Kelvin Planck statement : No process is possible whose sole result is the absorption of heat from a heat reservoir, and the complete conversion of the heat (thus absorbed) into work.
Clausius statement : No process is possible whose sole result is the transfer of heat from a colder object to a hotter object.
This means that some work will be required to be done in the process. Some rejection of heat is also possible.
The second law implies that the efficiency of a heat engine is less than 1. There can be no perfect heat engine, no perfect heat pump or perfect refrigerator.
A reversible heat engine with multiple cycles on an ideal gas as the system is called Carnot machine. It operates between two temperatures.
efficiency of use of energy = η = W / Q₁ = 1 - T₂ / T₁ < 1
W = work done, Q₁ = heat absorbed by system at temperature T₁
Q₂ = Q₁ - W = heat rejected / released into reservoir at temperature T₂
efficiency in using energy = η = W / Q₁ = [ 1 - Q₂ / Q₁ ],
η = < 1
A refrigerator is like a heat pump. It extracts Q₂ from cold substances at temperature T₂, and work W is done on the system - refrigerant liquid - gas.
Q₁ = W + Q₂ = heat released to the heat reservoir at outside air temperature T₁
Coefficient of performance of refrigerator = α = Q₂ / W = Q₂ / (Q₁ - Q₂)
α = T₂ / ( T₁ - T₂ )
The second law states that , α < ∞.
To understand why entropy increases and decreases, it is important to recognize that two changes in entropy have to considered at all times. The entropy change of the surroundings and the entropy change of the system itself. Given the entropy change of the universe is equivalent to the sums of the changes in entropy of the system and surroundings:ΔSuniv=ΔSsys+ΔSsurr=qsysT+qsurrT(1.1)(1.1)ΔSuniv=ΔSsys+ΔSsurr=qsysT+qsurrT
In an isothermal reversible expansion, the heat q absorbed by the system from the surroundings isqrev=nRTlnV2V1(1.2)(1.2)qrev=nRTlnV2V1
Since the heat absorbed by the system is the amount lost by the surroundings, qsys=−qsurrqsys=−qsurr.Therefore, for a truly reversible process, the entropy change isΔSuniv=nRTlnV2V1T+−nRTlnV2V1T=0(1.3)(1.3)ΔSuniv=nRTlnV2V1T+−nRTlnV2V1T=0
If the process is irreversible however, the entropy change isΔSuniv=nRTlnV2V1T>0(1.4)(1.4)ΔSuniv=nRTlnV2V1T>0
If we put the two equations for ΔSunivΔSunivtogether for both types of processes, we are left with the second law of thermodynamics,ΔSuniv=ΔSsys+ΔSsurr≥0(1.5)(1.5)ΔSuniv=ΔSsys+ΔSsurr≥0
where ΔSunivΔSuniv equals zero for a truly reversible process and is greater than zero for an irreversible process. In reality, however, truly reversible processes never happen (or will take an infinitely long time to happen), so it is safe to say all thermodynamic processes we encounter everyday are irreversible in the direction they occur.
apply the formula weight =force x speed
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