Engineering - Thermodynamics Flashcards
In the equation ΔQ = Δu + ΔW
What does each stand for?
ΔQ: heat transfer to gas
Δu: increase in internal energy
ΔW: work done by gas
In the equation ΔQ = Δu + ΔW
What does it mean if Q is negative?
Energy transfers out of the system
In the equation ΔQ = Δu + ΔW
What does it mean if u is negative?
Internal energy has decreased
In the equation ΔQ = Δu + ΔW
What does it mean if w is negative?
Work is done on the system by external action
Energy transfers into the system
State the first law of thermodynamics
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)
Define adiabatic
No heat energy is transferred
What are the requirements for an adiabatic change?
Action is quick
Negligible transfer to surroundings
ΔQ=0
Outline an adiabatic compression by a piston on a gas
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
Why does temperature of a gas fall when it expands adiabatically?
Work is done at the expense of internal energy
Outline an isothermal compression by a piston on a gas
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
In an ideal gas, what is u?
The Ek of its molecules
Where does pressure in a piston come from?
Gas molecules colliding with external walls
Prove that W=pΔV
Gas expands by very small Δx without pressure change
volume change is AΔx
F=pA
W=Fd=pAΔx=pΔV
What does W=pΔV mean for compression or expansion?
If volume decreases, we gain a negative change and a negative value for work done
So work is done on the gas
What is the equation involving pressure and volume for an adiabatic compression?
pV^γ = constant
γ changes on type of gas (diatomic etc)
Using a p against V graph, how are adiabatic and isothermal processes identified?
Adiabatic have steeper gradients than isothermal
How is work done calculated from a p against V graph?
Area under the curve
What is the gradient for adiabatic?
Δp/ΔV=-γp/V
Why is mechanical efficiency always less than 100%?
Friction between moving parts
Oil for lubrication is viscous so causes some resistance to motion
Why is thermal efficiency lower than in theory?
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
What are the steps of the petrol engine cycle?
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
Why is the petrol cycle a four stroke cycle?
Piston moves four times
Drawing in air (piston down), compressing air (piston up), gas expansion (piston down), removing air (piston up)
What are the steps of the diesel engine cycle?
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
As in the diesel engine cycle, fuel is burnt inside the cylinder, what kind of engine is it?
Internal combustion engine
What occurs once a heating coil placed in flowing water, reaches a steady state?
Coil’s state does not change
All electrical energy going to coil goes to heating water
How can the steady state of a heating coil in flowing water be applied to mechanical work overcoming friction?
All mechanical energy is converted to heat
What is a device called that does heat to work?
Heat exchange
How can the idea of a heat exchange be applied to gas in a piston system?
If heat is added the gas expands, piston is pushed
So heat to work
Process stops when piston pressure is atmospheric
State the 2nd law of thermodynamics
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
Consider an engine between a hot reservoir at T1 and a cold reservoir T2, what would the process be for heat exchange?
Heat is taken in from the hot reservoir, Q1
Work is done, Q1 - Q2
Reject some heat at lower temperature, Q2
Consider an ideal engine between a source and a sink, outputting work. How is the efficiency found?
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%
Explain the process used by a fridge to keep the inside cool
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
Explain the process used by a heat pump
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
What is the effectiveness of a refrigerator/heat pump determined by?
Coefficient of performance
What is the coefficient of performance?
Ratio of heat extracted or supplied compared to work done by external action
