Thermodyanmics I Flashcards
what is thermodynamics?
science of energy
what is energy?
ability to change or do work
define entropy
measurement of disordeness
name two systems of dimensions
english system and metric SI system
name different types of units
primary
secondary (can be derived)
define system
quantity of matter or region of space defined by a boundary
define boundary
anything that separates a system from its surroundings
define stationary system
system with constant velocity and elevation
define isolated system
neither mass or energy can across the boundary, an example is the universe
define closed system (or fixed mass, control mass or CV)
mass cannot cross the boundary but energy can, the boundary can either be fixed or moveable
define open system (or control volume or CV)
both energy and mass can cross the boundary
define volume flow rate
volume of fluid flowing through cross section per unit time (represented by a V with a dot above it)
define mass flow rate
amount of mass flowing through cross section per unit time (represented by m with a dot above it)
define energy flow rate
amount of energy flowing through cross section per unit time (represented by E with a dot above it)
define macroscopic energy
energy wrt to outside energy frame (what you’re measuring frame)
define microscopic energy
energy related to molecular or atomic structure and activity
define internal energy (U)
the sum of all microscopic forms of energy of a system
define total energy
microscopic energy + macroscopic energy
name the different forms of macroscopic energy
kinetic energy + potential energy
equation and definition for kinetic energy
associated with a system’s motion
1/2mv^2
equation for potential energy
associated with a systems elevation
mgz (z representing elevation/height)
what is mechanical energy?
sum of the potential energy and kinetic energy
different types of microscopic energy?
sensible, latent, chemical, nuclear
definition of sensible energy?
associated with motion of molecules, it is proportional to temperature
definition of latent energy?
associated with intermolecular forces and phases of a system
gases have a higher latent energy
equation for total energy
E= U +1/2mv^2 +mgz
define heat/heat transfer (Q)
energy transferred due to temperature difference
what is heat transfer rate represented as and what are the units?
Q with a dot above it
Units: J/s or W
name heat transfer mechanisms
conduction, convection, radiation
define adiabatic process
a process without heat transfer
define work (W)
energy interaction not caused by temperature difference
define power
rate of doing work
define mechanical work
energy transferred by a force, requires force and displacement
what is boundary work (Wb or PdV work)?
one form of mechanical work transferred by a force
name some types of mechanical work
shaft work, electrical work, spring work
define dynamic energy and examples
energy that crosses the boundary, eg. heat, work
define static energy and examples
energy that does not cross the boundary, e.g internal energy (U)
define property
any characteristics of a system e.g temperature, volume, pressure and mass
define intensive property and examples
NOT dependant on the amount of material in the system e.g temperature, pressure, density
define extensive property and examples
IS dependant on amount of material in the system, e.g mass, volume, energy
what is specific property?
extensive property per unit mass
define state
condition at an instant time as described/measured by its properties
eg. temperature, pressure, energy
define thermodynamic state
a set of physical quantities describing properties of a system
define path function and examples
value of the variable depends on transition/path of system to change, e.g work (depends on amount of force applied) and heat, (depends on temperature differences)
define point function and examples
value of the variable is not dependant on the transition/path of a system, e.g temperature and pressure
specific weight definition and units
weight per unit volume
density * gravitational value measured in Nm-3
specific/relative gravity and units
density per density value of water (Nm-3)
define ‘state’
condition at an instant time as described/measured by its properties
define ‘thermodynamic state’ and examples
a set of physical quantities describing properties of a system
examples: temperature, pressure, energy
3 conditions of thermodynamic state properties
values of the properties define the state of a system
each property has a single value at each state
all properties are point, not path, functions
define ‘path function’ and examples
value of the variable depends on transition/path of a system to change, e.g work, heat
define ‘point function’ and examples
value of variable is not dependant on the transition/path of a system. e.g temperature and pressure
define ‘process’
a path/passage for a system to change from initial state to final state
what properties are being observed in a process?
initial and final states
energy and mass interactions during the process
define ‘natural process’
process where entropy increases, it is a quick irreversible reaction that occurs in nature
define ‘unnatural process’
total entropy decreases when reaction occurs, it is a logically conceivable reaction
define ‘flow process’
mass is entering/leaving an open system
define ‘non-flow process’
closed system, no mass is crossing the boundary
define ‘steady flow process’
flow remains constant, does not change with time
define ‘equlibrium’
stable, balanced or unchanging condition with all acting, opposing, competing forces or influences that cancel one another out
change process continues until gradients are nullified/balanced
what can occur only under equilibrium?
only under equilibrium are system properties fixed, so they can be measured and calculated
define ‘thermal equilibrium’
same temperature throughout a system
define ‘phase equilibrium’
no or balanced phase changes
define ‘internal thermodynamic equilibrium’ (ITE)
equilibrium state within a system, if undistributed no changes occur within the system
define ‘global thermodynamic equilibrium’ (GTE)
equilibrium within whole system and the intensive parameters remain homogeneous throughout the system
define ‘cycle’
process or series of processes whose initial and final states are identical
what do you need to know in a cycle?
initial and final states of a system
energy and mass interactions
what is Carnot Engine?
a cycle formed from a series of process
what occurs during a cycle?
system returns to original state. Properties are exactly the same including energy, temperature
why is a cycle important?
repeating nature allows for continuous operation
what is a heat engine?
heat from a hot source, converted into work, part disposed into a cold sink
what is a heat pump?
used to remove heat from cold source to warm sink
define ideal cycle
for the purpose of analysis and design, used to model real devices by making assumptions
e.g quasi static process + no friction is occurring
allows you to decide the highest efficiency to get from the cycle
define entropy
measurement of disorder or chaos of energy distribution, the higher level of entropy= the higher amount of wasted energy
how to calculate entropy
heat change divided by temperature, delta Q/ time
is entropy intensive or extensive property
extensive property
does entropy change in an isolated property?
nope, it never decreases
define pressure
named force exerted by a fluid per unit of contact surface area
units of pressure
pascal (Pa) or Nm-2 or kgm-1s-2
define ‘zero absolute pressure’
perfect vacuum where absolute pressure is zero (no single molecules in order to apply pressure)
define ‘atmospheric pressure’
pressure within the atmosphere, caused by the weight of above the measurement point, it decreases with increasing elevation
how to calculate positive gauge pressure
absolute pressure- atmospheric pressure
how to calculate negative gauge pressure
atmospheric pressure- absolute pressure
which should you always use, gauge or absolute pressure?
absolute pressure
define temperature
a thermal state to distinguish a hot body from a cold body
it is proportional to the average kinetic energy (sensible energy) of the molecules
name the two types of temperature scales
freezing/ice point- e.g. ice and water mixture in equilibrium at 1 atm pressure
boiling/steam point- e.g water, liquid and vapour mixture in equilibrium at 1 atm
temperature value in degree F from degree C equation
T(F)+ 1.8(C) +32
temperature difference in degree F from degree C equation
d(F) = 1.8 dt’(C)
what is the Kelvin scale?
temperature measured with absolute zero as basis in degrees Centigrade
temperature value of Kelvin equation
T(K)= t(C) +273.15
temperature difference of Kelvin equation
dT(K)=dt(C)
what is the Rankine scale?
absolute temperature scale in English system, Units R
temperature value of Rankine equation
T(R)= T(F) + 459.17 T(R)= 1.8t(K)
temperature difference of Rankine equation
dT(R)= dT(F) dT(R)= 1.8dT(K)
what is the zeroth law of thermodynamics?
if two bodies are in thermal equilibrium with a third body, they are in thermal equilibrium with each other
define ‘pure substance’
element/compound of only one component with definite, chemical properties and composition
define ‘latent heat’
energy absorbed or released during phase changes
what are the three principle gases?
solid, liquid and gas
describe ‘gas phase’ (2 principles)
the molecules are at a higher energy level than liquid and solid phases
gases releases large amount of energy when condensed and frozen
describe ‘solid phase’
large amount of energy needed to melt/vaporise
define ‘compressed liquid’/’sub-cooled liquid’
at 1 atm pressure, temperatures above freezing point and below point
the liquid is compressed if it is not about to vaporise
define ‘saturated liquid’
liquid that is about to vaporise
Q=> U (sensible energy) +PdV
define ‘saturated liquid and vapour’
a mixture of saturated liquid and vapour and they exist in equilibrium
define ‘superheated vapour’ and the heat transfer equation
vapour not about to condense
Q=>U (sensible energy) +PdV
define ‘enthalpy’
internal energy + pressure*volume
define ‘saturated vapour’
vapour about to condense
Q= U (latent energy) + PdV
define ‘saturation pressure’
at a given temperature, the boiling pressure is saturation pressure.
this is because boiling temperature depends on pressure
define ‘saturation temperature’
at a given pressure, the boiling temperature is called saturation pressure
What is quality (x)?
Ratio of the vapour mass to the mass of the mixture
X= m(vapour)/ m (mix)
Define quality
One of the independent variables to describe the state of a system
Define ideal gas
Theoretical gas with randomly moving point particles not interacting except colliding elastically
Examples of gases treated as ideal gases
Air, nitrogen, oxygen, helium, argon, neon, krypton
Example of dense gases
Water vapour
Define specific heat (C) and what is it related to
Energy required to raise temperature of unit mass by 1 degree
It is related to internal energy, U( sensible energy)
What is Cp and Cv and what are they related to
Cv is specific heat at constant volume (sensible energy)
Cp is specific heat at constant pressure (sensible energy + boundary work)
Which out of Cp and Cv is greater and why
Cp is always greater than CV because h= u + Pv
Total internal energy change in the system ( delta e of system)
delta e (in) - delta e(out) = du Delta e(in) - delta (out) = CvdT
Equation for change in internal energy with temperature at a constant volume
Cv = (du/dT) v
Equation for change in enthalpy with temperature at a constant pressure
Cp= (dh/dT)p as h=u + Pv
What is different about C values for monoatomic gas?
Specific heat (C) of monoatomic gases (eg argon, helium) remain constant over the temperature range
specific heat relations for ideal gases and how to derive
differentiate enthalpy equation to get dh=du +rdT dh/dt= du/dT + R replace dh by CpdT and du by CvdT Cp= Cv +R k= Cp/Cv
