 The
Open Door Web Site |
||||||||||||||
|
Thermal
Physics The Carnot Cycle The sequence below (called the Carnot cycle) is not supposed to describe a practical engine. Carnot was attempting to find the theoretical maximum for the efficiency of a heat engine. Consider a quantity of gas in a cylinder having a piston which can move without friction. The walls of the cylinder are perfect insulators except at the base.
1. Gas at temperature TH 2. Gas at temperature TH placed in contact with heat source at temperature TH. 3. Isothermal expansion at TH (slow expansion). Work done by the gas. 4. Gas removed from source and insulated. 5. Adiabatic expansion until temperature reaches TC. Work done by the gas. 6. Gas at temperature TC placed in contact with heat sink at temperature TC. 7. Isothermal compression at TC (slow compression). Work done on the gas. 8. Adiabatic compression until temperature reaches TH.
Work
done on the gas. back to 1…and so on, and so on… The expansions are done at high temperature and the compressions are done at lower temperature. Therefore, the work done by the gas is greater than the work done on the gas. The net effect of the cycle is that a quantity of work, w (= QH – QC) has been done and a quantity of thermal energy (QC) has been transferred from a hot body (the source) to a cold body (the sink). The cycle is reversible. This means that if the sequence is followed in the reverse order the net result will be that an external agent does a quantity of work, w, transferring a quantity of internal energy, Q, from the sink to the source. It was stated that there was no friction and that the internal energy transfers took place under conditions very close to thermal equilibrium (this is because thy took place very slowly, in a perfectly conducting container). If there is friction and/or if the energy transfers take place far from equilibrium (rapid compression and expansion) the cycle will not be reversible (in the sense defined above). After considering this cycle of operations Carnot proposed the following theorem, now called Carnot’s theorem.
|
© David Hoult 2009 |
||||||||||||
