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Cyclic processes and their effeciency, Carnot cycle Presentation.pptx
How does a steam engine work?
Topic: Cyclic processes and their efficiency, Carnot cycle
Эта фотография, автор: Неизвестный автор, лицензия: CC BY-SA
Эта фотография, автор: Неизвестный автор, лицензия: CC BY-SA
Эта фотография, автор: Неизвестный автор, лицензия: CC BY
Lesson objectives:
Assessment criteria
What is Heat Engine?
The mechanics of a basic heat engine involve the following steps:
Эта фотография, автор: Неизвестный автор, лицензия: CC BY-SA
Efficiency
The efficiency of an engine is define as the ratio of net work performed to input heat; this is the fraction of heat energy converted to mechanical energy by the engine:
Example
A heat engine takes in 900 J of heat from a high temperature reservoir and produces 300 J of work in each cycle. What is its efficiency?
What can be the maximum efficiency for a heat engine?
An ideal engine, the most efficient theoretically possible, is called a Carnot Engine.
Its efficiency is given by the following formula, where the temperatures are, respectively, the temperature of the exhaust environment and the temperature of the heat input, in Kelvins.
Эта фотография, автор: Неизвестный автор, лицензия: CC BY-SA
Sadi Carnot -French military engineer and physicist
Carnot cycle
The Carnot cycle consists of two adiabatic and two isothermal processes.
2-3 and 4-1 are adiabatic. No heat is transferred.
1-2 – gas expands isothermally, absorbing heat QH from a hot reservoir of temperature TH.
3-4 – gas is compressed isothermally, releasing heat QC to a cold reservoir of temperature TC
Эта фотография, автор: Неизвестный автор, лицензия: CC BY-SA
No engine can be more efficient than a Carnot engine operating between the same temperatures.
Example
A heat engine operates at a temperature of 650K. The work output is used to drive a pile driver, which is a machine that picks things up and drops them. Heat is then exhausted into the atmosphere, which has a temperature of 300K. What is the ideal efficiency of this engine?
Mark Scheme Carnot Cycle
Parts | Isoprocesses | Q | ∆T | Description |
a - b | Isothermal expansion | W | 0 | The gas is first expanded isothermally, with the addition of heat QH, along the path “ab” at temperature TL. |
b - c | Adiabatic expansion | 0 | - | Next, the gas expands adiabatically from “b” to “c” – no heat is exchanged, but the temperature drops to TL. |
c - d | Isothermal compression | -W | 0 | The gas is compressed at constant temperature TL, path “cd”, and heat QL flows out. |
d - a | Adiabatic compression | 0 | + | Finally, the gas is compressed adiabatically, path “da”, back to its original state. |
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