12. cyclic processes, entropy and order

Question 1

Define cyclic process

Answer 1

Defined as processes which begin and end at the same point in the p-V diagram

Question 2

How to calculate work done per cycle of a cyclic process

Answer 2

Area inside the loop of the p-V diagram

Question 3

What is the carnot process?

Answer 3

Carnot combined 4 thermodynamic processes to construct a cycle that provides an upper limit on the maximum efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work.

Question 4

Carnot process assumptions

Answer 4

1. the engine is completely insulated - there is not transfer of heat with en/v
2. the heat reservoirs and exchange are ideal
3. we are dealing with 1 mole of gas
4. the piston’s motion is frictionless

Question 5

Describe the isothermal expansion - carnot process

Answer 5

step 1
• the system is in thermal contact with the heat reservoir at high temp
• heat causes gas to expand - doing work on surroundings
• lowering the pressure, increasing the volume while keeping T constant (∆U constant)

Question 6

Describe the adiabatic expansion - carnot process

Answer 6

Step 2
• the gas is thermally insulated from both reservoirs
• the gas continues to expand by reduction of pressure - doing work on the surroundings
• loses internal energy to the work done and causing its temperature to decrease

no heat; Q = 0

Question 7

Describe the isothermal compression of carnot process

Answer 7

Step 3
• the system is in thermal contact with the heat reservoir at T(low)
• the surroundings do work on the gas - pushing the piston down causing an amount of heat energy to leave the system to the low temperature reservoir
∆U = 0 (constant tem)

Question 8

Describe the adiabatic compression of carnot process

Answer 8

Step 4
• The gas is thermally insulated from both reservoirs
• the surroundings do work on the gas, pushing the piston down further increasing its internal energy - compressing it
• causes temperature to rise back up
Q=0

Question 9

What are the steps in the carnot process (in order)?

Answer 9

1. isothermal expansion
2. adiabatic expansion
3. isothermal expansion
4. adiabatic compression

Question 10

Define reversible process

Answer 10

• passes through a continuous series of equilibrium states
• can be stopped at any stage and reverse so that the system and surroundings are exactly restored to their initial states
• produces maximum work in engines and requires minimum work in devices such as heat pumps

Question 11

Some examples of reversible processes?

Answer 11

• expansion/compression of a spring
• adiabatic expansion or compressions
• elastic stretching of solid

Question 12

Define irreversible processes

Answer 12

• passes through a series of non-equilibrium states
• difficult to predict the properties of the system over time
• when an irreversible process is made to proceed backwards it does not reach its original state - system reaches a new state

Question 13

P-V diagrams of irreversible processes

Answer 13

Usually represented by dotted lines

Question 14

What are the factors which causes a process to be irreversible?

Answer 14

A presence of any of these factors makes a process irreversible
• friction
• free expansion
• diffusion
• electric resistance
• inelastic deformation
• chemical reactions

Question 15

What is the second law of thermodynamics?

Answer 15

• efficiency of a system
• thermal energy is distinct from all other energy forms because it cannot be converted completely into work
• impossible to take heat from a reservoir an change it into work without a fraction of the heat being released

Question 16

Define entropy

Answer 16

•Defined as the property of a system which measures the degree of disorder or randomness in the system

Question 17

In what kind of system is ∆S = 0?

Answer 17

Applies to
• closed system with reversible processes
or
• systems that are continuously in an equilibrium state

Question 18

In what kind of systems is ∆S > 0?

Answer 18

Applies to close systems with irreversible processes or systems that are non equilibrium state

Question 19

How to calculate the number of accessible microstates in a system?

Answer 19

s = Kb x ln(W)

Kb = boltzmann's constant
W = number of accessible microstates of the system

Question 20

What are the different ways entropy can be increased?

Answer 20

1. adding particles
2. adding energy
3. increasing volume
4. dissociation