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AQA A-Level Physics > Engineering > Flashcards

Engineering Flashcards

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1
Q

Define angular speed

A

the rate of change of angular displacement

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2
Q

Define angular acceleration

A

the rate of change of angular speed

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3
Q

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

A

I = m(r^2)

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4
Q

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

A

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

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5
Q

What does T represent in the equation T=Iα

A

RESULTANT torque

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6
Q

What is meant by torque?

A
  • turning effect

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

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7
Q

What does I represent in rotational dynamics?

A

moment of inertia

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8
Q

What is a flywheel?

A

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

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9
Q

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

A
  • I (moment of inertia)

- ω (angular speed)

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10
Q

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

A
  • I = m(r^2), use a hoop
  • large m, dense material
  • large r, as much as possible
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11
Q

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

A
  • magnetic bearings - no friction
  • evacuated chamber - no air resistance
  • stiff material - no deformation
  • strong material - avoids breaking apart
  • balanced - avoid uneven forces
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12
Q

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

A
  • have to compromise

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

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13
Q

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

A

GPE = linear KE + rotational KE (+ heat)

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14
Q

Angular momentum is conserved unless…

A

external torques act on the system

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15
Q

What is the first law of thermodynamics?

A

Q = ∆U + W

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16
Q

What does Q represent in the first law of thermodynamics?

A

heat energy into the system

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17
Q

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

A

change in internal energy (change in temperature)

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18
Q

What does W represent in the first law of thermodynamics?

A

work done by the system

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19
Q

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

A
  • W is positive

- the gas expands

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20
Q

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

A
  • W is negative

- the gas contracts

21
Q

What is meant by an isothermal change?

A

no temperature change

22
Q

What does ∆U equal in an isothermal change?

A

∆U = 0

23
Q

Give an example/situation when an isothermal change would occur

A
  • slow change

- e.g. ‘frictionless’ piston warmed slowly

24
Q

What is meant by an adiabatic change?

A

no heat change to or from the surroundings

25
Q

What does Q equal in an adiabatic change?

A

Q = 0

26
Q

Give an example when an adiabatic change would occur

A
  • quick/insulated change

- e.g. fire piston

27
Q

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

A

W = 0

28
Q

What is meant by an isobaric change?

A

a change that occurs at constant pressure

29
Q

Give 3 aspects of the second law of thermodynamics

A
  • 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
30
Q

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

A

1:9

31
Q

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

A

1:20

32
Q

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

A
  • 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
33
Q

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

A
  • intake
  • compression
  • power
  • exhaust
34
Q

Efficiency of a heat engine is always…

A

less than 100%

35
Q

What is the equation for mechanical efficiency?

A
  • η = Pout / Pindicator

- mechanical efficiency = (brake power output) / (indicated power)

36
Q

What is the equation for thermal efficiency?

A
  • ε = Pindicator / Pinput

- thermal efficiency = (indicated power) / (input power)

37
Q

What are the equations for overall efficiency?

A
  • η x ε

- Pout / Pinput

38
Q

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

A
  • divide the value they give by 2

- as the power stroke happens once every 2 cycles

39
Q

What is the range of values for maximum theoretical efficiency?

A

maximum theoretical efficiency < 100%

40
Q

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

A

measured in Kelvin [K]

41
Q

Why can theoretical efficiency never be 100%?

A
  • for 100% efficiency, Tc = 0K or Th = ∞K

- neither of which are possible

42
Q

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

A
  • ideally Th as high as possible

- (can’t usually change Tc as this is usually the temperature of the surroundings)

43
Q

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

A
  • would result in performance issues

- metals become more ‘sticky’ when they are heated, increasing friction and mechanical inefficiency

44
Q

State and explain 3 causes of thermal inefficiency

A
  • 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
45
Q

State and explain 2 causes of mechanical inefficiency

A
  • friction between moving parts wasted as heat energy

- oil is viscous and therefore has a resistance to motion (of the moving parts)

46
Q

List the 6 steps for a refrigerator

A
  • 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
47
Q

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

A

by keeping it under pressure

48
Q

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

A
  • 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
49
Q

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

A
  • 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

AQA A-Level Physics (21 decks)

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