Engineering Flashcards by Sam Hillman

Define angular speed

the rate of change of angular displacement

How well did you know this?

Not at all

Perfectly

Define angular acceleration

the rate of change of angular speed

How well did you know this?

Not at all

Perfectly

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

I = m(r^2)

How well did you know this?

Not at all

Perfectly

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

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

How well did you know this?

Not at all

Perfectly

What does T represent in the equation T=Iα

RESULTANT torque

How well did you know this?

Not at all

Perfectly

What is meant by torque?

turning effect

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

How well did you know this?

Not at all

Perfectly

What does I represent in rotational dynamics?

moment of inertia

How well did you know this?

Not at all

Perfectly

What is a flywheel?

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

How well did you know this?

Not at all

Perfectly

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

I (moment of inertia)

- ω (angular speed)

How well did you know this?

Not at all

Perfectly

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

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

How well did you know this?

Not at all

Perfectly

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

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

How well did you know this?

Not at all

Perfectly

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

have to compromise

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

How well did you know this?

Not at all

Perfectly

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

GPE = linear KE + rotational KE (+ heat)

How well did you know this?

Not at all

Perfectly

Angular momentum is conserved unless…

external torques act on the system

How well did you know this?

Not at all

Perfectly

What is the first law of thermodynamics?

Q = ∆U + W

How well did you know this?

Not at all

Perfectly

What does Q represent in the first law of thermodynamics?

heat energy into the system

How well did you know this?

Not at all

Perfectly

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

change in internal energy (change in temperature)

How well did you know this?

Not at all

Perfectly

What does W represent in the first law of thermodynamics?

work done by the system

How well did you know this?

Not at all

Perfectly

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

W is positive

- the gas expands

How well did you know this?

Not at all

Perfectly

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

Study These Flashcards

W is negative

- the gas contracts

What is meant by an isothermal change?

Study These Flashcards

no temperature change

What does ∆U equal in an isothermal change?

Study These Flashcards

∆U = 0

Give an example/situation when an isothermal change would occur

Study These Flashcards

slow change

- e.g. ‘frictionless’ piston warmed slowly

What is meant by an adiabatic change?

Study These Flashcards

no heat change to or from the surroundings

What does Q equal in an adiabatic change?

Q = 0

Give an example when an adiabatic change would occur

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

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

W = 0

What is meant by an isobaric change?

a change that occurs at constant pressure

Give 3 aspects of the second law of thermodynamics

- 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

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

1:9

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

1:20

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

- 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

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

- intake - compression - power - exhaust

Efficiency of a heat engine is always...

less than 100%

What is the equation for mechanical efficiency?

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

What is the equation for thermal efficiency?

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

What are the equations for overall efficiency?

- η x ε | - Pout / Pinput

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

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

What is the range of values for maximum theoretical efficiency?

maximum theoretical efficiency < 100%

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

measured in Kelvin [K]

Why can theoretical efficiency never be 100%?

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

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

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

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

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

State and explain 3 causes of thermal inefficiency

- 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

State and explain 2 causes of mechanical inefficiency

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

List the 6 steps for a refrigerator

- 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

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

by keeping it under pressure

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

- 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

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

- 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

Engineering Flashcards

(49 cards)