Lecture 7 - Carnot Cycle Flashcards by Janelle Reyes

Heat, Q, and work, W, are path dependent.

True

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State functions are path dependent; therefore, heat and work are state functions.

FALSE! Heat and work are path dependent and are therefore not state functions.

State functions are path INDEPENDENT

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Internal energy is a state function and is therefore …

independent of the path taken

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For an isothermal process, the work done by the system is …

the area under the curve, which can only be solved by calculus and is

W = nRT*ln(V2/V1)

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The internal energy of a gas, U, is proportional to …

the temperature

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If the temperature does not change, the internal energy …

remains the same

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Efficiency, η, is equal to …

η = (work in + heat in)/work out

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What is a reversible process?

a process that can be reversed without leaving any trace on the surroundings

a process where the net heat and work exchange between the system and surroundings is 0.

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In thermodynamics, we look at work with reversible processes and consider them as ideal because …

they are easier to work work

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All real processes (processes in reality) are irreversible processes because …

mechanical friction (pressure drop, surfaces rubbing together)

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All real processes (processes in reality) are irreversible processes because …

mechanical friction (pressure drop, surfaces rubbing together)
non - quasi equilibrium expansion and compression processes
heat transfer across “large”/finite differential temperatures

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Thermal energy is easy to recover

false

thermal energy is difficult to recover, which is why processes that release thermal energy are irreversible

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quasi equilibrium expansion processes are those that …

go very very slowly in order to maintain equilibrium

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non - quasi equilibrium expansion and compression processes are those that …

move very quickly and are therefore not in equilibrium

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quasi equilibrium expansion and compression processes are those that …

go very very slowly in order to maintain equilibrium

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A reversible process has heat transfer occurring across …

an infinitesimally small differential temperature

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All heat cycles studied are revesible

FALSE!

they do not meet the criteria for reversibility (opposite of list for irreversibility)

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We study reversible processes because …

they give us a good idea on how real world processes are performing with respect to the idealization

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Reversible processes deliver …

the largest amount of work (engines/turbines) and require the least amount of work (pumps/ compressors)

for this reason, they are used as theoretical limits

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The devices that give us the largest amount of work are …

engines and turbines

The devices that use the least amount of work are ..

pumps and compressors

What is a carnot heat engine?

An engine that operates on 4 reversible processes

The processes of the carnot heat engine are …

reversible isothermal expansion from state 1 to state 2 at T (hot/ source)
reversible adiabatic expansion from state 2 to 3
reversible isothermal compression from state 3 to 4
reversible adiabatic compression from state 4 to 1

If a gas expands rapidly …

it gets cold

During reversible isothermal expansion from state 1 to 2 in the Carnot heat engine cycle, we have ...

heat addition to prevent the gas from cooling

During reversible adiabatic expansion of the Carnot heat cycle, we have ...

no heat transfer while the gas expands, so we drop to a lower temperature

If there is no heat transfer when you are expanding a gas, the gas ...

gets cooler

During reversible isothermal compression,

the gas gets hotter as the gas compresses, and that heat is ejected to the surroundings from the cycle so the process can remain at the same temperature

During reversible adiabatic compression,

the gas gets hotter, but the heat is not released, causing the temperature to increase as well, taking us from one isotherm to another, higher isotherm.

Although the Carnot cycle is an idealization, we can write an equation for the efficiency of the cycle, η. η =

η =

Although the Carnot cycle is an idealization, we can write an equation for the efficiency of the cycle, η. η =

η = (Qh/Ql)rev = Th/Tl where Qh= heat going into the cycle Ql = heat leaving the cycle Th = the higher temperature (in Kelvin) Tl = the lower temperature (in Kelvin)

The efficiency of a reversible heat cycle, η is equal to ...

η = (Qh/Ql)rev = Th/Tl where Qh= heat going into the cycle Ql = heat leaving the cycle Th = the higher temperature (in Kelvin) Tl = the lower temperature (in Kelvin)

The Carnot engine is a reversible heat engine.

True

The thermal efficiency of a reversible cycle is ....

η = 1 - Ql/Qh = 1 - Tl/Th

What are the Carnot principles?

- the thermal efficiency of an irreversible heat engine is always less than a reversible heat engine operating between the same two reservoirs as the irreversible engine ηirrev < ηrev - the thermal efficiency of all reversible engines operating between the same two reservoirs are the same ηrev(a) = ηrev(b)

Draw the Carnot Heat Cycle on a P-v diagram.

``` 1 ↙ Qh \ ↘ Th \ 2 \ \ (slightly curved inwards in the center) 4←(Tl) 3 ↓ Ql ``` P on y and v on x

What are the processes in a Carnot heat pump/refrigerator?

1. reversible adiabatic expansion from state 1 to state 2 2. reversible isothermal expansion from state 2 to 3 3. reversible adiabatic compression from state 3 to 4 4. reversible isothermal compression from state 4 to state 1

In compression, the pressure goes .... | In expansion, the pressure goes ...

up | down

In reversible adiabatic expansion,

the pressure goes down

In reversible isothermal expansion ...

the gas gets cold, so to keep it isothermal (at the same temperature), we add heat from the low temperature source, Ql

In reversible adiabatic compression, ...

the pressure increases and we go from a "cold" isotherm to a "hot" isotherm

In reversible isothermal compression, ...

the gas heats up, causing the pressure to go up, and to remain isothermal (at the same temperature), we must eject heat

Draw the Carnot refrigerator/heat pump cycle on a P-v diagram.

``` 1 ↗ Qh \ ↖ Th \ 4 \ \ (slightly curved inwards in the center) 2→(Tl) 3 ↑ Ql ```

What is the coefficient of performance (COP) for a reversible heat pump cycle?

A ratio of required work to useful heat | ``` COPr)rev = 1/(Th/Tl -1 (COPhp)rev = 1/(1 - Tl/Th) ```

Higher COP mean ... | low COP mean ....

high efficiency | low efficiency

Coefficients of performance is used for heat engines.

FALSE! Thermal efficiency, η is for heat engines COP are for heat pumps/refrigerators

Heat pumps and refrigerators work the same way, so what is the difference between a heat pump and a refrigerator?

Heat pumps are a reverse refrigerator. They warm buildings by pumping heat from outside (where it's cold; normally winter) into a building (where it is hot). Refrigerators take heat from food (where the environment is cold) and ejects that heat to surroundings outside the fridge (where it's hot)

True or False; (COP)rev < (COP)irr

(COP)rev > (COP)irr

Coefficients of performance, COP, are the best performance we can achieve with a heat pump or refrigerator.

True