Engineering physics

Question 1

moment of interia

Answer 1

a measure of an object’s resistance to rotational acceleration around a axis

Question 2

moment of inertia equation

Answer 2

I=mr^2

Question 3

factors which affect moment of inertia

Answer 3

• an objects total mass
• how mass is distributed about the axis of rotation

Question 4

equation for new moment of inertia

Answer 4

Inew = I + mr^2

Question 5

what is a flywheel?

Answer 5

• a heavy metal drisk that spins on an axis and has a large moment of intertia.
• its accumilates and stores energy

Question 6

uses of flywheels

Answer 6

• regenerate breaking
• wind turbines
• smoothing torque & angular velocity

Question 7

Regenerative breaking

Answer 7

when brakes are applied a flywheel is engaged and it will use the energy lost by braking to “charge up”

Question 8

Wind turbines

Answer 8

• flywheels are used to store excess power on very windy days or when electricity is not being used up as quickly as it is being made.
• when there is a reduction in electricity produced, the flywheel can be used to ourput power.

Question 9

smoothing torque and angular velocity

Answer 9

• some systems do not produce power continuously but rather in bursts
• torque supplied will fluctuate
• flywheels will charge up the burst of power and can output it smoothly to the system

Question 10

Factors which affect the amount of energy that can be stored in a flywheel:

Answer 10

• mass of flywheel
• angular speed of flywheel
• friction
• using a flywheel that is spoked / has more mass concentrated as it edges

Question 11

using a flywheel that is spoked / has more mass concentrated as it edges

Answer 11

• increase the amount of energy stored
• moment of inertia of the object will be larger if most masses are far from the axis of rotation

Question 12

how to reduce friction of a flywheel

Answer 12

• lubricating bearings
• use bearings made of superconductors → allows the flywheel to levitate and have no contact with bearing
• use flywheels in vacuums or sealed containers to reduce air resistance

Question 13

Torque (T)

Answer 13

the product of a force and its distance from its axis of rotation (Nm)

Question 14

equation for torque

Answer 14

T = Fr

Question 15

how can angular acceleration be increased?

Answer 15

• increasing m and so increasing the torque
• using a lighter wheel, which will decrease the moment of inertia

Question 16

angular momentum

Answer 16

the product of the moment of inertia and angular velocity of an object (Nms)

Question 17

angular momentum equation

Answer 17

angular momentum = Iω

Question 18

law of conservation of momentum

Answer 18

when no external torque acts, the angular momentum of a system remains constant.

Question 19

angular impulse

Answer 19

the product of torque and its duration

Question 20

angular impulse equation

Answer 20

TΔt = Δ(Iω)

Question 21

what is the area of a torque-time graph>

Answer 21

angular impulse

Question 22

work done (W)

Answer 22

the force causing a motion multiplied by the distance travelled

Question 23

Power (P)

Answer 23

rate of energy transfer

Question 24

First law of thermodynamics

Answer 24

The increase in internal energy of a system is equal to the sum of the thermal energy added to the system and the work done on it

Question 25

thermodynamics equation

Answer 25

Q=ΔU+W

Question 26

Internal energy (U)

Answer 26

the sum of all kinetic energies and potential energies of all its particles

Question 27

Non-flow processes

Answer 27

- changes which occur in closed systems

Question 28

assumptions for the 1st law of thermodynamics to non-flow processes

Answer 28

- the gas in the system is ideal

Question 29

adiabatic change (Boyles law) equation

Answer 29

pV^γ = constant

Question 30

Isothermal change (Boyles law) equation

Answer 30

pV = constant

Question 31

adiabatic process

Answer 31

no energy is transferred in or out of the system

Question 32

adiabatic process first law application

Answer 32

Q = 0 ΔU = -W

Question 33

isothermal process

Answer 33

the temperature of the system is kept constant

Question 34

isothermal process first law application

Answer 34

ΔU = 0 Q = W

Question 35

Constant pressure

Answer 35

the pressure of a system is kept constant

Question 36

constant volume

Answer 36

the volume of the system is kept constant

Question 37

constant volume first law application

Answer 37

W = 0 Q = ΔU

Question 38

what's a cyclic process

Answer 38

the system undergoes two or more processes one after another and returns to its initial volume, temperature and pressure

Question 39

net work done during cyclic process equation

Answer 39

work done by cycle = area of loop

Question 40

four-stroke petrol engine

Answer 40

burns fuel every four strokes of the piston (every two revolutions)

Question 41

What are the four strokes of a petrol engine, in order?

Answer 41

Induction → Compression → Expansion (power) → Exhaust.

Question 42

What happens during the induction stroke?

Answer 42

Piston moves down, inlet valve open, fuel–air mixture drawn in at nearly constant pressure.

Question 43

What happens during the compression stroke?

Answer 43

Inlet valve closes, piston moves up, mixture compressed, pressure rises sharply; spark ignites fuel at almost constant volume.

Question 44

What happens during the expansion (power) stroke?

Answer 44

Gas expands, does work on piston, pushing it down; pressure decreases as volume increases.

Question 45

What happens during the exhaust stroke?

Answer 45

Exhaust valve opens, piston pushes burnt gases out.

Question 46

Key difference in ignition: petrol vs diesel?

Answer 46

Petrol uses a spark; diesel ignites fuel by high compression (no spark).

Question 47

Formula for indicated power?

Answer 47

Actual output power at the engine shaft. Equation: P = Tω.

Question 48

What is friction power?

Answer 48

Indicated power − Brake power.

Question 49

Thermal efficiency?

Answer 49

Indicated power / Input power.

Question 50

Overall efficiency?

Answer 50

Brake power / Input power.

Question 51

Mechanical efficiency?

Answer 51

Brake power / Indicated power.

Question 52

What is a reversed heat engine?

Answer 52

A device that uses work to move heat from cold → hot region.