Reversible and Irreversibleprocesses&2nd law of thermodynamicsPresented to:Ma’m Bushra ShafaqPresented by:Muhammad Tayyab Bukhari 30Muhammad Mohsin Rasool 41Muhammad Ibraheem 28Nausherwan Haider 082
Reversible and irreversible processReversible processThe system undergoing the reversible process can be returnedto its initial conditions along the same path.i. Reversible processes are equilibrium processes.ii. The system always remains in thermodynamic equilibrium.iii. In the system, no change of state would take place.iv. Heat would not flow into or out of a system.v. The system would not do work against its surroundings.3
Ocean:-The water evaporated from ocean move towards hillyareas and falls in the form of hail, snow or rain. Thiswater ultimately come back to ocean4
Rubbing of hands:Your left and righthands are normally at thesame temperature. Rubbingyour hands together to warmthem increase thetemperature for a while andafter some time theirtemperature will retain itsposition.5
Irreversible process:A process that can’t be retraced.Examples:i. Dropping of stone from certain height.ii. Cooling down of coffee.iii. Ink drop in water.8
Dropping of stone: Cooled coffee:A stone dropped fromsome height can’t go to thatposition until an externalforce act on it.A cup of coffee lefton your desk graduallycools down. It never getshotter all by itself.9
Ink drop in Water:The molecules of ink in a glasseventually spread uniformly through out thevolume of water. They never regroup into adrop shaped clump of their own accord.10
2nd law of thermodynamicsLimitations of 1st Law of Thermodynamicsi. Important distinction b/w 1st and 2nd is that it is impossibleto convert internal energy completely into work done.ii. It does not make distinctions between the processes thatoccur spontaneously and non spontaneously.iii. It does not tell us the direction of flow of heat.11
Definitions:Clausius saying,Heat can never pass from acolder to a warmer body withoutsome other change, connectedtherewith, occurring at the sametimeKelvin Statement:It is impossible to takeheat from a single source and istotally changed into work done12
Heat Engine:Any device that converts heat partially intomechanical energy or work is called Heat EngineHeat engine absorb heat fromhot reservoir and uses partiallyto do mechanical work W andthe remaining heat is rejectedas Qc to the cold reservoir13Two reservoirs are necessary forthe flow of heat. (Ocean and atmosphere)
14From 1st lawQh - Qc = ∆U + W∆U = 0 (engine undergoes cyclic process)Qh - Qc = WEfficiency of engine:Ratio of work done by engine to the energy gained bythe engine.
15For 100% efficiencyQc = 0It is impossible thatQh =WSo Kelvin plank’s statement becomes true thatIt is impossible to construct a heat engine that,operating in a cycle, produces no effect other than theabsorption of energy from a reservoir and the performanceof an equal amount of work.
16Refrigerator & Heat Pumps:Refrigerator and heatspumps work on the reverseprinciple of heat engines i.e.they absorb energy Qc fromcolder region and expel anamount of energy Qh to hotterregion. But this is onlyaccomplished by the help ofsome work that is to be done onthe engine to transfer this heat.
17From the first law, we know that the energy givenup to the hot reservoir must equal the sum of the workdone and the energy absorbed from the cold reservoir.Qh - Qc = ∆U + W∆U = 0(engine undergoes cyclic process)Qh - Qc = WQh = Qc +WIt is desirable to construct anengine in which work has not to be doneto transfer energy from cold to hot body.
18The existence of such machine will bethe violation of the 2nd law ofthermodynamics.And Clausius’ statement becomes true thatEnergy does not flow spontaneously froma cold object to a hot object.