Transcript Thermal_Physics_summary_SL_and_HL

Thermal Physics
04/08/2016
Heat flows from hot to cold
Net energy flow stops when their temperatures are the same
i.e. They are in thermal equilibrium
Tc = Tk - 273
04/08/2016
Tk = Tc +273
The Mole and Molar mass
One mole of “anything” contains 6x1023 (the Avogadro
constant NA) number of atoms (or molecules)
One mole of carbon
One mole of green atoms
contains 6x1023 atoms
contains 6x1023 atoms
Definition of the mole: the amount of
substance containing as many elementary
particles as there are in 12 g of Carbon-12
Molar mass = mass of one mole
04/08/2016
Temperature, Internal Energy
and Thermal energy (Heat)
Temperature: A measure of the average
random kinetic energy per molecule.
The internal energy of a substance is the
sum of the molecular kinetic and potential
energies.
Kinetic energy arises from the translational and rotational motions
Potential energy arises from the forces between the molecules
The term heat represents energy transfer
due to a temperature difference resulting in
an increase in the kinetic energy of the
molecules.
Thermal (HEAT) Capacity
 Heat
capacity = Q / T in JK-1
◦ Q = the change in thermal energy in joules
◦ T = the change in temperature in Kelvin


Defined as the amount of energy needed to
change the temperature of a body by unit
temperature.
Applies to a specific BODY
Specific Heat Capacity
Specific Heat Capacity = Q / (mT) in J kg
where m is the MASS of the material
-1
Heat Capacity = m x Specific Heat Capacity
K-
Microscopic Characteristics
Characteristics
Solid
KE
Vibrational
PE
High
Liquid
Gas
Mostly
Vibrational Translational
Rotational
Higher
Some
Rotational
Higher
Translational
Vibrational
Higher
Highest
Heating ice
04/08/2016
Temp/OC
150 This flat line shows where energy is being
100
used to break the temporary bonds for
boiling. The amount of energy needed to
turn 1kg of a liquid into a gas is called the
Specific Latent Heat of Vaporisation L.
50
0
-50
Time/s
This flat line shows where energy is being
used to break bonds – this has to be done
during melting. The amount of energy
needed to turn 1kg of a solid into a liquid is
called the Specific Latent Heat of Fusion L.
04/08/2016
Latent heat (changes
in potential energy)
Energy = mass x specific latent heat
Evaporation and Boiling
04/08/2016
• Boiling occurs at a fixed temperature
• Evaporation occurs at any temperature
• Boiling happens throughout the body of the
liquid
• Evaporation only happens at the surface of
the liquid.
•The average kinetic
energy of the
remaining particles
drops
•The temperature of
the remaining liquid is
lowered
Assumptions of the kinetic theory
of gases
04/08/2016
1. Molecules behave as if they were hard, smooth,
elastic spheres. (i.e. the collisions are perfectly
elastic)
2. Molecules are in continuous rapid, random motion.
3. The average kinetic energy of the molecules is
proportional to the absolute temperature of the
gas.
4. The molecules do not exert any appreciable
attraction on each other.
5. The volume of the molecules is infinitesimal when
compared with the volume of the gas.
6. The time spent in collisions is small compared with
the time between collisions.
Because the collisions are perfectly elastic there is no loss
of KE as a result of the collisions
Pressure = Force / Area
Pressure can be explained by the collisions
with the sides of the container
 If the temperature increases, the average KE
of the particles increases
 The increase in velocity of the particles leads
to a greater rate of collisions and hence the
pressure of the gas increases as the
collisions with the side have increased.
 When the volume of a gas decreases
collisions are more frequent with the sides of
the container leading to an increase in
pressure and/or temperature.

AHL
Topic 10
Ideal Gases and Real Gases
An Ideal Gas: Is a theoretical gas that obeys the
gas laws and thus fits the ideal gas equation
exactly.
Real gases behave as ideal gases at
room temp and pressure.
 The gas molecules become
“interacting” at high temperatures
and high pressures. Therefore they
lose there ideal properties.
 Ideal gases cannot be liquefied.
 Ideal gases obey the gas equation.

Combining the gas laws gives
PV = (a constant)×T
Therefore, for n moles of an ideal gas;
P = Pressure
PV = nRT
V = Volume
n = number of moles
R = Universal gas constant
T = Temperature (KELVIN)
Deduce an expression
for the work involved in a
volume change of a gas
at constant pressure.
The work done by this force is w = Fs = PAs,
since F=PA
but As is the change in the volume occupied by the gas, ΔV.
therefore;
W = PV
State the first law of thermodynamics.
We can add energy to a gas by heating Q
(temperature gradient)
Or by working (mechanical energy) = W
Q = ΔU + W
Q = Heat energy added to the
ΔU = Internal energy
increase of the gas
W = Work done by the gas.
Students should be familiar with
the terms system and
surroundings. They should also
gas appreciate that if a system and
its surroundings are at different
temperatures and the system
undergoes a process, the
energy transferred by nonmechanical means to or from
the system is referred to as
thermal energy (heat).
Change of p (and T) at
constant
volume;
an
isovolumetric change.
2. Change of V (and T) at
constant
pressure;
an
isobaric change.
3. Change in p and V at
constant temperature; an
isothermal change.
4. Change in p and V in an
insulated container (no
heating of the gas); an
adiabatic change.
1.
The product of pressure and volume represents a quantity of
work. This is represented by the area below a p-V curve.
Therefore, the area enclosed by the four curves represents the
net work done by the engine during one cycle.
Second Law of Thermodynamics: It is impossible to
extract an amount of heat QH from a hot reservoir
and use it all to do work W . Some amount of heat QC
must be exhausted to a cold reservoir.
Entropy is a measure of the disorder (of the
energy) of a system
Every time we change energy from one form to
another, we increase the entropy of the
Universe even though local entropy may
decrease.