Engineering Physics Flashcards
Advantages of flywheels
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)
Disadvantages of flywheels
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
Increasing the energy stored in a flywheel
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
Reducing friction in flywheels
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
Uses of flywheels - potters wheels
Controlled by a pedal so hard to apply a constant force
A flywheel is used to keep the speed of the wheel constant
Uses of flywheels - regenerative braking
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
Uses of flywheels - wind turbines
Flywheels can be used to store excess power on windy days or during off-peak and give power on days without wind
Uses of flywheel - power grids
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
Uses of flywheels - riveting machines
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
Flywheels smoothing torque and angular velocity
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
Conservation of angular momentum
The angular momentum of a system remains constant unless external torque acts on the system
Isothermal changes
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
Adiabatic processes
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
Work done and constant pressure
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
Work done and constant volume
No work is done W = 0 so Q = dU
So by transferring heat energy to he system you only increase its internal energy
P-V curves - isothermal
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
Adiabatic p-V diagrams
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
Four stroke engine stages: induction
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
Four-stroke engine stages: compression
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
Four-stroke engine stages: expansion
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
Four-stroke engine stages: exhaust
Piston moves up the cylinder and the burnt gas leaves through the exhaust valve
The pressure remains almost constant just above atmospheric pressure
Four-stroke Diesel engines
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
Assumptions for theoretical models for engine cycles
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
First law of thermodynamics
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