A-LEVEL PHYSICS: 11.2.9: Second Law of Thermodynamics (SaveMyExams)

Question 1

heat engine:

Answer 1

system that converts heat to usable energy which is then used to do mechanical work

Question 2

second law of thermodynamics:

Answer 2

a heat engine requires a source and a sink to operate

Question 3

source:

Answer 3

a high-temperature reservoir

has a high temperature TH
heat energy from it is QH

Question 4

sink:

Answer 4

low-temperature reservoir

has a low temperature TC
heat energy going to it is QC

Question 5

what would happen if the heat engine reaches the temperature of the source? why? what does this mean?

Answer 5

no heat would flow
bc would have reached thermal equilibrium
∴ no work would be done (W=0)

this means that it is impossible for a heat engine to work solely on the first law of thermodynamics (∆U=Q-W)

Question 6

why is it impossible for a heat engine to work solely on the first law of thermodynamics? (2)

Answer 6

-bc if the heat engine reaches the temperature of the source, no heat would flow

-bc would have reached thermal equilibrium

Question 7

if a heat engine only obeyed first law of thermodynamics (no friction), the source-sink diagram would look like:

Answer 7

source (TH)
↓ (QH)
↓
→ W

Question 8

source-sink diagram for an actual heat engine (obeys second law of thermodynamics):

Answer 8

source (TH)
↓ (QH)
↓
↓ → W
↓
↓ (QC)
↓
sink (TC)

Question 9

explain the source-sink diagram of actual heat engine (obeys second law of thermodynamics): (3)

Answer 9

-heat energy (QH) is transferred from source at temperature TH

-some of this energy is transferred into work W

-remaining energy (QC) is transferred to the sink at temperature TC