increased on such a system, particles move into higher and higher energy states, and as the temperature increases, the number of particles in the lower energy states and in the higher energy states approaches equality. (This is a consequence of the definition of temperature in statistical mechanics for systems with limited states.) By injecting energy into these systems in the right fashion, it is possible to create a system in which there are more particles in the higher energy states than in the lower ones. The system can then be characterised as having a negative temperature.
c) Isentropic Expansion. The vapor is expanded in the turbine, thus producing work which may be converted to electricity. In practice, the expansion is limited by the temperature of the cooling medium and by the erosion of the turbine blades by liquid entrainment in the vapor stream as the process moves further into the two-phase region. Exit vapor qualities should be greater than 90%.
d)
introducing fuel into the engine
In most engines spark ignition (Otto cycle), air and fuel are introduced into the combustion chamber in the form of gaseous mixture. The mixture is carried into the carburetor, and the regulation of the quantity of mixture introduced is obtained by means of a butterfly valve.
In compression ignition engines (diesel cycle), air is introduced into the chamber through ducts combusted will suction valve while the fuel is introduced directly by means of an injector. The air-fuel mixture takes place in the combustion chamber, there is regulation of the amount of air, but only a regulation of the amount of fuel added.
On the fuel
The spark ignition engine (Otto cycle) requires an ignition system to generate in the combustion chamber a spark between the electrodes of a spark plug, in order that the combustion can be initiated.
The compression ignition engine (diesel cycle) uses high temperature and pressure obtained by compressing the air in the cylinder to begin combustion when the fuel is injected.
Compression ratio
The value of the compression ratio in spark ignition engines (Otto cycle) varies from 6-10, except in exceptional cases, while in compression ignition engines (cycle diesel) ranges between 14 and 22.
In the spark ignition engine (Otto cycle), the upper limit of the compression ratio is essentially determined by the fuel antiknock quality in the market for compression-ignition engines n (diesel cycle) is determined mainly by the weight of the engine structure, which increases with increasing compression ratio, of a special mode with large displacements.
Motor Weight
The compression ignition engine (diesel cycle) is usually heavier than a spark ignition engine (Otto cycle) of the same capacity, because it works considerably higher pressure.
e)
The Carnot cycle is a theoretical thermodynamic cycle proposed by Nicolas L
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