engineering physics Flashcards
what is inertia?
a measure of how much an object resists a change in velocity (more inertia= more it resists)
what is the moment of inertia?
how difficult it is to rotate an object or change its rotational speed
what is angular displacement?
the angle through which a point has been rotated
what is angular velocity?
the angle a point rotates through per second
what is angular speed?
the magnitude of the angular velocity
what is angular acceleration?
the rate of change of angular velocity
what is a couple?
a pair of forces that cause no resultant linear motion but cause and object to turn
what is torque?
the turning effect from a force causing an object to turn
what is a flywheel?
a heavy wheel that has a high moment of inertia. designed to store as much energy as possible
what happens to the input torque in flywheels?
its converted to rotational kinetic energy which is then stored for later use
how can you increase the energy storage capacity of a flywheel?
increase its mass- the moment of inertia and sense KE stored is directly proportional to the mass
increase its angular speed- the energy stored increases with angular speed squared
use a wheel with spokes- compared to a solid wheel with the same mass, moment of inertia will be higher as distance is further
why are there limits on energy storage capacity of flywheels?
a giant heavy wheel, can take up over half the machined is impractical
if you increase angular velocity too much, the centrifugal force can increase to a point where it breaks
whats the best material to make flywheels out of and why?
carbon fibre. allows high angular speeds to be used without the flywheel disintegrating
how do we reduce friction in flywheels?
lubricate to reduce friction between bearings and wheel
levitate with superconducting magnets
operate in vacuums or inside sealed chambers to reduce drag from air resistance
what are the most common uses for flywheels?
potters wheels, to keep the speed constant
regenerative braking- store energy when you break then uses the energy to move the wheels faster when you accelerate
power grids- store power during low demands and then provide extra energy at high demand times
wind turbines- store excess on windy days then give power on days without wind
riveting machines
advantages of flywheels
very efficient
last a long time without degrading
recharge time is short
they can charge and discharge quickly
environmentally friendly
disadvantages of flywheels
much larger and heavier then other storage methods
pose a safety risk as the wheel could break apart
energy can be lost through friction
if used in moving objects, can oppose changes in direction
what is angular impulse?
the change in angular momentum
what is angular impulse?
the change in angular momentum
what’s the first law of thermodynamics?
describes how much energy is conserved in a system through heating, cooling and doing work
Q=change in U + W
WHERE
Q=energy transferred to the system
U=increase in internal energy
W=work done by the system
what is a system?
a volume of space filled with gas
can either be open or closed
what’s a closed system?
they don’t allow gas to enter or escape
what’s an open system?
allows gases to flow in, out and through them
how can you work out the internal energy of a system?
it’s the sum of the potential and kinetic energies of all the particles in the system
when is Q negative for thermodynamics?
if energy is transferred away from the system
when is W negative for thermodynamics?
if work is done on the gas, compressing it
what are non-flow processes?
in a closed system, the gas doesn’t go anywhere it cannot flow in and out of the system
how can you apply the first law of thermodynamics to changes in closed systems?
-you have to assume the gas in the system is an ideal gas
-so internal energy is only dependant on temperature
-you need to assume that if any work is done, this causes a change in volume
-a change in heat energy can cause a change in volume, temperature and pressure
-you can use the ideal gas equation and R and n are constant in a closed system so p1V1/T1=p2V2/T2
what does isothermal mean?
the temperature stays the same so the change in internal energy is 0
what is an adiabatic process?
no heat is transferred in or out of the system Q=0
how do you work out work done at a constant pressure?
pressure x change in volume
when the volume of a system is kept constant, what is the work done?
0
what’s an isotherm?
a p-V curve for a isothermal process
how does a p-V curve for an isothermal process change with heat?
as it gets hotter, further away from axis
how does the p-V graph differ for adiabatic processes compared to isothermal?
adiabatic has a steeper gradient
-more work is done for compression but less work done during expansion
how do you find the net work done during a cyclic process?
find the difference between the work done by the system and the work done on the system
-this is equal to the area of the loop
what are 4 stroke engines?
engines that burn fuel once every four strokes of a piston
what are indicator diagrams?
p-V diagrams to illustrate the strokes on an engine
what are the four strokes on a piston of a four stoke petrol engine called?
- induction
- compression
- expansion
- exhaust
what happens during the induction stroke in a 4 stroke petrol engine?
the piston starts at the top of the cylinder and moves down
-increasing the volume of the gas above it
-sucks in a mixture of fuel and air through an open inlet valve
-pressure of the gas in the cylinder remains constant, just below atmospheric pressure
-indicator diagram is a straight horizontal line
what happens during the compression stroke of a 4 stroke petrol engine?
the inlet valve is closed and the piston moves back up the cylinder
-this 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 indicator diagram shows a decrease in volume and increase in pressure, almost vertical after spark
what happens during the expansion stroke of a 4 stroke petrol engine?
the hot air-fuel mixture expands and does work on the piston, pushing it downwards
-the work done by the gas as it expands is more then the work done to compress it as its now at a higher temperature
-just before the piston is at the bottom of the stroke, the exhaust valve opens and pressure reduces
-indicator diagram shows an increase in volume and decrease in pressure
what happens during the exhaust stroke of a 4 stroke petrol engine?
the piston moves up the cylinder and the burned gas leaves through the exhaust valve
-pressure remains almost constant, just above atmospheric pressure
-indicator diagram shows a horizontal straight line as volume decreases at constant pressure
how does a four stroke diesel engine differ from a four stroke petrol engine?
-on the induction stroke, only air is pulled into the cylinder not a fuel-air mix
-don’t have a spark plug, so in the compression stroke the air is compressed until it reaches a temperature high enough to ignite the diesel
-just before the end of the stroke, diesel is sprayed into the cylinder through a fuel injector and ignites
-indicator diagram differs as the peak isn’t sharp
what’s an Otto cycle?
the theoretical cycle for a four stroke petrol engine
whats a diesel cycle?
the theoretical cycle for a four stroke diesel engine
what are the assumptions made for a theoretical engine cycle?
-the same gas is continually taken round the cycle
-gas is pure with an adiabatic constant
-pressure and temperature changes can be instantaneous
-heat source is external
-engine is frictionless
for real engine indicator diagrams, why are the corners rounded?
the inlet and exhaust valves take time to open and close and let new air in and burnt gas out
in a real 4 stroke engine why doesn’t heating take place at a constant volume?
because then increase in pressure and temperature would have to be instantaneous
why is the temperature rise in a real four stroke engine never as much as a theoretical one?
they have a internal heat source, the fuel used to heat the gas is near completely burned in the cylinder so you can never get the maximum energy out of it
what is indicated power of an engine cylinder?
the net work done by the cylinder in one second
(maximal theoretical power generated by the gases in the engine cylinders)
how can you calculate indicated power?
(area of p-V loop) x (number of cycles per second) x (number of cylinders)
how is a piston connected to the crankshaft?
by a stiff rod
how do you calculate the output/ brake power of an engine?
(engine torque) x (angular velocity of the crankshaft)
how do you work out friction power?
indicated power- brake power
what is input power for an engine?
the amount of heat energy per unit time it could potentially gain from burning fuel
how can you calculate input power?
(calorific value of fuel) x (fuel flow rate)
what is mechanical efficiency effected by?
the amount of energy lost through moving parts
how do you calculate mechanical efficiency?
brake power/ indicated power
what does thermal efficiency describe?
how well heat energy is transferred to work done
how can you calculate thermal efficiency?
indicated power/ input power
how do you calculate overall efficiency for an engine?
brake power/ input power
what is the second law of thermodynamics?
heat engines must operate between a heat source and a heat sink
no engine can transfer all the heat energy supplied into useful work done
why can no engine transfer all heat energy into useful work done?
some heat always ends up increasing the temperature of the engine
what happens if the engine temperature reaches the temperature of the heat source?
then no heat flows and no work is done
what is a heat sink?
a region that absorbs heat from the engine
why is real het engine’s efficiency lower than its theoretical maximum?
energy is lost through the engines components due to friction and the energy that is needed to move them
and fuel doesn’t burn entirely so value of Qh is lower in practice
what are reversed heat engines?
taking heat energy from a colder space to a hotter space
what is the coefficient of performance?
a measure of how well the work is converted into heat transfer
it represents the amount of joules of heat energy transfers for every 1J of work done
how does COP effect running cost?
higher the COP, lower the running cost
if a refrigerator is running at max theoretical efficiency how can you calculate COP?
temp of cold space/ (temp of hot- cold space) in kelvin
