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Shantanu Nair

on 6 September 2012

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Transcript of Thermodynamics

THERMODYNAMICS Two Greek words What does THERMODYNAMICS mean?? therme meaning heat
dynamis, meaning power ZEROTH LAW LAWS OF THERMODYNAMICS If two systems are in thermal equilibrium with
a third system, they are also in thermal equilibrium with each other This Law is the basis of Temperature Measurement Temperature determines the 'hotness' of a body.
It determines direction of flow of heat when 2 bodies are place in thermal contact.Heat flows from the body with higher temperature , to the body with lower temperature. The flow stops when the temperatures equalise. The Energy of the Universe is CONSTANT. First law of Thermodynamics E represents the change in the systems internal energy,Q represents heat supplied to the system, and W the work done by the system on the surroundings. E = Q - W Specific Heat Capacity Energy required to raise the temperature of one gram of a substance by one degree Celsius. S = Q / T Second law of Thermodynamics Kelvin-Planck Statement Clausius Statement No process is possible whose sole result is the
absorption of heat from a reservoir and the
complete conversion of heat into work. No process is possible whose sole result is the transfer of heat from a colder object to a hotter object. Efficiency of a heat engine can never be unity. HEAT ENGINES Basic heat engine consists of a gas confined by a piston in a cylinder. If the gas is heated, it expands, moving the piston. This wouldn't be a particularly practical engine, though, because once the gas reaches equilibrium the motion would stop. A practical engine goes through cycles; the piston has to move back and forth. Once the gas is heated, moving the piston up, it can be cooled and the piston will move back down. A cycle of heating and cooling will move the piston up and down. In a full cycle of a heat engine, three things happen:
Heat is added. This is at a relatively high temperature, so the heat can be called QH.
Some of the energy from that input heat is used to perform work (W).
The rest of the heat is removed at a relatively cold temperature (QC).
Applying conservation of energy : QH = W + QC Refrigerators A device such as a refrigerator or air conditioner, designed to remove heat from a cold region and transfer it to a hot region, is essentially a heat engine operating in reverse. Step 1 - The fluid passes through a nozzle and expands into a low-pressure area. Similar to the way carbon dioxide comes out of a fire extinguisher and cools down, the fluid turns into a gas and cools down. This is essentially an adiabatic expansion.
Step 2 - The cool gas is in thermal contact with the inner compartment of the fridge; it heats up as heat is transferred to it from the fridge. This takes place at constant pressure, so it's an isobaric expansion.
Step 3 - The gas is transferred to a compressor, which does most of the work in this process. The gas is compressed adiabatically, heating it and turning it back to a liquid.
Step 4 - The hot liquid passes through coils on the outside of the fridge, and heat is transferred to the room. This is an isobaric compression process. A refrigerator is rated by something known as the coefficient of performance, which is the ratio of the heat removed from the fridge to the work required to remove it:
Coefficient of performance = QC/W By Shantanu Nair CLASS - XI F THE END !
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