Applied Flashcards
When rotating, which direction is positive?
clockwise
what is the symbol of angular acceleration?
α
what is torque equal to?
radius * force
what are the units for moment of inertia?
kg m²
what are the units for angular momentum?
kg m² s⁻¹
what is moment of inertia for an object turning in a cycle?
mass times radius squared.
what is the symbol for angular momentum?
L
How is angular momentum found from linear momentum?
linear * radius
what is change of momentum equal to?
time * ΔTorque
what is the angular equivalent of force?
Torque. Symbol of T
Torque = force * radius
If you had a rotating satellite in space, why would you need two rockets to stop it spinning?
Single would give unwanted linear acceleration.
Two would give a couple with only angular acceleration.
What is the internal energy of an ideal gas?
As there is no attraction between the particles, the only energy is the KE
what is the symbol for internal energy?
u
What is the first law of thermodynamics?
conservation of energy
Heat energy applied to a system either increases the internal energy of a system or enables it to do work
What is meant by the work done on a gas?
The gas is compressed and u increases.
In the equation on the formula sheet, the w will be negative.
What are the ways of finding the work done to a gas when compressed?
forcedistance the piston went
force = pressurearea
work=pressureareadistance
What is adiabatic change?
No heat flow into or out of the system. Q=0
if doing an experiment, perform It very quickly to prevent energy from being transferred.
Thus any increase in the internal energy is equal to the work done on it.
What is an isobaric process?
A process that takes place under constant pressure
What is the area under a pressure volume graph?
Area under curve = work done.
What is torque?
Product of a force and the perpendicular distance of the line of action of the force from a particular point of rotation.
What is Th and Tc?
Th= temperature of the hot reservoir and Tc is the temperature of the cold reservoir.
The first law of thermodynamics can be written
ΔQ=ΔU+ΔW
State the meaning of the terms
ΔQ heat energy added to the gas
ΔU increase in the internal energy of gas
ΔW work done by the gas
What is an isovolumetric change?
Constant volume
Work is 0
ΔU=ΔQ
what is an equation that relates energy in and out to temperature in and out?
Qout / Qin = Tc/Th
what does the P/V diagram for a petrol engine look like?
Starts at A. Curved line goes left and up to B
From B vertically up to C
Curved line right and down to D.
Vertically down from D to A. Should be a shorter distance than from B to C
Explain the compression stroke from A to B
The compression is adiabatic, and no heat enters or leaves the cylinder.
what does the P/V diagram of a engine look like in real life?
The corners are rounded
What does the P/V cycle for a diesel engine look like?
A curved line from A to B. A straight horizontal line to C. Curved line down to D. Vertical line straight down to A.
Why is an engine not 100% efficient?
Fuel air mixture doesn’t act as an ideal gas
Friction
Heat lost to the walls of the engine.
Explain the P/V cycle of a diesel engine
From A to B the piston goes up and adiabatic compression occurs. It gets hot enough to ignite diesel.
B to C. Fuel is spayed into cylinder and ignites. This is the first part of the power stroke.
C to D. Fuel supply is cut off and adiabatic expansion occurs. 2nd part of power stroke
D to A. exhaust has leaves.
Advantage of a diesel engine over petrol.
As no fuel is in the cylinder during the compression, it can achieve a higher compression ratio and so it is more efficient.
Disadvantage of diesel over petrol
Costs more to make as it has to hold larger pressure and it is heavier meaning a worse power to weight ratio.
When a engine has gone through one revolution, how many cycles have occurred?
0.5
What is thermal efficiency?
indicated power / input power
What is mechanical efficiency?
Output power(power at the wheels) / indicated power
What is overall efficiency?
Overall efficiency = output power ÷ input power of the fuel.
Why does mechanical efficiency fall as the engine goes faster?
The amount of friction in the engine increases.
What is a isovolumetric process?
one where volume is constant
work done by gas is 0
What is an isothermal process?
same temperature any change is slow pressure * volume= constant internal energy is constant change in heat=change in work
If a gas was compressed adiabatically and then isothermally, which process would have produced more work?
The isothermal one as there is a larger area under a p/v curve
what is the coefficient of performance?
Applies to heat pumps and fridges.
The ratio of the heat energy removed or added to the work done to move it.
What is a reverse heat engine?
Work is done to move heat energy from a cold sink to a hot sink
Same as a heat pump and refrigerator but they have different jobs.
A heat pump is meant to heat an area while a refrigerator cools an area.
What are the units of angular impulse?
Newton meter seconds or kg m^2 s^-1
Explain the terms in the coefficient of performance of a refrigerator
Qout = heat removed from the cold area Qin = heat energy given to the surroundings
In a heat pump, how much energy is sent to the hot sink?
Qin = work + Qout
Why is the COP of a reverse heat engine larger when working as a heat pump compared to a refrigerator?
Qin = W + Qout
Thus Qin is always larger than Qout
Look at the COP equations for both. As Qin is larger COPhp is larger.
Why in a Diesel engine, fuel starts to be injected in the cylinder before the piston reaches the highest height?
To allow the fuel to partially evaporate and burn before the head gets to the highest point for the greatest pressure.
If you draw the real P/V diagram on the theoretical one, what does it look like?
It should be completely within the theoretical and have rounded corners.
On a real P/V diagram, what does the horizontal line on the bottom represent?
The induction and exhaust stroke
Why is the real engine less efficient than the theoretical one?
Always some exhaust gas in cylinder, taking up space
Internal friction between moving surfaces is overcome
Area of loop is less.
Compression and expansion aren’t infinity quick, thus not adiabatic, thus less efficient.
Real engine has pumping loop which requires energy. Theoretical has same air taken through repeated cycles.
Explain the difference in the shape of the real and theoretical diesel engine loop
Heating doesn’t occur at constant pressure, impossible to control rate of burning of fuel during injection
Cooling can’t happen at constant volume as engine would have to stop
Corners are rounded because valves are needed and take a finite time to open and close.
