Engineering Physics 2 Flashcards
Equation showing the relationship between P,T and V
P1V1/T1 = P2V2/T2
PV/T = Const.
What is a property of an ideal gas interms of its energy
Energy of the molecules is entirely kinetic and depends only on the temperature
No forces acting between the molecules, no work done against these forces
Define a system
A fixed mass of some substance, enclosed by a boundary, outside of which are the surroundings
What is the internal energy (U) of a gas
The sum of the total kinetic energy of its constituent particles and (unless it is an ideal gas) the total potential energy of the particles.
What is the first law of thermodynamics
Q = ΔU + W
ΔU=Q-W
Q = Energy transferred to the system by heating
W = always negative
U = Internal Energy
Change in internal energy = Energy transferred to the system by heating - Work done by the system
What is W in the first law of Thermodynamics
Work done by the system
eg. energy transferred out by compression
Value is negative if energy is transferred out
Define a non flow process
A process during which the fluid does not move in or out of the system during the process
What is an isothermal process
So what is its restriction
A process through which a system stays at the same temperature
So Change in U = 0
A non-flow process
What is the first law of thermodynamics for an isothermal process
Why
Q=W
No change in temperature, so average kinetic energy of particles is constant
So total kinetic energy and therefore internal energy is constant, change in U = 0
What is the ideal gas equation for an isothermal process
pV=Const.
Boyles Law
n and T are constant
Use the first law of thermodynamics to explain how an isothermal change can happen
If a gas expands and does external work W, an amount of heat Q has to be supplied to the gas in order to maintain its temperature constant, and vice versa.
How does pV change as the temperature that the isothermal process takes place at changes
Graph gets further from the origin
Equation for work done on a gas during compression
W=pAΔs
Δs = distance moved by the piston
p = pressure
A = Area
What is the first law of thermodynamics for a constant pressure process
How do you use the equation for a compression
Q = ΔU + pΔV
For a compression
ΔV is negative as volume is decreasing
pΔV =W is negative as work is being done on the gas
What is the first law of thermodynamics for a constant volume process, explain it
ΔU = Q
When the volume is constant, no work has been done on/by the gas
So all thermal energy transferred to the gas will be transferred to the internal energy
What is an adiabatic process
An isolated system, where no energy is able to transfer in or out by heating from or to the external environment.
Any work being done on the system increases the internal energy so its temperature increases
What is the first law of thermodynamics for a adiabatic process
Explain why
−W = ΔU
As no heat is transfered to the system, Q=0
What is The constant of gas atomicity and its symbol
A constant for the particular gas that is dependent on the degrees of freedom
About the no. of atoms in each molecule of the gas, this may add rotational KE to particles, affecting the value of average KE used
Gamma sign
2 equations that can be used for an adiabatic process
pxV(Constant of atomicity) = const.
TV(Constant of atomicity) - 1 = const.
What is the restriction in an Adiabatic process
Q=0
What is the restriction in a constant volume process
W=0
No work is done on the gas as the volume doesn’t change - no compressed etc.
What is the restriction in
What is the ideal gas equation for a constant volume process
p/T = Const.
What is the ideal gas equation for a constant pressure process
V/T = Const.
pV graph for an isothermal and Adiabatic process
Explain it
How to calculate work done from a p-V graph
Area under graph
How to calculate area under pV curve
W = Area under curve = ∫VV2 dV
What is an indicator diagram
How do you interpret one
Just a pV graph with an arrow to show the direction of the change
So if the arrow is pointing right, volume is increasing so work is being done by the system, hence W is positive
If the arrow is pointing left, volume is decreasung so work is being done on the system, hence W is negative
Area under graph is W
What is a cyclic process
A process through which the system goes through 2 or more consecutive changes such that the final state is the same as the initial state.
Example of an indicator diagram for a cyclic process
Use the concept of cyclic processes to explain the basis on which an engine works by
An engine is meant to force the working substance into such a cycle, so that the work done by the gas exceeds the work done on the gas as it is returned to the starting point.
The net difference means that the engine is doing useful work
