Engineering Physics

Question 1

Advantages of flywheels

Answer 1

They are very efficient

They last a long time without degrading

The recharge time is short

They can react and discharge quickly

They are environmentally friendly (don’t rely on chemicals to store energy)

Question 2

Disadvantages of flywheels

Answer 2

They are much larger and heavier than other methods

The pose a safety risk as the wheel could break apart at high speeds

Energy can be lost through friction

If used in moving objects they can oppose changes in directions which can cause problems for vehicles

Question 3

Increasing the energy stored in a flywheel

Answer 3

Increase the mass - the moment of inertia and hence the stored kinetic energy is directly proportional to mass

Increase the angular speed

Make it spoked - compared to a solid wheel, a spoked wheel of the same mass stores about twice as much energy

Question 4

Reducing friction in flywheels

Answer 4

Flywheels lose energy to friction and air resistance

Lubrication

Levitate it with superconducting magnets - no contact with bearings

Operate them in vacuums or inside sealed cylinders to reduce drag from air resistance

Question 5

Uses of flywheels - potters wheels

Answer 5

Controlled by a pedal so hard to apply a constant force

A flywheel is used to keep the speed of the wheel constant

Question 6

Uses of flywheels - regenerative braking

Answer 6

In regular vehicles, applying the brakes causes the wheels to slow down, generating lots of heat

However, in some vehicles when the brakes are used a flywheel is engaged - charging it up

When the vehicles is ready to accelerate the flywheel can be used to turn the vehicle’s wheels

Question 7

Uses of flywheels - wind turbines

Answer 7

Flywheels can be used to store excess power on windy days or during off-peak and give power on days without wind

Question 8

Uses of flywheel - power grids

Answer 8

When lots of electricity is used in an area, the electricity grid can’t meet the demand

Flywheels can be used to provide the extra energy needed whilst backup power stations are started up

Question 9

Uses of flywheels - riveting machines

Answer 9

An electric motor charges up a flywheel, which then rapidly transfers a burst of power as the machine presses down on the river

Useful as it stops rapid changes in power going through the motor - which could cause a stall

Also a less powerful motor can be used

Question 10

Flywheels smoothing torque and angular velocity

Answer 10

In systems where the force supplied can vary a flywheel can be used to keep the angular velocity of rotating components constant

They deliver stored energy smoothly to the rest of the system

They can also smooth out force exertion by a system

Question 11

Conservation of angular momentum

Answer 11

The angular momentum of a system remains constant unless external torque acts on the system

Question 12

Isothermal changes

Answer 12

Changes that occur at a constant temperature so the internal energy of the gas doesn’t change

So the work done on or by a system is equal to the heat energy supplied

Q = W

Must take place very slowly in order for no energy to be transferred to dU

Question 13

Adiabatic processes

Answer 13

No heat is lost or gained by the system so Q = 0 and dU = -W

So if work is done by the system W will be positive so internal energy will decrease

A change in temperature occurs as the internal energy only depends on temperature

Must take place fast enough that no energy is able to be transferred to the surroundings

Question 14

Work done and constant pressure

Answer 14

For processes where the pressure doesn’t change W = p dV

For expansion the change in volume and the work done are positive

For compression the change in volume and the work done are negative

Question 15

Work done and constant volume

Answer 15

No work is done W = 0 so Q = dU

So by transferring heat energy to he system you only increase its internal energy

Question 16

P-V curves - isothermal

Answer 16

Smooth curve

area under the curve is work done

Curves are called isotherms

Position of isotherm depends on the temperature

Higher the temperature the further from the origin the isotherm will be

Question 17

Adiabatic p-V diagrams

Answer 17

Similar to isotherms but they have a steeper gradient

More work is done to compress gas adiabatically rather than isothermally

Less work is done to expand has adiabatically rather than isothermally

Question 18

Four stroke engine stages: induction

Answer 18

Piston starts at the top of the cylinder and moves down increasing the volume of gas above it

This sucks in a mixture of fuel and air through the open inlet valve

The pressure of the gas in the cylinder remains constant - just below atmospheric pressure

Question 19

Four-stroke engine stages: compression

Answer 19

The inlet valve is closed

The piston moves back up the cylinder and does work on the gas, increasing the pressure

Just before the piston is at the end of this stroke, the spark plug creates a spark which ignites the air-fuel mixture

The temperature and pressure suddenly increase at an almost constant volume

Question 20

Four-stroke engine stages: expansion

Answer 20

The hot air-fuel mixture expands and does work on the piston, moving it downwards

The work done by the gas as it expands is more than the work done to compress the gas, as it is now at a higher temperature. There is a net output of work

Just before the piston is at the bottom of the stroke, the exhaust valve opens and the pressure reduces

Question 21

Four-stroke engine stages: exhaust

Answer 21

Piston moves up the cylinder and the burnt gas leaves through the exhaust valve

The pressure remains almost constant just above atmospheric pressure

Question 22

Four-stroke Diesel engines

Answer 22

Undergo the same four strokes as petrol engines

But during the induction stroke only air is pulled into the cylinder, not an air-fuel mixture

Compression stroke - air is compressed to a high enough temperature that it ignites diesel fuel sprayed into the cylinder

For the indicator diagram there isn’t a sharp peak at the start of the expansion stroke - diesel burns it doesn’t explode

Question 23

Assumptions for theoretical models for engine cycles

Answer 23

Same gas is taken continuously around the cycle - pure air with adiabatic constant = 1.4

Pressure and temperature changes can be instantaneous

Heat source is external

The engine is frictionless

Question 24

First law of thermodynamics

Answer 24

Q = dU + W

Q is energy transferred by heating either to (+) or from (-) the system

dU is the change in internal energy

W is the work done to (-) or by (+) the system

Question 25

Differences between theoretical and real engine diagrams

Answer 25

Rounded corners - inlet and exhaust valves take time to open and close

Heating doesn’t take place at a constant volume - pressure and temperature increase isn’t instantaneous

Theoretical model doesn’t include small amount of negative work it assumes the same air cycles around continuously

Area inside the loop is slightly less due to friction

Fuel is never completely burnt - so you can’t get maximum energy out and pressures are higher in theoretical model

Engines have an internal heat source, not an external one - temperature rise isn’t as large in the theoretical model due to the assumption of an external heat source

Heat is lost - not adiabatic, heat is lost to metal surroundings

Question 26

Engine efficiency - mechanical efficiency

Answer 26

= brake power/indicated power

Question 27

Engine efficiency - thermal efficiency

Answer 27

Thermal efficiency = indicated power/input power

Question 28

Overall engine efficiency

Answer 28

Overall efficiency = brake power/input power

Question 29

Second law of thermodynamics applied to engines

Answer 29

Heat engines must operate between a heat source and a heat sink (a region which absorbs heat from the engine)

1) Heat energy transferred from the heat source to the engine is Qh
2) Some of this energy is converted into useful work, W
3) But some of this energy (Qc) must be transferred to a heat sink, which has a lower temperature (Tc) than the heat source
4) This means engines can never be 100% efficient

Question 30

CHP plants

Answer 30

Engines are very inefficient - lots of heat is is transferred to the surroundings

Combined heat and power plants (CHP) try to limit energy waste by using it for other purposes - e.g. Heating houses and businesses nearby

Markinch Biomass CHP plant in Scotland - excess heat is used to create steam to dry paper in the paper mill

Question 31

Reversed heat engines

Answer 31

Operate between hot and cold reservoirs

Direction of energy transfer - heat energy is taken from the cold reservoir and transferred to the hot reservoir

For reversed heat engines sources and sinks are instead called hot and cold spaces

To transfer heat from a colder space to a hotter space work must be done

Question 32

Refrigerators

Answer 32

Aims to extract as much heat energy from the cold space as possible for each joule of work done

The cold space is inside the refrigerator whilst the hot space is the room the refrigerator is in

Refrigerators keep enclosed spaces cool that can then be used to store perishable food to keep it fresh for longer

Question 33

Heat pumps

Answer 33

Aims to pump as much heat as possible into the hot space per joule of work done

Cold space is usually outdoors and the and the hot space is the inside of the house

Used to heat rooms and water in homes

Question 34

Coefficient of performance

Answer 34

A measure of how well the work done is converted into heat transfer

Similar to efficiency but can be above 1

E.g. Heat pump with coefficient of performance of 4 transfers 4J for every 1J of work done

Question 35

Uses of flywheels

Answer 35

Potters wheel 
Regenerative braking 
Power grids 
Wind turbines
Riveting machines
Smoothing torque, energy delivery and angular velocity

Question 36

Friction power

Net Torque

Answer 36

Indicated power - output power

Applied torque - frictional torque

Question 37

Function of the crankshaft

Answer 37

Converts the up and down motion of the piston into rotational motion

Question 38

What would happen to an engine if it didn’t operate between a heat source and a heat sink?

Answer 38

The engine will reach the same temperature as the heat source and so no more heat energy will flow

This is why the second law of thermodynamics and its application to heat engines so important

Question 39

Flywheels in crankshaft

Answer 39

Flywheel maintains angular momentum, helping to take the crankshaft over dead centres

There is a variation in engine torque due to changes in engine pressure, the flywheel is used to smooth out these changes

The greater the flywheels moment of inertia, the less fluctuations there are in angular velocity over one cycle

Flywheel stores energy when the gas expands, doing work on the piston and uses this energy to compress the gas

Question 40

How do you know if the overall change of internal energy of a cycle is zero?

Answer 40

The cycle will return to the same initial temperature

Change in internal energy is only dependent on temperature

Question 41

Unit for angular momentum

Answer 41

kg m^2 s^-1

Question 42

Law of conservation of angular momentum

Answer 42

Angular momentum of a system remains constant unless acted on by an external torque

Question 43

Flywheel design features to store maximum kinetic energy

Answer 43

Put more mass at a greater radius (e.g. make it spoked) - increases moment of inertia, use a thin axle

Use high density materials - larger mass for a given size

Low friction bearings and smooth outer surface

Rotational speed limited due to the tensile strength of the material - due to the centripetal force acting on the material

Needs to be perfectly balanced to avoid any adverse gyroscopic effects

Question 44

How does a heat pump obey the second law of thermodynamics and the law of conservation of energy?

Answer 44

Obeys 2nd law of thermodynamics as it operates between hot and cold spaces and work is done to move energy against the temperature gradient.

Obeys law of conservation of energy as although it delivers more energy than is supplied to it this is because the additional energy is the energy transferred from the hot space.