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weng cheong

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In summary, the change in entropy of the system + surroundings is zero if the process is reversible. This has to do with the way entropy is defined.

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weng cheong

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Andrew Mason

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An isothermal heat flow (infinitessimal temperature difference maintained) cannot be achieved in practice. It is a theoretical limit: a thermodynamic process taking place in a state that is arbitrarily close to equilibrium.weng cheong said:

AM

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weng cheong

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d Ssurr= Qsurr/Tsurr

= Qreversible/Tsurr

so what i don't understand is that, why it equates Qsurr to Qreversible, in which the transfer of heat is impossible to be reversible? and i can't figure out why it relates this to the fact that the surrounding is huge.

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Andrew Mason

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The reversible heat flow, Qweng cheong said:but in my textbook, in defining the change in entropy of surrounding,

d Ssurr= Qsurr/Tsurr

= Qreversible/Tsurrso what i don't understand is that, why it equates Qsurr to Qreversible, in which the transfer of heat is impossible to be reversible? and i can't figure out why it relates this to the fact that the surrounding is huge.

I am not sure I understand your question. In your question you say that Qsurr = Qrev. This is not correct. To calculate the change in entropy of the surroundings, you must use the heat flow in the reversible process between the initial and final states of the surroundings, not the actual heat flow.

But this does not mean that in calculating the change in entropy for a non-reversible process that you will get a 0 change in entropy using the reversible heat flows. You have to calculate the reversible heat flows of the system and of the surroundings

For example, to determine the change in entropy of the universe in an adiabatic free expansion of a gas into a vacuum, you do as follows:

1. calculate the change in entropy of the surroundings. No change in P,V, or T so dS = 0.

2. calculate change in entropy of the gas: change in P and V but no change in T. The reversible process between the initial and final states is a quasi-static isothermal expansion. Since such an expansion does work but results in no change of internal energy (constant T) there must be heat flow into the gas: dQ = 0 + dW = PdV. So in calculating the change in entropy in the reversible process, there is positive heat flow so there is an increase in entropy:

[tex]\Delta S = \int dQ_{rev}/T = \int PdV/T = \int nRdV/V = nR\ln\frac{V_f}{V_i}[/tex]

So, although in this process there is no heat flow at all, there is an increase in entropy since the reversible path between the beginning and end states involves a positive flow of heat.

AM

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Studiot

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I have to say Andrew Mason is good at this (the tricky bits).

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weng cheong

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i think now i have a better understanding. Thank you =)

Infinitesimal temperature difference refers to an extremely small temperature difference between two objects or systems. It is typically measured in fractions of a degree and is crucial in understanding the transfer of heat between objects.

Achieving infinitesimal temperature difference during heat transfer allows for a more accurate understanding of how heat is transferred between objects. It also allows for more precise calculations and predictions, especially in thermodynamics and engineering applications.

Temperature difference during heat transfer can be controlled by adjusting the rate of heat transfer, the amount of heat being transferred, and the properties of the materials involved. This can be achieved through various methods such as insulation, changing the flow rate of fluids, and using different types of materials.

Several factors can affect the ability to achieve infinitesimal temperature difference during heat transfer. These include the thermal conductivity of the materials involved, the surface area of the objects, the rate of heat transfer, and the presence of any insulation or barriers.

Infinitesimal temperature difference can be measured using highly sensitive temperature sensors and instruments such as thermocouples, thermistors, and infrared cameras. These devices can detect even the smallest temperature differences and provide accurate readings for analysis.

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