Thermal Physics Flashcards
internal energy def
The sum of the randomly distributed kinetic and potential energies of the particles in a body
what is there internal energy of a system dependant on
- Temperature (higher temperature, higher kinetic energy and vice versa)
- The random motion of molecules
- The phase of matter: gases have the highest internal energy, solids have the lowest
- Intermolecular interactions between the particles
how can the internal energy of a system be increased or decreased
The internal energy of a system can increase by:
- Doing work on it
- Adding thermal energy to it (heating it)
The internal energy of a system can decrease by:
- Losing thermal energy to its surroundings
- Changing state from a gas to a liquid or liquid to a solid
what is the first law of thermodynamics
The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it (and vice versa)
what happens to the internal energy of a gas when it is expanded or compressed
When a gas expands (its volume increases), work is done by the gas on the surroundings
This decreases the internal energy of the gas
When a gas is compressed (its volume decreases), work is done on the gas by the surroundings
This increases the internal energy of the gas
Specific Heat Capacity def
The amount of thermal energy required to raise the temperature of 1 kg of a substance by 1 °C (or 1 K) without a change of state
what does a value of high or low specific heat capacity relate to
- If a substance has a low specific heat capacity, it heats up and cools down quickly
- If a substance has a high specific heat capacity, it heats up and cools down slowly
-The specific heat capacity of different substances determines how useful they would be for a specific purpose eg. choosing the best material for kitchen appliances
describe how the specific heat capacity of a fluid is measured using a continuous-flow calorimeter
- A fluid flows continuously over a heating element where energy is transferred to the fluid
- It is assumed that the heat transferred from the apparatus to the surroundings is constant
- For this experiment, the flow rate and the potential difference is changed, keeping the change in temperature of the fluid constant
what are the different state changes
Melting = solid to liquid
Evaporation / vaporisation / boiling = liquid to gas
Sublimation = solid to gas
Freezing = liquid to solid
Condensation = gas to liquid
what happens to temperature when there is a state change
nothing
what is the definition of the energy needed to chance a substances state (latent heat)
The thermal energy required to change the state of 1 kg of mass of a substance without any change of temperature
what are the two types of latent heat
- Specific latent heat of fusion (melting)
- Specific latent heat of vaporisation (boiling)
what is the definition of the latent heat of fusion and what does it apply to
The thermal energy required to convert 1 kg of solid to liquid with no change in temperature
Latent heat of fusion applies to:
Melting a solid
Freezing a liquid
what is the definition of the latent heat of vaporisation and what does it apply to
The thermal energy required to convert 1 kg of liquid to gas with no change in temperature
Latent heat of vaporisation applies to:
Vaporising a liquid
Condensing a gas
what are the values of latent heat for water
Specific latent heat of fusion = 330 kJ kg-1
Specific latent heat of vaporisation = 2.26 MJ kg-1
what happens to the kinetic and potential energies of molecules as they change state
The potential energies of the molecules change, but not their kinetic energies
how does heat absorption or release affect intermolecular forces
- The heat absorbed in melting and boiling causes the molecules to move further apart by overcoming the intermolecular forces of attraction
- The heat released in freezing and condensation allows the molecules to move closer together and the intermolecular forces of attraction become stronger
This is because the kinetic energy is proportional to the temperature
-If there is no change in temperature, there must be no change in kinetic energy either
Absolute zero def and what is it equal to
The lowest temperature possible. Equal to 0 K or -273.15 °C
absolute zero definition
The temperature at which the molecules in a substance have zero kinetic energy
what is a change in a temperature of 1 K equal to in degrees Celsius
a change in temperature of 1 °C
what do the ideal gas laws investigate
- The ideal gas laws are the experimental relationships between pressure (P), volume (V) and temperature (T) of an ideal gas
- The mass and the number of molecules of the gas is assumed to be constant for all of these
what does a higher temperature of gas relate to
a higher pressure
Describe Boyle’s Law, it’s equation, and draw its graph
Describe Charles’ law and its equation and graph
Describe the pressure law and give its equation and graph
What is an ideal gas
pressure of a gas def
the frequency of collisions of the gas molecules per unit area of a container
what are the properties of an ideal gas
- Has molecules with negligible volume
- Collisions which are elastic
- Cannot be liquified
- Has no interactions between the molecules (except during collisions)
- Obeys the (ideal) gas laws (Boyles law, Charles’ law and Pressure law)
(All of these can occur at any temperature or pressure)
what do the components of the two gas law equations represent
how can a gas do work on its surroundings by expanding
When a gas expands, it does work on its surroundings by exerting pressure on the walls of the container it’s in
equation for the work done when a volume of gas changes at constant pressure
W = pΔV
Where:
W = work done (J)
p = external pressure (Pa)
V = volume of gas (m3)
definition of the Avogadro’s constant (NA)
The number of atoms of carbon-12 in 12 g of carbon-12
what is one mole of any element equal to
One mole of any element is equal to the relative atomic mass of that element in grams
what is molar mass and what is the equation that relates to it
what is the equation which defines the Boltzmann’s constant
what does the Boltzmann’s constant relate to
- The Boltzmann constant relates the properties of microscopic particles (e.g. kinetic energy of gas molecules) to their macroscopic properties (e.g. temperature)
- This is why the units are J K-1
- Its value is very small because the increase in kinetic energy of a molecule is very small for every incremental increase in temperature
adiabatic def
A reaction is described as being adiabatic if no heat passes to or from the gas.
what is the constant for a fixed mass of ideal gas
what unit is the temperature T in all ideal gas equations in
in Kelvin.
Celsius to kelvin
What is the combined gas law
What are the 5 assumptions needed for the gas laws to work
Differences between gas laws and kinetic theory
Gas Laws:
- The gas laws are empirical in nature which means they are based on observation and evidence
- These are all based on observations of how a gas responds to changes in its environment, namely volume, pressure and temperature from experiment
Kinetic Theory:
- Kinetic theory is based on theory (as stated in its name)
- This means it is based on assumptions and derivations from existing theories
- These are then used to explain why the gas laws behave the way they do
What are the two property types of particles
- microscopic properties of particles (mass and speed) - macroscopic properties of particles (pressure and volume)
What are the assumptions made by the kinetic theory model
- Molecules of a gas behave as identical (or all have the same mass)
- Molecules of gas are hard, perfectly elastic spheres
- The volume of the molecules is negligible compared to the volume of the container
- The time of a collision is negligible compared to the time between collisions
- There are no intermolecular forces between the molecules (except during impact)
- The molecules move in continuous random motion
- Newton’s laws apply
- There are a very large number of molecules
Describe, in words, how the kinetic theory of gases equation can be found/what it represents
Describe how the kinetic theory of gases equation is derived
- Determine the change in momentum as a single molecule hits a wall perpendicularly
One assumption of the kinetic theory is that molecules rebound elastically
This means there is no kinetic energy lost in the collision
If the particle hits one side of the wall and rebounds elastically in the opposite direction to their initial velocity, their final velocity is –c
The change in momentum is therefore:
p = mc
Δp = final p – initial p = −mc − (+mc) = −mc − mc = −2mc
Where:
Δp = change in momentum (kg m s-1)
m = mass of the molecule (kg)
c = speed of the molecule (m s-1)
- Calculate the number of collisions per second by the molecule on a wall
The time between collisions of the molecule travelling to the opposite facing wall and back is calculated by:
dividing a distance of 2L with speed c.
Note: c is not taken as the speed of light in this scenario
- Calculate the force exerted by the molecule on the wall
The force the molecule exerts on one wall is found using Newton’s second law of motion:
mc (squared), divided by L
The change in momentum is +2mc since the force on the molecule from the wall is in the opposite direction to its change in momentum
- Calculate the total pressure for one molecule
The area of one wall is L2
The pressure is defined as the force per unit area:
mc (squared) divided by L (cubed)
This is the pressure exerted from one molecule in a particular direction
- Consider the effect of N molecules moving randomly in 3D space
The pressure equation still assumes that all the molecules are travelling in the same direction and collide with the same pair of opposite faces of the cube
In reality, all molecules will be moving in three dimensions equally and randomly
By splitting the velocity into its components cx, cy and cz to denote the amount in the x, y and z directions, c2 can be defined using Pythagoras’ theorem in 3D:
c2 = cx2 + cy2 + cz2
Since there is nothing special about any particular direction, it can be deduced that:
cx2 = cy2 = cz2
Therefore, cx2 can be defined as:
Speed in x Direction Equation
Where c2 is the sum of the squared speeds of all the molecules
c2 = c12 + c22 + c32 +… + cN2
- Consider the speed of the molecules as an average speed
Each molecule has a different speed and they all contribute to the pressure
Therefore, the square root of the average of the square velocities is taken as the speed instead
This is called the root-mean-square speed or crms
crms is defined as:
Root Mean Square Speed Equation
Therefore
N(crms)2 = c12 + c22 + c32 +… + cN2
- Consider the volume of the box
The box is a cube and all the sides are of length l
This means L3 is equal to the volume of the cube, V
Substituting N(crms)2 and L3 back into the pressure equation obtains the equation:
pV= 1/3 Nm (c (rms) squared)
This is the pressure parallel to the x (or y or z axis)
Multiplying both sides by the volume V gives the final Kinetic Theory of Gases Equation:
Kinetic Theory Final Equation
Where:
p = pressure (Pa)
V = volume (m3)
N = number of molecules
m = mass of one molecule of gas (kg)
crms = root mean square speed of the molecules (m s-1)
What is the equation for the pressure of an ideal gas with density
What is the equation for average kinetic energy
What is the phenomenon of Brownian motion
Small particles (such as pollen or smoke particles) suspended in a liquid or gas are observed to move around in a random, erratic fashion
What does Brownian motion provide evidence for
the existence of molecules in a gas or liquid
What is meant my ‘random motion’
A range of speeds
No preferred direction of movement
Describe how Brownian motion can be observed in smoke
- The observable particles in Brownian motion are significantly bigger than the molecules that cause the motion
- In most cases, these were observed as smoke particles in air
- The air particles cause the observable motion of the smoke particles that we see
- This means that the air particles were small and light and the smoke particles were large and heavy
- The small molecules are able to affect the larger particles in this way because:
They are travelling at a speed much higher than the larger particles
They have a lot of momentum, which they transfer to the larger particles when they collide
Describe how our knowledge of gases has changed over time
- Democritus (2000 years ago)
Ancient Greek and Roman philosopher Democritus named the infinitesimally small pieces of matter atomos meaning ‘indivisible’
This is the etymology of the word ‘atom’
Both of the two most well-known Greek philosophers, Aristotle and Plato, rejected his theories
Due to their influence, Democritus’s theories were not accepted until almost 2000 years later
- Robert Boyle (1662)
Robert Boyle discovered Boyle’s Law
- Guillaume Amontons (1699)
Amontons, and later also by Joseph Louis Gay-Lussac (1809), discovered the Pressure Law
- Jacques Charles (1787)
This was then followed by Charles who discovered the Charles’s Law
- Daniel Bernoulli (18th Century)
Bernoulli assumed that gases were made up of tiny particles which sparked the beginning of kinetic theory
- Robert Brown
Brown discovered Brownian Motion, the random motion of particles in a fluid, which helped support kinetic theory
- Albert Einstein (1905)
In Einstein’s miracle year of 1905, he produced a paper on how kinetic theory was used to make predictions for Brownian motion
Only then did the atomic and kinetic theory of particles start to become more widely accepted.
What is the root mean square speed
How to work out mass of one atom if given mass of 1 mol of the substance
Isothermal process def
a thermodynamic process in which the temperature of a system remains constant.
What do the components of this equation mean
