Thermal Physics Flashcards
Define internal energy
The sum of the randomly distributed kinetic energies and potential energies of the particles in a body
When is the internal energy of a system increased
When energy is transferred to it by heating or when work is done on it (& visa versa)
Eg: a qualitative treatment of the first law of thermodynamics
What happens to the particle ensemble during a change in state
The potential energies of the particle ensemble are changing but not the kinetic energies
What is the formula for a change in state
Q = ml
Where L is the specific latent heat.
What is the proof for existence of atoms
Brownian motion
What is potential energy in particles due to
The separation between the molecules
What is the internal energy of a system determined by
Temperature
Random motion of molecules
The phase of matter - gases have the highest internal energy, solids have the lowest.
Intermolecular forces between particles
(Greater intermolecular forces, higher potential energy)
State 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)
1st Law: what happens when a gas expands?
WORK is done BY the gas ON the surroundings
Decreasing the internal energy of the gas
What happens when a gas is compressed
Work is done ON gas BY surroundings
Define specific heat capacity
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
How can the specific heat capacity of a fluid be found?
By a CONTINUOUS-FLOW CALORIMETER
Describe the use of 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
A fluid flows through an electrical heating wire. The rise in temperature of the fluid is measured using the electric thermometers and is calculated by:
Δθ = T2 – T1
For mass of fluid: flow rate recorded, multiplied bu time taken to give mass of fluid that flows (M1)
The current I and potential difference V are also recorded
The flow rate is then altered to give a mass m2 and the potential difference of the power supply is changed so the temperature difference, Δθ stays the same
The specific heat capacity is found by assuming the thermal losses to the surroundings are constant for both flow rates
Define 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 2 types of latent heat
Specific latent heat of fusion (melting)
Specific latent heat of vaporisation (boiling)
Define the specific latent heat of fusion
The thermal energy required to convert 1 kg of solid to liquid with no change in temperature
Define the specific latent heat of vaporisation
The thermal energy required to convert 1 kg of liquid to gas with no change in temperature
What does latent heat of fusion apply to
Melting a solid
Freezing a liquid
What does latent heat of vaporisation apply to
Vaporising a liquid
Condensing a gas
What happens during a change in state
- no change in temperature
- the potential energies of the molecules change, but not their kinetic energies
What does the heat in melting and boiling cause the molecules to do
Move further apart by overcoming the intermolecular forces of attraction
In freezing and condensation molecules move closer together and intermolecular forces of attraction become stronger
As
Kinetic energy is proportional to temp
If no change in tempo, no change in kinetic energy either
Define absolute zero
The temperature at which the molecules in a substance have zero kinetic energy
What does boule’s law state
pressure is inversely proportional to the volume of a gas
State charles’s law
the volume is proportional to the temperature of a gas
At constant pressure
State pressure law
pressure is proportional to the temperature
What is an ideal gas
An ideal gas is one that obeys the relation:
pV ∝ T
How does the temperature of a gas relate to the average speed of the molecules
The hotter the gas, the faster the molecules move
Hence the molecules collide with the surface of the walls more frequently
Since force is the rate of change of momentum:
Each collision applies a force across the surface area of the walls
The faster the molecules hit the walls, the greater the force on them
Since pressure is the force per unit area
Higher temperature leads to higher pressure
If the volume V of the box decreases, and the temperature T stays constant:
There will be a smaller surface area of the walls and hence more collisions
This also creates more pressure
Since this equates to a greater force per unit area, pressure in an ideal gas is therefore defined by:
The frequency of collisions of the gas molecules per unit area of a container
Define the 5 qualities 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)
Equation for ideal gas
Define an ideal gas
A gas which obeys the equation of state pV = nRT at all pressures, volumes and temperatures
Work done by a gas formula
W = pΔV
Where:
W = work done (J)
p = external pressure (Pa)
V = volume of gas (m3)
Derive work done by a gas formula
Derivation
The volume of gas is at constant pressure. This means the force F exerted by the gas on the piston is equal to :
F = p × A
Where:
p = pressure of the gas (Pa)
A = cross-sectional area of the cylinder (m2)
The definition of work done is:
W = F × s
Where:
F = force (N)
s = displacement in the direction of force (m)
The displacement of the gas d multiplied by the cross-sectional area A is the increase in volume ΔV of the gas:
W = p × A × s
This gives the equation for the work done when the volume of a gas changes at constant pressure:
W = pΔV
Where:
ΔV = increase in the volume of the gas in the piston when expanding (m3)
This is assuming that the surrounding pressure p does not change as the gas expands
This will be true if the gas is expanding against the pressure of the atmosphere, which changes very slowly
When the gas expands (V increases), work is done by the gas
When the gas is compressed (V decreases), work is done on the gas
Define molar mass
The molar mass of a substance is the mass, in grams, in one mole
Its unit is g mol-1
What is Boltzmann and molar gas constant formula
Where:
R = molar gas constant
NA = Avogadro’s constant
