Engineering - Thermodynamics

Question 1

In the equation ΔQ = Δu + ΔW

What does each stand for?

Answer 1

ΔQ: heat transfer to gas
Δu: increase in internal energy
ΔW: work done by gas

Question 2

In the equation ΔQ = Δu + ΔW

What does it mean if Q is negative?

Answer 2

Energy transfers out of the system

Question 3

In the equation ΔQ = Δu + ΔW

What does it mean if u is negative?

Answer 3

Internal energy has decreased

Question 4

In the equation ΔQ = Δu + ΔW

What does it mean if w is negative?

Answer 4

Work is done on the system by external action

Energy transfers into the system

Question 5

State the first law of thermodynamics

Answer 5

The change in internal energy of a system (Δu) is equal to the sum of the energy entering the system by heating (ΔQ) and the energy entering the system through work being done on it (ΔW)

Question 6

Define adiabatic

Answer 6

No heat energy is transferred

Question 7

What are the requirements for an adiabatic change?

Answer 7

Action is quick
Negligible transfer to surroundings
ΔQ=0

Question 8

Outline an adiabatic compression by a piston on a gas

Answer 8

Perfect insulator between gas and surroundings
When piston pushed down, gas is compressed and work is done on the gas
Δu = ΔW
Temperature increases as u increases

Question 9

Why does temperature of a gas fall when it expands adiabatically?

Answer 9

Work is done at the expense of internal energy

Question 10

Outline an isothermal compression by a piston on a gas

Answer 10

Work is done on the gas as the piston moves down
Perfect conductor between gas and surroundings which allows free flow of heat
As gas is compressed, heat escapes so there is no change in temperature (Δu=0)
-ΔQ=ΔW

Question 11

In an ideal gas, what is u?

Answer 11

The Ek of its molecules

Question 12

Where does pressure in a piston come from?

Answer 12

Gas molecules colliding with external walls

Question 13

Prove that W=pΔV

Answer 13

Gas expands by very small Δx without pressure change
volume change is AΔx
F=pA
W=Fd=pAΔx=pΔV

Question 14

What does W=pΔV mean for compression or expansion?

Answer 14

If volume decreases, we gain a negative change and a negative value for work done
So work is done on the gas

Question 15

What is the equation involving pressure and volume for an adiabatic compression?

Answer 15

pV^γ = constant

γ changes on type of gas (diatomic etc)

Question 16

Using a p against V graph, how are adiabatic and isothermal processes identified?

Answer 16

Adiabatic have steeper gradients than isothermal

Question 17

How is work done calculated from a p against V graph?

Answer 17

Area under the curve

Question 18

What is the gradient for adiabatic?

Answer 18

Δp/ΔV=-γp/V

Question 19

Why is mechanical efficiency always less than 100%?

Answer 19

Friction between moving parts

Oil for lubrication is viscous so causes some resistance to motion

Question 20

Why is thermal efficiency lower than in theory?

Answer 20

No sharp corners like in the diagram so less work is done
Maximum temperature not reached
Not perfectly adiabatic compression and expansion
Work done by engine to cause induction and exhaust strokes

Question 21

What are the steps of the petrol engine cycle?

Answer 21

Inlet valve opens, exhaust valve closes
Induction stroke - air drawn into cylinder as piston moves down
Inlet valve closes
Compression stroke - piston moves up, gas compressed adiabatically, temperature rises
Spark ignites mixture, supplying heat Q1 and increasing p at constant V, temperature rises
Power stroke - increased p pushes piston down and gas expands adiabatically, p and T decrease
Exhaust valve opens and gas leaves cylinder, removing heat Q2, p and T decreases
Exhaust stroke - rest of gas is expelled as piston moves up
Cycle repeats

Question 22

Why is the petrol cycle a four stroke cycle?

Answer 22

Piston moves four times

Drawing in air (piston down), compressing air (piston up), gas expansion (piston down), removing air (piston up)

Question 23

What are the steps of the diesel engine cycle?

Answer 23

Inlet valve opens and exhaust valve closes
Induction stroke - air in, piston down
Inlet valve closes
Compression stroke - adiabatic compression, piston up, T increases
Power stroke part 1 - Fuel enters cylinder, ignited by hot air, supplies heat Q1, piston forced down, constant p
Power stroke part 2 - Fuel supply cut off, gas expands adiabatically, piston down, T decreases
Exhaust valve opens, most gas leaves, heat Q2 removed, p and T decrease
Exhaust stroke - Piston up, gas expelled
Cycle repeats

Question 24

As in the diesel engine cycle, fuel is burnt inside the cylinder, what kind of engine is it?

Answer 24

Internal combustion engine

Question 25

What occurs once a heating coil placed in flowing water, reaches a steady state?

Answer 25

Coil's state does not change | All electrical energy going to coil goes to heating water

Question 26

How can the steady state of a heating coil in flowing water be applied to mechanical work overcoming friction?

Answer 26

All mechanical energy is converted to heat

Question 27

What is a device called that does heat to work?

Answer 27

Heat exchange

Question 28

How can the idea of a heat exchange be applied to gas in a piston system?

Answer 28

If heat is added the gas expands, piston is pushed So heat to work Process stops when piston pressure is atmospheric

Question 29

State the 2nd law of thermodynamics

Answer 29

It is not possible to continuously convert heat into work without at the same time transferring some heat from the warmer body to the colder body

Question 30

Consider an engine between a hot reservoir at T1 and a cold reservoir T2, what would the process be for heat exchange?

Answer 30

Heat is taken in from the hot reservoir, Q1 Work is done, Q1 - Q2 Reject some heat at lower temperature, Q2

Question 31

Consider an ideal engine between a source and a sink, outputting work. How is the efficiency found?

Answer 31

``` Source temperature Th and sink temperature Tc can be defined as ratio Qh/Qc Qh is heat entering, Qc is heat leaving So Th/Tc = Qh/Qc Efficiency = W/Qh x 100% Therefore efficiency = Th-Tc/Th x 100% ```

Question 32

Explain the process used by a fridge to keep the inside cool

Answer 32

Takes in low temp air that is cooled by expansion Air takes in energy from inside fridge to get to same temp as before Compressed to hotter temp and ejected Fridge effectively removes heat from low temp reservoir

Question 33

Explain the process used by a heat pump

Answer 33

Take in low temp and compress to high temp Surrounding is heated up Eject heat at high temp External action needed like a motor or pump action working on substance Heat pump effectively supplies heat to high temp reservoir

Question 34

What is the effectiveness of a refrigerator/heat pump determined by?

Answer 34

Coefficient of performance

Question 35

What is the coefficient of performance?

Answer 35

Ratio of heat extracted or supplied compared to work done by external action