Topic 9- Thermodynamics Flashcards
specific heat capacity
the amount of energy needed to raise the temperature of 1kg of the subtance by 1K
specific heat capacity equation
Δe = mcΔθ
measuing specific heat capacity experiment
- insulate the material and place and heater and thermometer inside it (solid or liquid)
- heat the substance for a set amount of time
- measure the change in temperature of the substance.
- using the power of the heating appliance use E = Pt to calculate the energy
- use Δe = mcΔθ to calculate the value value of c
units of specfic heat capacity
J/kg/K
Why is the calculated value of c always too large?
energy from the heater did not all go to increase the internal energy. Some will have escaped to increase the internal energy of the room.
Specific Latent heat
the energy needed to change the state of 1kg of the substance without changing the temperature
What is specific latent heat?
the energy needed to break the bonds a subtance melts or boil (changes state)
Latent heat of fusion
solid -> liquid (melting)
Latent heat of vapourisation
liquid -> gas (boiling)
Latent heat equation
E = LΔm
latent heat unit
J/kg
measuring latent heat
- place a beaker of cold water on a top pan balance with a heater in it
- heat the water for a set time
- measure the change in mass during the heating
- work out the energy inputted from the power of the heating appliance E=pt
- Work out the latent heat using E = LΔm
Why doesn’t the temperature change when a substance is changing state?
the energy is being used to change the state
Internal energy
the sum of the kinetic and potential energy within a system
Heating
the process when energy is transferred from a higher temperature object to a lower temperature object
heat
the energy transferred by heating
1st law of thermodynamics
energy in a system is conserved: change in internal energy = heat transfer + work done (U=Q+W)
0th law of thermodynamics
if two systems are at the same temperature, there is no resultant flow of heat between them, the system is in thermal equilibirum
thermal equilibirum
two systems are at the same temperature so there is no resultant flow of heat between themWhat
What can happen when the internal energy increases?
- increase in temperature
- change state
What does doubling the number of atoms do?
doubles the time to heat up
As the temperature of a gas increases:
- average particle speed increases
- the maximum particle speed increases
- the distribution curve becomes more spread out
how is energy transferred between particles
via momentum in collisions
kelvin -> celsius
θ = T - 273
Celsius -> kelvin
T = θ + 273
Boyles Law
for a fixed mass of gas at constant temperature pressure, p, is inversely proportional to volume, V.
Work done, w =
pressure, p * Change in volume, V
Pressure volume graph
1/x curve
pressure, 1/v graph
straight line graph through the origin
PV =
a constant
P1V1 =
P2V2
As the Pressure, P, doubles to 2P the volume, V, ….
halves to 1/2V
Boyles law experiment
- slowly use the foot pump to increase the pressure
- record the volume for different pressure level
- plot p agaisnt 1/v, if the graph is directly proportional it follows boyles law
Charles Law
for a fixed mass of gas at constant pressure the volume, V, is (directly) proportional to the temperature, T (in kelvin)
V/T =
a constant
V1/T1 =
V2/T2
Volume Temperature graph
straight line
- if in kelvin directly proportional
- if in celsius cuts the y axis
Charles law experiment
- record the volume and temperature of the cold water
- heat the water and record the temperatures
- plot volume agaisnt temperature. If the graph is straight line and approximately crosses the x-axis at -273 degrees it follows Charles law
Gay Lussacs pressure law
far a fixed mass of gas at constant volume pressure is proportional to temperature
P/T =
a constant
P1/T1 =
P2/T2
Gay Lussacs experiment
- heat is applied to the cylinder
- measure the temperature and volume at regular intervals
- plot P agaisnt T if its a straight line graph then it obeys the law
How does gay lussacs and charles experiments give evidence for an absolute zero temperature
They should be directly proportional however they are not and both intersect the x axis at -273. This gives evidence for the kelvin temperature scale where 0K is absolute zero.
ideal gas
a gas where the molecules do not interact
why do real gases no obey the gas laws as well
- if too cold they condense
- if under too much pressure they condense
- molecule interactions result in imperfect result
How did they find the exact value of absolute zero
extrapolating the graph from charles law and gaylussacs law
absolute zero
the lowest possible temperature where all particles have the minimum possible kinetic energy
how are kelvin and celsius linked
they have the same interval
ideal gas equation
pV = NkT
what is k in the ideal gas equation
boltzman constant
pressure in a container of gas
particles move about and collide with the walls of the the container the change in momentum causes a force on the side of the container which results in pressure P = F/A
A gas is ideal if:
- molecules have negligible size
- molecules are identical
- collisions are perfectly elastic
- no inter-molecular forces
- enough molecules for statistical analysis
- motion is random
derive pV = 1/3Nm
- when the loecules bounce off the side of the box its momentum changes from +mv to -mv so change in momentum is -2mv
- the time for the molecule to get back to its original position is 2L/v (v=st)
- the force on the side of the cube is calculated by: F = Δp/Δt = 2mv/ (2L/v) = mv^2/L
- P = F/A where A = L^2 so P= mv^2/V
- if the box contained N molecules the pressure would increase by N times so P = Nmv^2/V
- since the molecules move in all directions we can assume there is a third of each direction so P = Nmv^2/3V
- all the molecular speed vary so we used the root mean square P = Nm/3V
What does a maxwell-boltzmann distribution look like for lower temperatures
close together higher peak
What does a maxwell-boltzmann distribution look like for higher temperatures
spread out lower peak
Whats on the axes of a maxwell-boltzmann distribution
Number of molecules against speed
What does the peak of a maxwell-boltzmann distribution show?
the most probable speed
how to calculate the root mean square
- square all the values
- take the mean
- square root
what can 1/2m = 3/2RT be used for
to calculate the mean kinetic energy
how to derive 1/2m = 3/2RT
equate pV = NkT and pV = 1/3Nm
black body
a body that absorbs and emits all incident radiation of all wavelengths
it is a perfect emitter and a perfect radiator
Why is the predicted behaviour of a black body not true
due to the ultraviolet catastrophe
stefan boltzmann law
L = σAT^4
What does the stefan boltzmann law do?
links the factors of a black body
wiens law
λmax T = 0.0029
What does wiens law explain
the relationship between peak wavelengths and the temperature of a body
in terms of molecular energy changes why does the temperature remain constant when something boils?
The average kinetic energy is constant and any input energy increases the potential energy causing molecules to move further apart
What does an intensity against wavelength graph show?
The peak wavelength/ distribution of wavelengths for a black body
If there is a higher temperature how does intensity against wavelength graph change?
The peak is higher and shifted to the left (the peaks do not align)
Theorectically what should an intensity against wavelength graph look like? Why doesn’t it look like this?
exponential decay, its not due to the ultraviolet catastrophe
What causes line spectra
After an electron has been exicted it drops back down the energy levels and releases energy in the form of photons which are then detected and put into line spectra.
What gives line spectra?
all elements
The hotter the object…
the more energy it emits
When drawing a blackbody curve:
- The left hand side must be drawn steeper than the right hand side
The energy per second must decrease towards (but not reach) zero as the wavelength increases. - The line must not cross the energy per second axis (y-axis)
Higher temperature for boltzmann vs blackbody curves
boltzmann - shifts right
blackbody - shifts left
axes for a blackbody curve graph
y axis - energy per second per m^2 (intensity)
x axis - wavelength
area under a boltzmann distrubution
number of particles
boltzmann distrubution axes
y axis - number of molecules
x axis - kinetic energy / speed
the greater the photon energy (hf)…
the greater then number of energy levels the electron moves up
internal energy of a gas
Just kinetic energy (no PE)
KE = 3/2kT
Why does pressure decrease as it cools
- reduced average kinetic energy
- travels slower / less collisions with wall
- change in momentum is less therefore force on walls is less
- therefore pressure is less
condition to use PV/T = PV/T
- one variable is constant
- mass constant
- acts as an ideal gas
relationship between SHC and Number of atoms
The energy required to raise the temperature of 1kg is proportional to the number of atoms in 1kg.
