thermodynamics

Question 1

Adiabatic process

Answer 1

• no heat enters or leaves the system
• thick walls
• rapid change

Expansion:
- work done by expanding gas
W= −ΔU
- Internal energy and temperature falls

Compression:
ΔU=−W
- work done on compressing gas

Question 2

Isothermal process

Answer 2

• slow change
• thin walls
• large surroundings so temperature of surroundings remains constant

Expansion:
ΔU=0 , therefore Q=W

Compression:
ΔU=0, therefore −W= −Q

Question 3

Isovolumetric process

Answer 3

• constant volume
  ΔV = 0, therefore W=PΔV = 0
  NO WORK DONE

Question 4

Isobaric process

Answer 4

• constant pressure
• work is done

Question 5

first law of thermodynamics

Answer 5

energy can’t be created or destroyed

Question 6

second law of thermodynamics:

Answer 6

Clausius’ statement: heat spontaneously flows from hotter to colder bodies. It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a colder body to a hotter body.

Kelvin: impossible to devise a cyclically operating heat engine that produces net work while exchanging heat with only a single thermal reservoir. A heat engine must reject some heat to a colder reservoir to operate. The perfect conversion of heat into work in a cycle is impossible

Entropy -> Total entropy of an isolated system can only increase over time or remain constant in a reversible process. It never decreases.

Question 7

The Carnot cycle

Answer 7

Theoretical cycle- two isothermal and two adiabatic processes
sets an upper limit for the efficiency of a heat engine.
effeiciency η = 1 - Tc/Th

for 100% efficiency to happen, Tc would have to be 0 kelvin which would be impossible.
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Question 8

entropy meaning

Answer 8

degree of disorder of the particles in a system

Question 9

Entropy decreasing locally

Answer 9

entropy can decrease locally in a non isolated system, but that is compensated by an equal or greater increase of the entropy of the surroundings

Question 10

example of entropy decreasing locally

Answer 10

refrigerator system.
refrigerators actively move heat from a colder region to a hotter region, decreasing the entropy of the cold space, but they require work (energy input), which is dissipated as heat to the surroundings. thus increasing the total entropy of the universe.

Question 11

What is a closed system?

Answer 11

A system in which no mass can be transferred in or out, but energy can be transferred in both directions as heat or as work

Question 12

What is an isolated system?

Answer 12

A system in which neither mass nor energy can be transferred in or out

Question 13

What are cyclic gas processes?

Answer 13

they are designed to convert thermal energy into mechanical work.
the working gas returns to its initial thermodynamic state after each sequence, thus a cycle.

Question 14

thermodynamic cycle

Answer 14

1. Heat intake from hot reservoir = increase in internal energy, temperature and pressure, leading to expansion
2. Expansion: Work output pushing piston outwards, does work out on the surroundings, therefore gas expands: leading to a decrease in temperature and pressure
3. heat exhaust -> ‘spent’ gas returns to its initial state. Rejecting remaining heat to a cold reservoir, leading to a decrease in temperature, pressure and internal energy
4. Compression: Work is done on the gas to compress it back to original volume, temperature and pressure ready for next cycle. This requires input of mechanical work.