Module 3: Thermodynamics Flashcards
Minimum value of Kelvins
0K
How can temperature be changed?
By transfer of thermal energy
How can a constant-volume gas thermometer be used?
By comparing with a triple point cell
T/Ttp = P/Ptp
Secondary thermometers and how they are made accurate
One which is practical for general use
Calibrated against a primary thermometer
Thermal energy
Energy of matter due to the vibration (and translational motion- just gas) of molecules
When two non-identical objects are joined, which property is made equal?
Temperature (not thermal energy)
Heat transfer
Movement of thermal energy
Higher nervy molecules share energy with lower energy molecules
Occurs until thermal equilibrium is reached
Thermal equilibrium
When both objects are the same temperature
Why does thermal expansion occur?
Molecules move further apart as they start moving faster due to increase in temperature. This causes expansion of the object
α (thermodynamics)
Expansion coefficient
What are the expansion coefficients of 2D and 3D objects?
2α and 3α
Ideal gases all have the same ____?
Volume expansion coefficient
Why do ideal gases have the same volume expansion coefficient? (2)
Volume of atoms/ molecules is very small compared to the volume available
Atoms/ molecules don’t stick together
What is the relationship between number of particles and volume when pressure is increased?
The more particles, the greater the increase in volume
Which two laws contribute to the ideal gas law?
Charles’ law and Boyle’s law
What does N stand for in the ideal gas law PV = NkT ?
Total number of atoms/ molecules
What does k stand for in the ideal gas law PV = NkT ?
Boltzmann constant
1.38 x 10^-23 J K^-1
n = N/?
NA (Avogadros constant)
Does the ideal gas law apply to pure gases or mixtures?
Both
Dalton’s law
P = P1 + P2
ppm
Parts per million
Partial pressure
The pressure one gas contributes to the total pressure in a mixture
Temperature is a measure of (apart from heat)?
Average kinetic energy per atom/ molecule
________
1/2mv^2
Average kinetic energy per molecule
U (thermodynamics)
Total kinetic energy
What does the Maxwell-Boltzmann distribution show?
The molecules of a gas have a wide range of speeds
What is the most common speed for O2 gas molecules?
400ms^-1
For real gases, when does the ideal gas relationship fail?
When T is low enough
Or V is small enough
What happens when T or V is low enough for a real gas?
Attractive forces between atoms of the gas cause them to stick together
Gas condenses into a liquid or solid
Called a phase change
Does T change when phase changes?
No
Saturated vapour pressure
The pressure at which a substance can co-exist as a liquid and vapour at a certain temperature
Does each substance have more than one vapour pressure?
Yes- at different temperatures
How can we boil water at room temperature?
Decrease the pressure
Why is thermal energy needed for phase change?
To overcome inter-atomic forces
Q (thermodynamics)
Thermal energy input
L (thermodynamics)
Latent heat: the thermal energy required for 1kg of a substance to melt or vapourise
At a specific temperature
kWh
Kilowatt hour
Equal to 3.6MJ
First law of thermodynamics
Energy of an isolated system is constant
Q = mcdT
Q thermal energy input
m mass
c specific heat capacity
dT change in temperature
Specific heat capacity
(c) amount of heat required to raise temperature of 1kg of a substance by 1K (or 1 degree Celsius)
Dew-point temperature
The temperature at which a certain (partial) pressure of water starts to condense/ form dew
Is the partial pressure of water vapour usually more or less than the saturated vapour pressure?
When are they equal?
Less- the saturated vapour pressure is the max amount a volume can hold
It is equal to the saturated vapour pressure of water of a lower temperature
Moisture content in the air
The mass of moisture per kg of dry air
Dry air
Air in our atmosphere containing nitrogen oxygen etc.
Other names for moisture content (2)
Absolute humidity
Humidity ratio
mw/ma
Humidity ratio
Mass of water / mass of air
Dry-bulb temperature
Normal air temperature
Relative humidity
Partial water vapour pressure/ saturated vapour pressure at same T
(Percentage humidity of what is possible)
If given dew-point T and dry-bulb T
How do you find the relative humidity in a psychometric chart?
Dew-point is at 100% relative humidity- so find the moisture content that corresponds with this data point
Find the data point that connects the moisture content to the dry-bulb temperature
Find the RH line closest to second point
Wet-bulb temperature
The equilibrium temperature at which the rate of heat lost by the wet bulb is equal to the rate of heat gained from the surroundings
Basically how cool a wet cloth over a thermometer can become
When does wet-bulb temperature = dry-bulb temperature?
When RH= 100%
Three forms of heat transfer
Conduction
Convection
Radiation
dQ/dt
Rate of heat transfer
h conduct
Conduction heat transfer coefficient
h cond= k/d
Where d = thickness
What is k in thermodynamics?
Thermal conductivity
Conduction
Heat transfer occurring through collisions of electrons and molecules in which they exchange energy.
Hot fluid _____ while cold fluid ____
Rises
Sinks
Convection
Like conduction, but at least one medium is a moving fluid.
What would happen to the rate of convection if the fluid stopped moving?
It would decrease to zero because the temperature difference would decrease to zero at the interface
What is the equation below for?
dQ/dt= AσεT^4
Radiation from one surface
What is the equation below for?
dQ/dt~ h rad A dT
Radiation between two surfaces
Radiation
The electromagnetic waves emitted and absorbed by all objects
Emissivity
The measure of an object’s ability to emit infrared energy
From 0-1.0
Blackbody
An object with an emissivity of 1.0
Which have higher emissivity, dull black surfaces, or shiny metallic surfaces?
Dull black surfaces
Which region is the radiation emitted by objects at body temperature in?
Infrared
In the equation below, what do the symbols stand for?
dU= Q - W + E
dU is the change in internal energy
Q is the heat input (usually negative for humans)
W is the work done
E is the energy input (food)
Metabolic energy input
E - dU
Energy input of food - change in internal energy
