Practical & Engineering Flashcards
(110 cards)
1
Q
What is meant by the Moment of Inertia of an object?
A
A measure of the opposition of a body to having its angular velocity changed by an applied torque
2
Q
What factors affect the moment of inertia of an object?
A
- The mass of the object
- How the mass is distributed
- The axis that it will be rotated about
3
Q
Use of Flywheels as a Rotational Kinetic Energy Store
A
- Large fly wheels can be used to stabilise the energy supply to a system
4
Q
State the Law of conservation of angular momentum
A
- The total angular momentum of a system remains constant provided no external torque acts on the system
5
Q
State 3 uses of a Flywheel
A
- To smooth out fluctuations in rotational speed
- To store rotational kinetic energy
- To smooth fluctuations in torque/power
6
Q
State the equation for Moment of Inertia
A
- I = Σmr^2
for a flywheel = Imr^2
for a solid sphere = 2/5 mr^2
for a hollow sphere = 2/3mr^2
for a cylinder = 1/2mr^2
7
Q
What is the equation for rotational kinetic energy?
A
KE = 1/2 I ω^2
where ω = Δθ/Δt
8
Q
Operation of Flywheels
A
- A wheel with a high moment of inertia (high mass, large radius)
- Resists changes to rotation
- It stores rotational energy, and only just enough power is transferred to the wheel to overcome the frictional torque
- When power is needed to the rest of the machine, the flywheel reduces its speed and transfers some power
9
Q
Factors affecting energy storage of Flywheels
A
- KE = 1/2 I ω^2
- mass
- higher rotational speed
- by adding spokes the amount of stored energy increases also
10
Q
Types of Systematic Errors
A
Parallax Error
- reading a scale at the wrong angle
Zero Error
- when an instrument is badly calibrated
Environmental Factors
- temperature in a room changed and affects your readings
11
Q
Different kinds of Errors
A
- Systematic Errors
- caused by a fault with the method or the apparatus
- they effect every reading
- they usually can be identified and removed
- Random errors
- random fluctuations which cannot be avoided
12
Q
How to reduce uncertainty
A
- using higher resolution measuring equipment e.g. digital rather than analog, greater no. of significance
- helps avoid systematic errors
- minimises the effect of random errors - take repeat readings if possible
- more reliable mean value
- allows for you to identify anomalous readings
- helps better estimate uncertainty of readings
13
Q
How do you improve accuracy?
A
- accuracy is how close to the true value you are when measuring
- to improve accuracy you must be calibrating measuring instruments
- if uncalibrated will lead to systematic errors in all readings
-
14
Q
What is the resolution?
A
- the smallest scale division
15
Q
What is precision?
A
- How reproducible your measurements are
- A set of measurements that are close together show high precision
16
Q
What limits rotational kinetic energy?
A
- the structural integrity of the object
17
Q
How do fly wheels smooth torque and speed
A
- power is not produced continuously but only in the form of ‘power strokes’
- as a result the power systems/ engines produce a torque that fluctuates
- uneven torque causes jerky motions in vehicles ( a waste of energy)
- adding a fly wheel which speeds up or slows down over a period of time due to its intertial will take that KE as rotational KE
- this smooths out the fluctuations in torque
- to increase power and torque combustion engines have multiple combustion cylinders
18
Q
KERS (kinetic energy recovery systems)
A
- regenerative breaking
- (finish using textbook)
19
Q
Advantages and Disadvantages of Flywheels
A
Adv:
- High Efficiency
- Short recharge time
- Environmentally friendly
Dis:
- Large and heavier
- Safety risk as they can reach their breaking point
20
Q
What is the equation for angular momentum and angular impulse?
A
J = I ω
J = T Δt / J = τ Δ t
for non constant torque -> Δ(Iα)t
21
Q
What is the First Law of Thermodynamics?
A
Q = ΔU + Q
where Q, heat energy transferred
where U, change in internal energy
where W, Work done BY the system
22
Q
What is an Adiabatic Process? What does it show?
A
- A process where there is no net gain or loss of heat
- i.e. Q = 0
thus ΔU = -W
therefore any changes done to a system will be caused by work done
23
Q
For an Adiabatic Process, describe what will happen to the internal energy when work is done for a system
A
Work done BY the system
- +W meaning decrease in internal energy
- ΔU = -W
Work done ON the system
- -W meaning increase in internal energy
- ΔU = -(-W)
- ΔU = W
24
Q
What is an Isothermal Process? What does it mean for a system?
A
- They happen at a constant temperature
- a constant temperature means no change in internal energy (for an ideal gas)
- Q = ΔU + W
- Q = 0 + W
- Q = W
pV = nRT, T is constant so p1V1 = p2V2
25
Work done at constant pressure
W = p ΔV
26
What is the Work done on a system when there is constant volume?
- = 0
- W = p ΔV , if no change in volume then no work is done
27
At constant volume what happens to the internal energy of a gas?
- The first law of thermodynamics states = Q = ΔU + W
- when at constant volume W = 0 ∵ W = p ΔV, where ΔV = 0
- Therefore Q = ΔU + 0
- so Q = ΔU
- ALL The heat energy supplied to gas at constant volume goes towards raising the internal energy of the gas
28
For a gas of constant volume how can pressure be found? For a gas of constant pressure how can volume be found?
P1/T1 = P2/V2
V1/T1 =V2/T2
29
Isothermal Curves
(insert pic)
30
Adiabatic Curves
- Steeper gradient than Isothermal curves
31
Curves at Constant pressure and Constant Volume for a System
(insert pic)
32
Four-Stroke Engine Cycle Process
Four-stroke enginee -> burns fuel **once** for every 4 strokes of the piston
**Induction**
- Piston goes down, the volume of the base above increases
- Fuel and air mixture gets into engine from inlet valve
- pressure of the gas’s remains constant
**Compression**
- Ths inlet valve is closed. The piston moves back up
- The pressure increases
- Just before the end of the stroke, a spark occurs
- The pressure and volume increase
dramatically at a constant volume
**Expansion**
- The air fuel mixture expands does work on the piston. The work it does is more than the work required to compress the gas
- thus a net output of energy
**Exhaust**
- piston moves up the cylinder and the burnt air-fuel mixture leaves through the open exhaust valve
33
Describe the Induction stage of a 4-strokes engine
- Piston goes down, the volume of the base above increases
- Fuel and air mixture gets into engine from inlet valve
- pressure of the gas’s remains constant
34
Describe the Exhaust stage of a 4-strokes engine
**Exhaust**
- piston moves up the cylinder and the burnt air-fuel mixture leaves through the open exhaust valve
35
Describe the Expansion stage of a 4-strokes engine
**Expansion*
- The air fuel mixture expands does work on the piston. The work it does is more than the work required to compress the gas
- thus a net output of energy
36
Describe the Compression stage of a 4-strokes engine
**Compression**
- Ths inlet valve is closed. The piston moves back up
- The pressure increases
- Just before the end of the stroke, a spark occurs
- The pressure and volume increase
dramatically at a constant volume
37
Diesel Engine cycle
(to be completed)
38
Where on the _ Curve is the Compression stroke? How can you tell
- Compression stroke is where the spark occurs due to the spark plug
- It initiates the combustion of the fuel-air mixture within the cylinder
39
Ideal engine cycle
( to be continued)
40
Reverse Engine Cycles (fridge)
41
Reverse Engine cycles (heat pump)
42
What is a system?
- A fixed mass of any substance enclosed by a boundary
43
What is Q when heat energy is transferred out of system? (first law)
‘ -Q ‘
44
What is work done when work is done on a system? (first law)
-W
45
- If energy (Q) is added to a system then the internal energy will increase
- If however the gas does work against the environment, then there is a loss in internal energy of the system
46
The absolute value of internal energy can not be determined because you would have to know all the potential and kinetic energies of ** every particle **
47
Internal energy only depends on the intial and final states of a system
48
- In order to specify values of the pressure, volume and temperature of the system at any point in time, we need changed to occur **very slowly** (or infinitely slowly)
- otherwise pressure and temp will not be uniform throughout the system
49
Why are changes in real engines not reversible?
- the processes usually happens far too quickly
- for a reversible change the pressure and temperature must change uniformly, which only occurs for very slow processes
50
first law equation for a system under constant temperature (Isothermal)
Q = W
delta U = 0
assuming ideal gas then no change in temperature means no change in KE so no change in internal energy
51
What kind of vessel must a system undergoing an isothermal process be in?
- gas must be kept in a ** thin-walled vessel**
- of excellent **conducting material**
- surrounded by a constant-temperature bath
- any expansion or compression of the system must be incredibly slow
52
equation for reversible isothermal change
PV = constant
note: very slow and reversible process
53
What is a non-flow process?
- a thermodynamic process for which the fluid does not move in or out of the system
- essentially a closed system
54
isothermal
- same temperature system
55
equation for the first law of thermodynamics incorporating pressure and volume
Q = delta U + p x delta V
for a compression work is done ON the gas so delta V and W will be negative
vice versa for expansion
56
first law equation for a constant volume process
delta U = Q
no work is done when there is no change in volume
if heat energy us absorbed by the share then the internal energy must increase
57
Define an Adiabatic Process & state its first law equation
- An isolated system where no energy is able to transfer in or out by heating (from or to the external environment)
- -W = delta U
58
59
State what the internal energy is for an ideal gas
- The total molecular **translational ** kinetic energy
60
Why do most gases within systems not have an ideal gas internal energy equation = to translational KE?
- some gases are diatomic or poly atomic
- this means they have both rotational and vibrational kinetic energy
- the vibrational KE typically occurs for very high temperatures
61
What component of molecules of gas is the internal energy dependent on?
- The atomicity
62
How can our knowledge on atomcity of gases be used to calculate volume or gas pressures
- if a gas is in other ways ideal ( so internal E is = KE)
- then it undergoes a **reversible** adiabatic expansion or contraction
- PV^y = constant
- P1V1^y = P2V2^y
where y is a constant for particular gases
63
Why are adiabatic curves more steep than isothermal curves?
- an adiabatic expansion is as a result gas doing work against the environment
- this will cause it to lose internal energy so it’ll cool down
64
Condition for an adiabatic change of a system?
- System must be perfectly insulated
- or the system must have a rapid process such that there is insufficient time for heat to transfer in or out
65
Adiabatic Equations
P1V1^y = P2V2^y
T1V1^(y-1) = T2V2(y-1)
66
Non-Flow processes and equations
67
what directions are the arrows on a P-V/indicator diagram
—-> = by gas
<—— = by us
68
How can the area under an indicator graph and direction tell us what work is being done on or by a system
- If volume is increasing
- work is being done BY the gas
- If the volume is decreasing
- work is being done ON the gas
can identify this by looking at direction of the arrows
69
What is a Cyclic Process?
- One which a system undergoes two or more **consecutive changes** such that the **final** state is the same as the **initial state**
- cycle is repeated continuously
70
What is the purpose of an engine?
- The purpose of an engine is to force the working substance into such a cycle, so that the work done by the gas **exceeds** the work done on the gas once it has returned to its original position
- the net work done by the gas is the energy we use to power things
71
describe the processes occurring at CD and AB
- drop in pressure at constant volume, there has been a drop in temperature so heat energy has been lost from system -AB
- increase in pressure at a constant volume. The temperature has increased as heat energy has been added to the system. -CD
72
Can a perpetual motion machind be made? and why?
No!!
- work cannot be produced without an energy input (first law)
- heat energy is not totally converted to mechanical work (second law)
- assumes no frictional affects which is not possible
73
What is a Heat engine?
- A device or system that extracts energy from its environment in the form of heat and converts it into useful work
74
What is a working substance?
a substance (usually a fluid) in which the thermodynamic processes are performed in an engine (by changes in temperature pressure and volume)
75
exams of heat engines
- internal combustion engine (petrol-air)
- diesel engine (diesel-air)
- steam engine (uses water)
76
What is the Otto cycle?
- a four-stoke cycle
77
Difference between indicator diagram and real piston cycle
- One thermodynamic cycle on the indicator diagram corresponds to **two up-and-down motions** of the real piston, and hence **two revolutions of the engine** !!
78
What is a Diesel engine?
- an engine able to compress air and raise it’s temperature above ignition point without a spark plug
79
Why are diesel engines used?
- produce higher torque and require heavier construction than your standard internal combustion engine
80
How does a diesel engine differ from your standard four stroke petrol engine?
- no spark plug needed
- no fuel in the cylinder during compression
81
Why are Diesel engines more efficient than petrol engines? what are a disadvantage of diesel engines?
- Much higher compression ratio than a petrol engine
- also supply a much greater torque
- the disadvantage is that they operate at higher working pressures then petrol engines, becomes they are more robust
- thus having a lower power to weight ratio
- also emit a lot of unbridgeable hydrocarbons from exhaust
82
when referring to “power of an engine” what are we referring to?
- The output power of the engine /driving power
83
What are the three key power indicators?
1. input power - power delivered by burning fuel
2. indicated power - theoretical power that the engine can deliver (based on indicator diagram)
3. output power/brake power
84
How do you find the input power of an engine? (equation)
P(input) = calorific value of fuel (J kg-1) x fuel flow rate (kgs-1)
85
What is the calorific value of fuel a measure of?
- it’s the energy density
- how fast energy is being consumed
what is it dependent on?
- how fast the car is moving (air resistance too)
- type of engine
- condition of tires
86
How do you find the indicated power?
(theoretical power capability of an engine)
P(ind) = area of p-V loop (J) x no. of cycles x no. of cylinders
e.g. 6 cylinders , area = 450, no. of cycle 3
3x6x450 =8,100
it assumes frictionless motion
hence theoretical power output
remember that for real engines two complete cycles -> one power stroke
thus to find no. of cycles take the (number of revolutions)/2
87
How do you find the output / brake power?
- power delivered to engines crankshaft (aka flywheel) ( there are still other loses via auxiliary components etc)
- P(out) = T ω
88
conversion for horse power to watts
1 horse power = 746W
89
How can Frictional power of an engine be found?
P(friction) = indicated power - output/brake power
90
What are the efficiency equations?
mechanical E = output/indicated
thermal E = indicated/input
overall = mechanical E x thermal E
thus overall E = P(out)/P(input)
91
why adiabatic curves more steeper than isothermal curves
-due a change in both temperature and pressure vs just pressure
92
Can the effeciency of a heat engine = 1?
- no!
- even if the mechanical efficiency is 1 (aka removed all losses due to friction)
- there will always be a maximum thermal efficiency so overall efficiency can never be 1
- because a heat engine cannot turn heat energy totally in to work
93
What is a source?
- A high temperature reservoir
94
What is a sink?
- A low temperature reservoir
95
96
what is the equation for the maximum theoretical efficiency?
- = 1 - Tc/Th
- (Th - Tc)/Th
Th -> temp of hot reservoir
Tc -> temp of cold reservoir
in KELVIN
97
Limitations of Heat engines
- energy is dissipated from the system
- there is no equilibrium with the surroundings as the process is too quick
- inlet and exhaust take a finite amount of time to open and close
- combustion is not instantaneous
- heating not achieved at constant volume in a petrol engine because pistons always moving
- expansion and compression trikes are not truly adiabatic due to heat energy loss
98
Assumptions on heat engine calculations
1. petrol-air mixed is an ideal gas
2. heat energy is taken in entirely in the compression phase
3. process that form the engine cycle is reversible
99
What is the Second Law of Thermodynamics
- It is not possible to convert heat continuously into work without at the same time transferring heat from a warmer to a colder body
- (because heat always flows from a hot body to a cold body and no heat engine can completely convert heat into work)
100
(this flashcard is about 2nd law )
because some energy must be involved in the ordering process
this is rejected at cooler temperatures
hence cold sink is essential
101
What are the consequences of the 2nd law of thermodynamics on heat engines?
- A heat engine needs to operate between a hot source and a cold sink
102
How do reverse what engines work
- take in heat energy at a low temperature and reject heat at a high-temperature
- aka refrigeration
- reverse of a heat engine
- to achieve this work needs to be done to transfer energy from a sink to a source
- external devices are needed for this
103
How do heat pumps and refrigerators differ?
- the purpose of the refrigerator is to **remove** heat from the cold reservoir
- the purpose of the heat pump is to **supply** heat to the high temperature reservoir
104
what is COP?
Coefficient of performance
ratio of heat extracted or supplied and work done in supplying in
COPfridge = Qc/(Qh-Qc) and Tc/(Th-Tc) (kelvin)
(also applies to air conditioning)
COPheatpump = Qh/(Qh-Qc)
and Th/(Th-Tc)
105
Why do we use heat pumps?
- They provide an extremely low-cost and efficient form of heating
because the heat supplied is MUCH greater than the work done by external devices
W = Qh - Qc
106
When do heat pumps operate best?
- the value of the COP is higher the closer the temperatures of the two reservoirs are to each other
- as the temp difference becomes smaller, less work is needed to transfer heat from cold to hot
- hence why heat pumps work more effectively in temperate climates rather than climates with temperatures that vary massively
107
Why do we like flywheels?
- smooths out torque
- stores energy lost that would otherwise be lost from a system i.e through braking
108
- power not produced continuously
- only at “power stroke”
- this means torque produced fluctuates
109
problem with uneven torque produced due to fluctuating torque from non-continuous power production?
- torque is what makes the wheels rotate
- uneven torque will cause a jerky motion and unwanted vibration
- this will cause an unwanted loss in energy
- flywheels act by speeding up and slowing down due to their inertia so sharp fluctuations in torque will be smoothed
110
how do real engines hack this?
- having multiple pistons/cylinders
- these increase total power and further control the fluctuations due to power strokes
- this works by staggering them in their operation and also while being attached to a flywheel
