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Thermodinamica > Lecture 7 - Carnot Cycle > Flashcards

Lecture 7 - Carnot Cycle Flashcards

1
Q

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

A

True

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2
Q

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

A

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

State functions are path INDEPENDENT

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3
Q

Internal energy is a state function and is therefore …

A

independent of the path taken

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4
Q

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

A

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

W = nRT*ln(V2/V1)

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5
Q

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

A

the temperature

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6
Q

If the temperature does not change, the internal energy …

A

remains the same

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7
Q

Efficiency, η, is equal to …

A

η = (work in + heat in)/work out

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8
Q

What is a reversible process?

A

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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9
Q

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

A

they are easier to work work

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10
Q

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

A

mechanical friction (pressure drop, surfaces rubbing together)

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11
Q

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

A
  • 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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12
Q

Thermal energy is easy to recover

A

false

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

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13
Q

quasi equilibrium expansion processes are those that …

A

go very very slowly in order to maintain equilibrium

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14
Q

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

A

move very quickly and are therefore not in equilibrium

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15
Q

quasi equilibrium expansion and compression processes are those that …

A

go very very slowly in order to maintain equilibrium

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16
Q

A reversible process has heat transfer occurring across …

A

an infinitesimally small differential temperature

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17
Q

All heat cycles studied are revesible

A

FALSE!

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

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18
Q

We study reversible processes because …

A

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

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19
Q

Reversible processes deliver …

A

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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20
Q

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

A

engines and turbines

21
Q

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

A

pumps and compressors

22
Q

What is a carnot heat engine?

A

An engine that operates on 4 reversible processes

23
Q

The processes of the carnot heat engine are …

A
  1. reversible isothermal expansion from state 1 to state 2 at T (hot/ source)
  2. reversible adiabatic expansion from state 2 to 3
  3. reversible isothermal compression from state 3 to 4
  4. reversible adiabatic compression from state 4 to 1
24
Q

If a gas expands rapidly …

A

it gets cold

25
Q

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

A

heat addition to prevent the gas from cooling

26
Q

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

A

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

27
Q

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

A

gets cooler

28
Q

During reversible isothermal compression,

A

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

29
Q

During reversible adiabatic compression,

A

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.

30
Q

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

η =

A

η =

31
Q

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

η =

A

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)

32
Q

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

A

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)

33
Q

The Carnot engine is a reversible heat engine.

A

True

34
Q

The thermal efficiency of a reversible cycle is ….

A

η = 1 - Ql/Qh = 1 - Tl/Th

35
Q

What are the Carnot principles?

A
  • 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)
36
Q

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

A 
1        ↙ Qh
\    ↘ Th
  \         2
    \         \ (slightly curved inwards in the center)
     4←(Tl)  3 
          ↓ Ql

P on y and v on x

37
Q

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

A
  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
38
Q

In compression, the pressure goes ….

In expansion, the pressure goes …

A

up

down

39
Q

In reversible adiabatic expansion,

A

the pressure goes down

40
Q

In reversible isothermal expansion …

A

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

41
Q

In reversible adiabatic compression, …

A

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

42
Q

In reversible isothermal compression, …

A

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

43
Q

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

A 
1        ↗ Qh
\    ↖  Th
  \         4
    \         \ (slightly curved inwards in the center)
     2→(Tl) ​3 
         ​↑ Ql
44
Q

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

A

A ratio of required work to useful heat

```
COPr)rev = 1/(Th/Tl -1
(COPhp)rev = 1/(1 - Tl/Th)
~~~

45
Q

Higher COP mean …

low COP mean ….

A

high efficiency

low efficiency

46
Q

Coefficients of performance is used for heat engines.

A

FALSE!

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

47
Q

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

A

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)

48
Q

True or False;

(COP)rev < (COP)irr

A

(COP)rev > (COP)irr

49
Q

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

A

True

Thermodinamica (10 decks)

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