Thermal Concepts Flashcards
What is temperature?
The average kinetic energy in a substance
What is heat?
A measure of energy inside an object
How do temperature and heat link?
Heat causes temperature to rise
What is absolute zero?
The minimum motion of a particle
How much is 0 degrees celsius in Kelvin?
273.15 K
Which way does heat flow?
From hot to cold areas
What is Avogadro’s hypothesis?
Equal volume of gas, at the same temperature and pressure, will contain the same number of molecules
What is a standard atom?
The standard atom is carbon-12. It is taken to have a mass of exactly 12 atomic units
What is a mole?
The mass of a mole of any substance = the relative molecular mass of the substance in grams
What is Brownian motion?
The random motion of particles
Choose the right words to finish the sentence (hint: think about Brownian motion):
The larger the object, the (more/less) it is being collided from all sides so the effect (adds on/cancels out)
more
cancels out
What is internal energy?
The total potential and kinetic energy of an object
What is kinetic energy?
The energy related to movement
What is potential energy?
The energy held in an objects’ intermolecular bonds
What does an increase in kinetic energy equal?
An increase in kinetic energy equals an increase in potential energy
Why does an increase in kinetic energy = an increase in potential energy? in a solid
This is because an increase in kinetic energy means the solid particles will vibrate more and move further away from each other, so the potential energy has to increase, to keep them together
Is there any IMF in a gas? why?
no. The average kinetic energy of the particles in a gas is great enough to overcome the forces of attraction between them.
What do you need for a scale?
- you need 2 fixed points (i.e. melting and boiling points)
- you need a measuring element
What is the unit for temperature at IB?
Kelvin
How is heat transferred?
- conduction
- convection
- radiation
What is conduction?
When energy is lost from fast-moving particles and passed onto slow-moving particles when they vibrate
What is convention?
Heat causes fluid (gas/liquid) particles to heat up. This causes an increase in pressure, which causes it to expand and displace the cold particles, so the particles move upwards. They rise to the top and cool down and sink to start the cycle again.
What is radiation? give an example
The heat passes from one body to another without heating the material in between
e.g. the sun radiating heat energy to the earth without heating up space in between
Why is conduction not effective in a gas?
Because the particles are more spread out, and do not touch each other often enough to pass on the energy
Can radiation go through a vacuum?
yes
What state is conduction the most effective in? why?
A solid. Solid particles are close together and vibrate more, so they can pass on the heat energy faster, making it more efficient
Why doesn’t heat rise?
Heat is a form of energy, so it doesn’t rise. Instead, heat moves from hot to cold areas
What happens when two areas are in equilibrium (in terms of convention)?
When they are in equilibrium, the temperatures are the same and no more heat transfers occur
Why does the leg of a stool feel colder than the table?
A stool is better at conducting heat (it has a better ability to take heat from you, making it seem cold, although the stool leg and the table are the same temperatures)
Why is foam a good insulator?
Foam is a solid with air pockets within it. Heat goes through the solid via conduction, but when it reaches an air pocket, the heat gets trapped by convention. This traps the heat and stops it from escaping
What is used as insulation in the walls of a loft? why?
Fiberglass. Same as foam, it has air pockets to trap heat
What is thermal conductivity?
Thermal conductivity is a measure of a material’s ability to pass heat through it
Does foam have a high or low thermal conductivity?
low thermal conductivity
What does a high thermal conductivity mean?
The material easily allows heat through and easily takes it in through from the environment
What does a low thermal conductivity mean?
The material acts as a good insulator, and prevents the transfer of heat.
Which one is better if I want the building to stay hot inside?
low thermal conductivity
How do you reduce radiation? why does this method work?
Use a light, shiny surface. It works because it reflects heat radiation back
What is thermal contact? Where will it go from?
The ability to transfer heat energy from one to another. It will go from a hot to a cold place
What is thermal capacity?
It is the amount of energy needed to raise a substance by 1C or 1K
What is a conductor?
A material that has a good ability to transfer heat
What is an insulator?
A material that transfers heat poorly
Finish the sentence?
The lower the SHC, the…
… less energy needed to raise its temperature
Why does an iron skillet take longer to heat up than an aluminium skillet if aluminium has a higher SHC?
An iron skillet has more mass than an aluminium skillet (take mass into account when thinking about SHC)
In the SHC practical, why does the value of SHC not be as accurate to the true value?
This is because, in the practical setup, half of the immersion heat is not in the metal block. Therefore, half of the heat is being transferred into the air and not into the block, so the value is not completely accurate
What happens to heat if a system is closed?
Heat is conserved
Finish the sentence:
If a cube of hot metal is dropped in a beaker of water, the water gains energy from the metal cube. Therefore, energy gained by the water = …
Energy lost by the metal cube
What does the heating curve look like?
A graph of temperature plotted against heat added. It shows the temperature of phase change and SHC
What do the straight, horizontal lines show on a heating curve?
The straight, horizontal lines show the SLH of a substance or its phase change
What do the straight, diagonal lines show on a heating curve?
Temperature is rising -> SHC
What is Specific Latent Heat? (SLH)
The amount of energy required for a substance to change state, however temperature must be constant
What does it mean if there is a shallower gradient on a heating curve?
A shallower gradient means more energy is required to raise the temperature of the substance
What does a longer line of SLH in a heating curve show? why?
The longer the line, the more energy is needed to change state. This is because energy is required for the bonds to break
What energy is related to temperature change?
Kinetic energy
What energy is related to phase change?
Potential energy
Why does it take more energy to change the phase between a liquid and a gas than a solid and a liquid?
Because it takes more energy to completely separate molecules than to enable them only to move past one another freely
What is the SHC formula? with units
Energy (J) = mass (kg) x c (J kg^-1 K^-1) x change in temperature (degrees C or K)
What is the SLH formula? with units
Energy (J) = mass (kg) x SLH (J kg^-1)
What is the total heat formula?
Q (total) = mc(change in)temp + mL
How much energy is needed to transform 0.5kg of ice and -20C into water at 50C?
-20 ice -> 0 ice - SHC
0 ice -> 0 water - SLH
0 water -> 50 water - SHC
total = SHC + SLH
SHC = 0.5 x 2090 x 20 = 20900
SHC = 0.5 x 4180 x 50 = 104500 +
SLH = 0.5 x 33400 = 167000
= 292400 J
What’s the difference between boiling and evaporation?
Boiling occurs at precise temperatures and bubbles form throughout the liquid. Evaporation happens at any temperature and it only happens at the surface
What is superheated water?
Water above 100C but has not boiled yet
How does evaporation work?
In water, some molecules have enough KE to escape and become a gas. When these faster molecules are lost, the average KE of the liquid decreases, cooling the object
If the temp is constant, but the phase changes, i.e. solid to liquid. What happens to the KE and PE
The KE stays the same (related to temp and temp is constant)
The PE is greater (related to phase change)
What are the assumptions we can make about gases?
- they are a large number of identical molecules
- the volume of molecules is negligible
- motion is random
- there are no forces between molecules
- All collisions are elastic
What happens if these assumptions are all true?
We get an ideal gas
What is the pressure formula?
P = F/A
Pressure = Force/ Area
Pa = N/ m^2
What is molar mass?
The mass of one mole of a substance
When is a gas no longer ideal?
-When it is compressed or close to a phase change
What is Boyle’s Law?
pressure is inversely proportional to volume at constant temperature
What is Pressure law?
pressure is directly proportional to temperature at constant volume
What is Charle’s Law?
Volume is directly proportional to temperature and constant pressure
What is the Ideal gas formula? for mols
pV = nRT
pressure(Pa) x volume(m^3) = mol(n) x real gas constant x temperature (K)
What is the Ideal gas formula? for number of particles
pV = nRT
pressure(Pa) x volume(m^3) = number of particles x Boltzmann’s constant x temperature (K)
What does isobaric mean?
Constant pressure
What does isochoric mean?
Constant volume
What does isothermal mean?
Constant temperature
What is the first law of thermodynamics?
The principle of energy conservation
what is the equation of the first law of thermodynamics? symbol and word
Q = U + W
heat energy = change in internal energy + external work done by the system
In the first law of thermodynamics equation, what happens if:
Q is positive/negative
U is positive/negative
W is positive/ negative
Q:
+ -> Heat flows into the system
- -> Heat flows out of the system
U:
+ -> increase in internal energy
- -> decrease in internal energy
W:
+ -> system loses energy through work
- -> system gains energy through work
What is entropy? (3 different definitions)
- A measure of the amount of disorder/chaos in a system
- A measure of the amount of energy that is unavailable to do work
- The number of arrangements of particles
What is the formula for entropy? Symbol and word
S =Q/T
Change in entropy = change in energy/ temperature
What is the unit for entropy?
J/K or JK^-1
What does adiabatic mean?
No heat is exchanged between the system and its surroundings. Q = 0
What is the equation for adiabatic expansion?
PV^5/3 = constant
Briefly describe what adiabatic expansion looks like on a graph
a downward sloped line going from a lower volume to a higher volume and going from a higher pressure to a lower pressure
Explain adiabatic expansion using the first law of thermodynamics
As the volume increases, work is done by the gas to expand. Therefore, W = +. Adiabatic means that no heat energy is exchanged between the system and the surroundings, therefore Q = 0. So U must be negative to cancel it out.
Q = U + W —> 0 = - +
How do you find the work done, using a graph?
Work Done = area under the graph
-> Wd = PV
What is the formula for thermal efficiency? symbol and word
n = 1 - Qc/Qh
Thermal Efficiency = 1 - heat transfer into the cold reservoir/ heat transferred out of the hot reservoir
Can thermal efficiency ever be greater than 1?
No. Because energy cannot be created or destroyed, so for the thermal efficiency to be greater than 1, we are saying that the energy output is greater than the energy input
What law is the Carnot cycle based around?
Boyle’s Law
Describe the steps in a Carnot cycle
1 -> 2: Work is done on the gas to compress it, pressure + temp increase
2 -> 3: Heat is gained by the gas, so it expands - work is done by the gas
3 -> 4: Work is done by the gas (adiabatic expansion) - and cools down
4 -> 1: Work is done on the gas, and brings it back to its original position
What is the formula for the efficiency of the Carnot cycle? symbol and work
n = 1 - Tc/Th
Efficiency = 1 - cold temp/ hot temp
Briefly describe how a fridge works
The fridge inputs work to make heat flow from a cold to a hot area, by pumping out the heat energy at the back of the fridge. This keeps the contents in your fridge cool
What are the 3 definitions for the second law of thermodynamics?
- It is not possible for a heat engine, working in a cycle, to absorb all heat energy and convert it all to work
- In a cyclic process, entropy will either stay the same or increase
- It is not possible for heat to be transferred from a cold body to a hot body without work being done
A freezer door is opened while switched on and placed in a sealed room.
The entropy of the room
A. equals zero
B. increases
C. decreases
D. does not change
Answer: B
A freezer is a heat pump, so thermal energy is transferred from inside the freezer and released at the back of the freezer
While it runs with the door open, the internal energy of the contents of the freezer decreases
The entropy of the contents of the freezer decreases because they are colder
But the entropy of the room increases because it is hotter
What is a black body radiator?
An object that is perfectly opaque and absorbs all energy
What is emissivity?
the ratio of the energy radiated from a material’s surface to that radiated from a perfect emitter, known as a blackbody. At the same temperature and surface area.
What is the formula for emissivity?
e = power radiated by a surface/ power radiated from a black body and the same temperature and surface area
What is the unit for emissivity?
There isn’t one, because it is a ratio
What two values does emissivity have to be in between?
0 and 1
What does 0 and 1 emissivity mean?
0 = perfect reflector
1 = perfect emitter
What is the emissivity of the sun? why
1 - because it emits all of the EM spectrum
Roughly what is the emissivity of the Earth? why
0.6 - it emits some of the EM spectrum
What is Stefan-Boltzmann Law?
The law for a perfect black body
What is the formula for Stefan-Boltzmann’s law? symbol and word (with units)
P = eoAT^4
Power (W) = emissivity (usually 1) x Stefan - Boltzmann constant (5.67x10^-8Wm^-2K^-4) x Surface area (m^2) x Absolute Temperature (K)
What is the surface area of a sphere?
4 x pi x r^2
Finish the sentence:
When a black body radiator is heated up, it emits ______
… a range of different wavelengths
Finish the sentence:
The hotter the star, the more ______ it emits and the ______ it becomes and it shifts to the _____ of the graph.
radiation
whiter
left
What is Wien’s Displacement Law?
Wien’s displacement law states that the black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature.
Briefly describe what Wien’s Displacement Law looks like on a graph
y - axis: radiation
x - axis: wavelength of radiation
it will peak at a certain point of the graph to signify the temperature of the star, and its wavelength
What colour is a shorter and longer wavelength?
shorter wv: violet
longer wv: red
A star has a high temperature (high peak) and a short wavelength (shifted to the left), what colour will it appear?
violet
Why can’t you have a green star?
If the graph is shifted to have green to be the most prominent, it will also mean there will be lots of red and blue, making the star look white. Changing the star’s temperature will make it appear slightly orange, yellow, red or blue, but you just can’t get green. Our eyes simply won’t see it that way. That’s why there are no green stars
What is Wien’s displacement law formula? word and symbol
ymax = 2.9x10^-3mK/T
Wavelength peak = 2.9x10^-3/Temperature
What does Wien’s Displacement law assume?
It assumes that this is a perfect black-body radiator
What should you assume the emissivity of a star is?
1
What is luminosity?
The power emitted by source
What is brightness?
how much light appears to shine from something. Dependant on distance
What is the formula for brightness? symbol and word
b = L/ 4 x pi x d^2
brightness = Luminosity/ 4 x pi x distance^2
What is the formula for intensity (Wm^-2)?
Power/Area
W/m^2
What is solar constant?
a measurement of the solar electromagnetic radiation available in a meter squared at Earth’s distance from the sun.
What is the value of solar constant?
1360 Wm^-2
or
1.36 x 10^3 Wm^-2
What is albedo?
the % reflected
What is the formula for albedo?
power scattered by a body/ incident power
(reflected/incident)
What should the value for emissivity and albedo add up to
1
If 80W of incident ray hit an object and 60W is reflected, what is the albedo, the emissivity, and the amount absorbed
albedo = 60/80 = 0.75
emissivity = 20/80 = 0.25
amount absorbed = 80-60 = 20W
The higher the albedo, the lower the ______
emissivity
Name 5 greenhouse gases that contribute to climate change
Water Vapour
CO2
Methane
Nitrogen oxides
Ozone
