Engineering

Question 1

Define angular speed

Answer 1

the rate of change of angular displacement

Question 2

Define angular acceleration

Answer 2

the rate of change of angular speed

Question 3

What is the equation for the moment of inertia for a hoop

Answer 3

I = m(r^2)

Question 4

How do you add different I (moment of inertia) together?

Answer 4

add I as a scalar (like you would for mass)

Question 5

What does T represent in the equation T=Iα

Answer 5

RESULTANT torque

Question 6

What is meant by torque?

Answer 6

• turning effect

- force x perpendicular distance from the axis to the line of action of the force

Question 7

What does I represent in rotational dynamics?

Answer 7

moment of inertia

Question 8

What is a flywheel?

Answer 8

a mechanical device which is used to store rotational/mechanical kinetic energy

Question 9

What 2 quantities determine how effective a flywheel is as a store of KE? (use formula sheet)

Answer 9

• I (moment of inertia)

- ω (angular speed)

Question 10

How can I (moment of inertia) be maximised for a flywheel? (3 things)

Answer 10

• I = m(r^2), use a hoop
• large m, dense material
• large r, as much as possible

Question 11

How can ω (angular speed) be maximised for a flywheel? (5 things)

Answer 11

• magnetic bearings - no friction
• evacuated chamber - no air resistance
• stiff material - no deformation
• strong material - avoids breaking apart
• balanced - avoid uneven forces

Question 12

For a flywheel on a vehicle, why may its KE not be maximised?

Answer 12

• have to compromise

- e.g. having a flywheel with a very large radius would not be practical for the vehicle

Question 13

For a disc or hoop rolling down a slope (initially from rest), what would the energy transfers be?

Answer 13

GPE = linear KE + rotational KE (+ heat)

Question 14

Angular momentum is conserved unless…

Answer 14

external torques act on the system

Question 15

What is the first law of thermodynamics?

Answer 15

Q = ∆U + W

Question 16

What does Q represent in the first law of thermodynamics?

Answer 16

heat energy into the system

Question 17

What does ∆U represent in the first law of thermodynamics?

Answer 17

change in internal energy (change in temperature)

Question 18

What does W represent in the first law of thermodynamics?

Answer 18

work done by the system

Question 19

What sign is W, and what happens to the gas, if work is done by the system?

Answer 19

• W is positive

- the gas expands

Question 20

What sign is W, and what happens to the gas if work is done on the system?

Answer 20

• W is negative

- the gas contracts

Question 21

What is meant by an isothermal change?

Answer 21

no temperature change

Question 22

What does ∆U equal in an isothermal change?

Answer 22

∆U = 0

Question 23

Give an example/situation when an isothermal change would occur

Answer 23

• slow change

- e.g. ‘frictionless’ piston warmed slowly

Question 24

What is meant by an adiabatic change?

Answer 24

no heat change to or from the surroundings

Question 25

What does Q equal in an adiabatic change?

Answer 25

Q = 0

Question 26

Give an example when an adiabatic change would occur

Answer 26

- quick/insulated change | - e.g. fire piston

Question 27

What does W equal if a change occurs at constant volume?

Answer 27

W = 0

Question 28

What is meant by an isobaric change?

Answer 28

a change that occurs at constant pressure

Question 29

Give 3 aspects of the second law of thermodynamics

Answer 29

- heat moves spontaneously from a hot body to a cooler body if they are in thermal contact - heat energy can do work - no heat engine can convert all the heat flow from a hot source to a cold sink

Question 30

What is the compression ratio for a four-stroke petrol engine?

Answer 30

1:9

Question 31

What is the compression ratio for a four-stroke diesel engine?

Answer 31

1:20

Question 32

What are the main differences between a four-stroke petrol engine and a four-stroke diesel engine?

Answer 32

- petrol engine has a spark plug, there is ignition - diesel engine has a much larger compression ratio - diesel engine has a slow burn whereas a petrol engine has an ignition

Question 33

What are the four strokes for a petrol and/or diesel engine?

Answer 33

- intake - compression - power - exhaust

Question 34

Efficiency of a heat engine is always...

Answer 34

less than 100%

Question 35

What is the equation for mechanical efficiency?

Answer 35

- η = Pout / Pindicator | - mechanical efficiency = (brake power output) / (indicated power)

Question 36

What is the equation for thermal efficiency?

Answer 36

- ε = Pindicator / Pinput | - thermal efficiency = (indicated power) / (input power)

Question 37

What are the equations for overall efficiency?

Answer 37

- η x ε | - Pout / Pinput

Question 38

What do you do to the cycles per second for a four-stroke engine?

Answer 38

- divide the value they give by 2 | - as the power stroke happens once every 2 cycles

Question 39

What is the range of values for maximum theoretical efficiency?

Answer 39

maximum theoretical efficiency < 100%

Question 40

For the equation εmax = 1 - Th/Tc, what are Th and Tc measured in?

Answer 40

measured in Kelvin [K]

Question 41

Why can theoretical efficiency never be 100%?

Answer 41

- for 100% efficiency, Tc = 0K or Th = ∞K | - neither of which are possible

Question 42

In order to maximise theoretical efficiency, what should be done?

Answer 42

- ideally Th as high as possible | - (can't usually change Tc as this is usually the temperature of the surroundings)

Question 43

Why in reality is Th not made as high as possible, even though doing so would increase theoretical efficiency?

Answer 43

- would result in performance issues | - metals become more 'sticky' when they are heated, increasing friction and mechanical inefficiency

Question 44

State and explain 3 causes of thermal inefficiency

Answer 44

- the valves cannot open/close instantaneously, rounding the corners of the indicator diagram, reducing the area - expansion and compression are not perfectly adiabatic, so some heat energy is lost, e.g. to the surrounding metal - the maximum theoretical temperature is not reached due to incomplete and imperfect combustion

Question 45

State and explain 2 causes of mechanical inefficiency

Answer 45

- friction between moving parts wasted as heat energy | - oil is viscous and therefore has a resistance to motion (of the moving parts)

Question 46

List the 6 steps for a refrigerator

Answer 46

- compressor does work on the coolant - coolant at high temperature and high pressure enters black coil behind the fridge - coolant loses heat to cooler surroundings and condenses and loses even more heat - coolant passes through expansion valve, lowering temperature and pressure (gas does work) - coolant absorbs heat from warmer surroundings inside the fridge and vapourises, drawing more heat to provide the latent heat needed to vapourise - coolant is about to be re-condensed by compression - increasing pressure

Question 47

How can you prevent a liquid from boiling when the ambient temperature is above its normal boiling point?

Answer 47

by keeping it under pressure

Question 48

Why may it seem that a heat pump contradicts the law of conservation of energy, and why is this not actually the case?

Answer 48

- heat pump does deliver more energy than is input as work, but there must also be energy input from the cold sink - obeys conservation of energy as the work done + the energy from the cold sink = energy by heat transfer to the hot source

Question 49

Why may it seem that a heat pump contradicts the second law of thermodynamics, and why is this not actually the case?

Answer 49

- heat is moving from the cold sink to the hot source | - however, the movement from the cold sink is NOT spontaneous - it is as a result of putting work in