thermal physics Flashcards
what is specific heat capacity?
The amount of energy needed to raise the temperature of 1kg of the substance by 1 degree C or 1K
what is the internal energy of a body?
the sum of randomly distributed Kinetic and Potential energies of all its particles
what temperature is absolute zero in degree C and K?
-273 degree C, 0K
how does KE change with temperature?
At 0K, molecules have no KE.
As temperature increases so does KE.
In kelvin, KE is proportional to temperature
how to convert degree C to K
add 273
speed of particles in a gas
gas particles travel at different speeds
speed distribution of gas particles depends on temperature
most will travel around average speed
what happens as the temperature of a gas increases to the:
speed
KE
PE
distribution curve
average particle speed increases
average KE increases
PE increases unless its an ideal gas
distribution curve becomes more spread out
what is a closed system?
one that doesn’t allow any transfer of matter in or out
what happens to internal energy and energy transfer in a closed system?
total internal energy is constant as long as it isn’t heated or cooled and no energy is transferred to or from the system.
energy is constantly being transferred between particles within a system through collisions
but total combined energy of all particles stays the same
what happens to the average speed of a closed system?
it will stay the same as long as the temperature stays the same and no work is done on the system.
how do you increase the amount of internal energy in a closed system?
heat it or do work to to transfer energy to the system by changing the shape. this will also increase average speed of particles.
how do you decrease the amount of internal energy in a closed system?
cooling the system or by doing work to remover energy. will lead to reduced average speed of the particles.
how is heat transferred in terms of temperature and energy?
higher temperature to lower
higher energy particles to lower
how does the difference in temperature between two substances effect heat transfer?
higher the difference, faster the transfer of heat
does a hotter or cooler substance radiate heat quicker?
hotter
what happens to a particles KE when its temperature rises?
KE increases
what is continuous-flow heating?
when a fluid flows continuously over a heating element. as it flows energy is transferred to the fluid. you can use this to find the specific heat capacity
what can you use to find out specific heat capacity?
continuous-flow calorimeter
as a substance changes state what happens to its internal energy, KE, PE and temperature?
internal energy and PE changes
KE and temperature stay the same
what is latent heat?
the energy needed to melt a solid or evaporate a liquid in order to break the bonds between the particles
what is specific latent heat?
the amount of thermal energy needed to be gained or lost to change the state of 1kg of a substance
what is Boyle’s law?
At a constant temperature, the pressure and volume of a gas are inversely proportional.
p1v1=p2v2
how does temperature change effect the curve on a pressure-volume graph?
the higher the temperature, the further away the curve is from the origin.
what’s Charles’ law?
at constant pressure, the volume is directly proportional to the absolute temperature
v1/T1=v2/T2
what does a (volume or pressure) vs temperature graph look like from the ideal gas laws?
straight line (through origin in Kelvin or intercept x at -273 in degrees)
what is the pressure law?
at constant volume, the pressure of an ideal gas is directly proportional to its absolute temperature
p1/T1=p2/T2
what is molecular mass?
the sum of the masses of all the atoms that make up a single molecule
what is relative molecular mass?
the sum of the relative atomic masses of all the atoms making up a molecule
what is the Avogadro constant?
the number of atoms in 12g of carbon 12.
gives the number of atoms in any volume of substance who’s mass in grams is the same as its relative atomic mass
what is molar mass for identical atoms in a molecule ?
the mass that one mole of the substance would have and is equal to its relative atomic or molecular mass.
what is molar mass for non identical atoms in a molecule?
the mass of one mole is often twice their relative masses because many gases exist as molecules which are more than two atoms bonded together.
in an ideal gas what is the equation involving pressure, temperature and volume?
(p1 x v1) /T1 = (p2 x v2) /T2
what’s the ideal gas equation for n moles?
pV = nRT
V= volume
p= pressure
n= number of moles
R= molar gas constant
T= temperature in K
what is the Boltzmann constant?
the gas constant for one molecule of gas
what’s the equation for the ideal gas equation for N molecules?
pV = NkT
what must happen for a gas to expand or contract at constant pressure?
work must be done so there must be a transfer in energy.
what does the area under a graph of pressure against volume show?
the energy transferred to change the volume of the gas (the work done)
deriving the pressure of an ideal gas
1) say molecule Q is moving towards wall A with velocity u
2) assuming the collisions are perfectly elastic, the wall pushes back on the molecule when it hits the wall causing it to rebound in the opposite direction with momentum -mu
3) change in momentum is 2mu
4) assume Q has no collisions with other molecules, time between collision of Q and A is (2 x length) /u.
5) number of collisions per second is therefore u/ (2 x length) so rate of change in momentum is 2mu x (u/ (2 x length))
6) force= rate of change of momentum N2 Law so force exerted on the wall by one molecule is (2mu^2) / (2 x length)= (mu^2) / length
7) Q is one of many so total force of all molecules on wall A= m(u^2+u^2+…) / length
8) mean squared speed is then (u^2+u^2+…) / N
9) put into equation before to get F=(Nm x mean squared speed) / length
10) pressure = force/area so p= (Nm x mean squared speed) / V
11) treat all N molecules the same so c^2= 3 mean squared c^2
12) pV= 1/3 (Nm x mean squared speed)
mean squared speed units
m^2 s^-2
if the volume of a container is fixed, why will increased temperature result in increased pressure inside the container?
there will be more collisions between the molecules and the walls of the container in a given amount of time.
on average, collisions will result in larger changes in momentum and exert a larger force on the walls of the container.
why does, at constant pressure, the volume of the container increase to compensate for the temperature change?
if the volume is larger, there will be a longer time between molecule-wall collisions so the rate of change of momentum causing an decrease in force on the walls of the container
as the volume increase, SA of the walls increases, so increasing the area stops the pressure from increasing.
assumptions in kinetic theory
all molecules of a gas are identical.
the gas contains a large number of molecules.
the molecules have negligible volume compared to the volume of the container.
the molecules continually mover around randomly.
the motion of the molecules follow newtons laws
collisions between molecules themselves or at walls of a container are perfectly elastic.
molecules move in straight lines between collisions
the forces that act during collisions last for much less time than the time between collisions
what is an ideal gas?
a gas that obeys the assumptions in kinetic theory model
for an ideal gas, what can you assume about the internal energy?
in the form of kinetic energy
how to rearrange equations to find average kinetic energy of gas molecules?
use pV= nRT and pV= 1/3 (Nm x mean squared speed)
equate formulas and then multiply by 3/2
How to calculate mass using the molar mass
mass= molar mass/ Avogadro’s constant
at a constant temperature, explain using the kinetic theory why pressure decreases as atoms leave though an exit hole?
fewer atoms means less collisions per second.
average momentum change per collision stays constant so total momentum change decreases as less collisions occur.
A number of assumptions are made when explaining the behaviour of a gas using the molecular kinetic theory model.
State one assumption about the size of molecules.
The volume/size of the gas molecules is negligible/point mass or point
molecule
Or molecules are point masses
Or small compared to the volume/size occupied by of the gas
