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State second law of thermodynamice . How is heat engine different from a refrigerator ? Explain. |
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Answer» Solution :Second Law of Thermodynamics : First law of thermodyamics is based on "Law of conservation of energy ". While second law of thermodynamics gives" information about the transformation of heat energy". So , there are two conventional statements of second law depending on common experience . 1) Kelvin - Plank statement : It is impossible foran engine working in a cyclic PROCESS to extract heat from a hot body and to convert it completely inot work . 2) CLAUSIUS Statement: It is impossible for a self acting machine , unaided by any external agency to transfer heat from a cold body to a hot RESERVOIR .In other words , heat cannot by itself flow from a colder body to a hotter body . Heat engine : A heat engine is a device used to convert heat energy into mechanical work . Generally heat engines will work in a cyclic process . Heat engine consists of three important units . 1) Source : which is an object or SYSTEM at high temperature . A heat enginewill absorb heat energy`Q_(1)`from source . 2) Working substance : Every heat engine requires a working substance to do work . Generally the working substance is like steam or fuel vapour and air mixture etc . A part of heat energy of working substance is converted into mechanical work . 3) Sink : In evergy heat engine heat energy content of working substance is not converted into worktotally . So some energy`(Q_(2))` is wasted or rejected by the engine . This rejectedenergy`(Q_(2))` is delivered to some other body or system at low temperature . This body with low temperature is called sink . EFFICIENCY of heat engine , `eta=("Work done")/("Heat energy supplied")=(W)/(Q_(1))` (But `W=Q_(1)-Q_(2))` (or)`eta=(W)/(Q_(1))=(Q_(1)-Q_(2))/(Q_(1))rArreta=1-(Q_(2))/(Q_(1))` Where `Q_(1)` = heat energy supplied by source `Q_(2)` = heat energy delivered to sink Block diagram of heat engine is as shown .  Refrigerator : A refrigerator works in the reverse process of heat engine . It extracts heat energy`Q_(2)` from sink i.e , from low temperature body with the help pf external work and delivers heat energy`Q_(1)` to high temperature body called source . work done`W=Q_(1)-Q_(2)` . In refrigerators external work is done on working substance . A block diagram of refrigerator is as shown .  Difference between heat enginc and refrigerator : Refrigeraote extracts heat energy from sink i. e .,from low temperature body with the help of external work and delivers heat energy to high temperature body called source . A heat engine will absorb heat energy from source and reject heat energy to then sink . Heat engine will work in a reversible process but the refrigerator works in the reverse process of heat engine . |
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