Topic 3 Thermal Physics Flashcards
temperature definition
a measure of the average random Ek of the molecules in the object (ºC/K)
what happens at absolute zero (0K)
- molecules of a substance have stopped moving
- Ek=0
- absolute temperature therefore cannot be negative value
what does internal energy consist of
- the total intermolecular potential energy of the molecules - due to force between molecules. (heating increases the intermolecular potential energy of the molecules by moving them further apart)
- the total random kinetic energy of the molecules - due to their random motion. hearing will increase the kinetic energy of the molecules
The molecules have two types of Ek:
- transitional (whole molecules move in a certain direction)
- rotational (molecules rotating about an axis)
energy will always flow from “____” to “_____” objects
“hot” to “cold”
what is thermal equilibrium
when two objects are in thermal contact, a transfer of energy will occur until the objects have reached the same temperature (thermal equilibrium)
what is thermal energy
when “hot” object loses internal energy to the “cold” object, the energy that is being transferred is known as thermal energy of heat (Q)
boiling definition
- takes place throughout a liquid
- always occurs at the same temperature (for a specific liquid)
evaporating definition
- takes place at a surface of a liquid
a greater surface area will have a greater rate of evaporation - can happen at all temperatures
only molecules with sufficient Ek leave the liquid, the liquid left behind will become cooler as it is losing the molecules with the highest Ek. the average Ek of the molecules of the liquid will decrease and so the temperature of the liquid decreases during evaporation
name of process from solid to gas
sublime
name of process from gas to solid
deposition
forces between molecules for solid, liquid and gas
solid: strong
liquid: strong
gas: very weak
volume of solid, liquid, gas
solid: fixed
liquid: fixed
gas: fills container
shape of solid, liquid, gas
solid: fixed
liquid: matches shape of container
gas: fills container
motion of molecules for solid, liquid, gas
solid: vibrating about a mean position
liquid: vibrating but no longer about a fixed position
gas: independent
thermal capacity definition
the amount of energy required to erase the temperature of an object by 1K (J K^–1 or J ºC^–1)
Used for OBJECTS
Remember this formula, it is not given:
Q = CΔT
specific capacity definition
the amount of energy required to raise the temperature of the unit mass of a substance by 1K (J Kg^–1 K^–1)
Used for substances
Change of temperature
why do substances have different specific heat capacities?
They have different:
- molecular structures
- forces between molecules and/or atoms
- densities
specific latent heat definition
the amount of energy per unit mass of a substance absorbed or released during a change of phase without a change in temperature (J Kg^–1)
(fusion = solid <––> liquid, vaporisation = liquid <––> gas)
Change of phase
why when a substance changes phase does its temperature not change?
- the molecules move further apart as bonds between molecules are broken (or closer together, as bonds are formed) but their speed does not change
- the molecules therefore gain or lose potential energy but not kinetic energy
- as temperature is dependent on the average kinetic energy of the molecules, and this does not change, temperature does not change
mole (mol) definition
the amount of a substance that contains the same number of elementary entities as the number of atoms in 0.012kg of 12C
Avogadro’s constant (Na) definition
the number of arms in 0.012kg of 12C (6.02 x 10^23)
molar mass definition
the mass of one mole of a substance (=atomic mass number in g) eg.
- one mole of Helium-4 has mass of 4g
- one mole of Uranium-238 has mass of 238g
ideal gases molecule assumptions
molecules:
- *undergo perfectly elastic collisions between themselves and the walls of their container (no kinetic energy is lost in collisions)
- are spheres
- are in random motion
- *have no intermolecular forces between them (the internal every of an ideal gas is therefore the sum of the kinetic energy of the molecules as Ep=0)
- have negligible volume (treated as points) compared to the volume of the gas as a whole
- spend negligible time in collisions compared to the time spent between collisions
under what conditions do real gases approximate to ideal gases?
low pressure, moderator temperature, low density
how can pressure change with gases
- force per collision
- number of collisions per unit area
- number of collisions per unit time
pressure law
“pressure is proportional to temperature if volume is constant (isovolumetric)”
at a constant volume, pressure is proportional to temperature (in K)
↑P=↑F/A
increase in number of collusions per unit time AND average force per collision
Charles’ law
“volume is proportional to temperature if pressure is constant (isobaric)”
at a constant pressure, volume is proportional to temperature (in K)
P=↑F/↑A
decrease in number of collisions per unit time on a unit area of the container BUT greater average force per collision between gas molecules and container
Boyle’s law
“pressure is inversely proportional to volume if temperature is constant (isothermic)”
at a constant temperature, pressure is inversely proportional to volume
↓P=F/↑A
average force per collision between gas molecules and container is unchanged BUT decrease in number of collisions per unit time on a unit area of the container
work done by an expanding gas (constant p)
work done = force x distance
work done = FΔx
as p = F/A, F = pA
so work done = pAΔx
but AΔx = change in volume (V)
therefore, work done = pressure x change in volume
W=PV
see pg 21 topic 3 booklet
gas diagrams
see pg 22 topic 3 booklet
equation of state for ideal gases rule
an ideal gas is one that follows the three gas laws for all values of p, V and T. This means that an ideal gas cannot be liquified
pV = nRT
Kinetic energy of gas molecules:
the pressure and volume of an ideal gas are dependant on two factors
- the number of gas molecules present (N)
- the average random kinetic energy per molecule (average Ek)
total internal energy of a sample of ideal gas =
N x average Ek
ideal gas - no potential energy, only Ek
