engineering- thermodynamics Flashcards
what is the Kelvin scale?
the absolute temperature scale
how do you calculate kelvin from degrees celcius?
add 273
what is Boyle’s law?
the pressure of a gas is inversely proportional to the volume
what is Charles’ law?
the volume occupied by a gas is proportional to its absolute temperature
what is the pressure law?
pressure of a gas is proportional to its absolute temperature
what is x?
p ∝ 1/x
V
what is x?
V ∝ x
T (or 1/p)
what law states p ∝ 1/V
Boyle’s law
for the pressure law, what is x?
p ∝ x
T
what is the ideal gas law?
pV = nRT
what equation links number of moles and number of molecules?
number of moles = number of molecules / avogadro’s constant
n = N/ NA
what are 5 assumptions made when modelling an ideal gas?
- its made of tiny spheres
- the spheres obey Newton’s laws of motion
- they occupy a negligible space in comparison with the volume occupied by the gas
- the spheres don’t exert non-contact forces on one another
- collisions involving the spheres are totally elastic
what is the first law of thermodynamics?
Q = ∆U + W
heat energy = internal energy + work done
what is the symbol for heat energy?
Q
what is the symbol for internal energy?
∆U
what is the symbol for work done?
W
which 3 ways can energy change in a closed system?
- heat energy can enter or leave
- work can be done on or by the system
- internal energy store of the system can increase or decrease
what has happened when a system expands?
work is done BY the system
what has happened when temperature decreases?
particles lose energy
when is the value for heat positive?
when heat goes into the system
what happens when work is done on the system?
it is compressed
what is constant during an isothermal process?
∆U = 0
in which non-flow process does the temperature stay the same?
isothermal
what does adiabatic mean?
without the transfer of heat
what happens in an adiabatic process?
no heat is gained or lost by the system
what does the 1st law of thermodynamics become for an adiabatic procces?
∆U = -W
or 0 = ∆U + W
in which non-flow process is Q = 0
adiabatic
what happens to the ideal gas equation for an isothermal process?
pV = constant
constant = pV = nRT
what has happened when a gas expands?
work has been done by the gas
what does the 1st law of thermodynamics become for a constant volume process?
Q = ∆U
which process has the equation:
work done, W = p∆V
constant pressure processes
what does γ (gamma) represent?
(DF + 2) / DF
DF= number of degrees of freedom
does an isothermal or adiabatic change have a steeper curve on a P/V graph?
adiabatic
which process moves from one isotherm to another on a P/V graph?
adiabatic change
what does the area under a graph show on a p/v graph?
work done
in what process is pV^γ = constant?
adiabatic
what is constant in an isothermal process?
pV
which way does the arrow point on a P/v graph for an expansion?
right
which way does the arrow point on a P/v graph for a compression?
left
which is happening to a gas if the arrow on a p/v graph is pointing right?
expansion
what =0 when the arrow on a p/v graph goes straight up or down?
W = 0
because its a constant volume process
how do you find work done per cycle on a p/v graph?
area inside the loop
what does it mean if a process is non-flow?
it occurs in a closed system
no fluid is added or lost in the process
they are reversible
what are 4 non-flow processes?
isothermal
constant pressure
constant volume
adiabatic
what is an isothermal change?
a change that occurs at a constant temperature
internal energy stays constant so Q = W meaning any transfer of energy would lead to work being done and vice versa
what law does an isothermal change obey?
Boyle’s law
if work is done by the system in an adiabatic change, what will happen to the internal energy?
it will decrease
what =0 in a constant volume change?
( Q, U or W )
W = 0
what does a graph of ‘constant volume’ change for heating on a p/v diagram look like?
arrow pointing straight up as temperature has increase
no work done as volume doesn’t change
what are indicator diagrams and what are they used for?
a p/v diagram for engines that allows identification of the different processes in the system
what are the basics of the four stroke petrol engine cycle?
piston moves down in the cylinder increasing the volume of gas above it,
air and petrol vapour are drawn into the cylinder through the open inlet valve,
pressure stays constant, volume increases
