Year 2 Chapter 4: Thermal Physics Flashcards
What is internal energy, U?
The sum of the randomly distributed kinetic and potential energies of the particles in a body
How can internal energy, U, be increased?
Transferring energy to the system by doing work on it or increasing its temperature
What is the first law of thermodynamics?
ΔU = ΔQ - ΔW
ΔU: increase in internal energy of system
ΔQ: thermal energy added to system
ΔW: work done by the system
How do kinetic and potential energy change during a state change?
Potential energy of the system changes however kinetic energy is constant, hence no change in temp during state change
How do kinetic and potential energy change when water is being heated to 100C and boiled?
Energy transferred to kinetic energy, increasing temp. Once reached 100C, energy is transferred to potential energy as bonds between water molecules are broken but NOT kinetic energy, so no increase in temp
What is the equation for thermal energy transferred?
Q= mcΔθ
Q: thermal energy/ J
m: mass/ kg
c: specific heat capacity/ J kg^-1 C^-1
What is specific heat capacity?
The amount of energy required to increase the temperature of 1kg of a substance by 1 Kelvin
What is specific latent heat?
The amount of energy required to change the state of 1kg of a substance without a change of temperature
What is the equation for specific latent heat?
Q= ml
Q: thermal energy/ J
m: mass/ kg
l: specific latent heat/ Jkg^-1
What are the types of specific latent heat?
Specific latent heat of fusion (solid to liquid)
Specific latent heat of vaporisation (liquid to gas)
What are the ideal gas laws?
Laws describing the experimental relationship between pressure, volume and temperature for a fixed mass of gas
What is Boyle’s Law?
pV= k (constant)
When temperature is constant, pressure and volume are inversely proportional
What is Charles’ Law?
V/T= k (constant)
When pressure is constant, volume is directly proportional to absolute temperature
What is the Pressure Law?
p/T= k (constant)
When volume is constant, pressure is directly proportional to absolute temperature
What is the absolute scale?
The Kelvin scale that starts at absolute zero.
K = C + 273
The Kelvin scale is used in all thermodynamic calculations, 1K change = 1C change
What is absolute zero?
The lowest possible temperature of a system, where no heat remains and the particles in the system have no kinetic energy, volume and pressure are zero
What is the ideal gas equation?
pV = nRT
p: pressure
V: volume
n: moles
R: molar gas constant (8.31)
T: temp in Kelvin
What is the ideal gas equation for a number of molecules?
pV = NkT
p: pressure
V: volume
N: number of molecules, n x avogadros
k: Boltzmann constant
T: temp in Kelvin
What is molar mass?
The mass of one mole of a substance, for atoms or molecules is the mass number in g
How do you find the work done on a gas when given a graph of pressure against volume?
Find the area under the curve
What is Brownian motion and what did it give evidence for?
The random motion of particles which gave evidence for the existence of atoms and molecules
How does the simple molecular model explain Boyle’s Law?
If you increase the volume of a fixed mass of gas, its molecules will move further apart so collisions will be less frequent therefore pressure decreases
How does the simple molecular model explain Charles’ Law?
When the temperature of a gas is increased, its molecules gain Ek meaning their velocity increases and because pressure is kept constant (so frequency of collisions is constant) the molecules move further apart and volume is increased
How does the simple molecular model explain the Pressure Law?
When the temperature of a gas is increased, its molecules gain Ek meaning their velocity increases. As volume is constant the frequency of collisions between molecules and their container increases and they collide at higher speeds so pressure is increased
What is the difference between the ideal gas laws and kinetic theory model?
The gas laws are empirical in nature, they come from observation and experimental evidence whereas the kinetic theory model only comes from theory
What are some assumptions for the kinetic theory model?
Motion of molecules is random
Collisions are perfectly elastic
Negligible duration of collisions compared to time between collision
Molecules follow Newton’s Laws of motion
What is the PMT derivation for the kinetic theory model?
(Box with sides l, particle mass m, velocity u)
Δp = mu - (-mu) = 2mu
time to collide with same wall again= 2l/ u
F= Δp/ Δt, 2mu/ (2l/u) = mu^2/ l
P = F/A, mu^2/ l / l^2 = mu^2 / V
Total P = m((u1)^2 + (u2)^2 … ) / V
P = Nm(u^2) / V (mean square speed)
crms^2 = 3ums^2
pV = 1/3 Nm crms^2
What are assumptions for an ideal gas?
Only internal energy is kinetic energy, no potential energy
No interactions other than perfectly elastic collisions
