Thermal Physics Flashcards
What is the internal energy of an object?
the total kinetic energy of the molecules and potential energy of the bonds added together
How can internal energy be increased?
internal energy of a system is increased when energy
is transferred to it by heating or when work is done on it
Why do molecules have kinetic energy?
due to their random motion
Why do molecules have potential energy?
the chemical bonds holding the molecules together
Why do gases have (almost) no potential energy?
their molecules are so far apart that there are no intermolecular interactions
Define temperature.
the measure of the average kinetic energy of all particles within the substance.
What is temperature measured in in Thermal Physics?
Kelvin
What does it mean to be in thermal equilibrium?
two objects are the same temp so no heat energy transfer between them
What is heat?
the type of energy which flows when there is a difference in temperature.
How does heat affect internal energy?
the more heat energy, the greater the internal energy
How does heat travel?
hot to cold
How do you convert from celsius to kelvin?
+ 273
How does the amount of kinetic energy change when an object changes state?
KE doesnt change
How does the amount of potential energy between the molecules change when an object changes state?
solid to liquid or liquid to gas = less potential energy
gas to liquid or liquid to solid = more potential energy
How do you calculate the energy change when an objects temperature change?
Q = mcΔθ
where c is the specific heat capacity
How do you calculate the energy change when an object changes state?
Q=ml
where l is the specific latent heat
What is meant by specific heat capacity?
heat energy required to raise the temperature of the unit mass of a given substance by a given amount. every substance has its own unique specific heat capacity
What is specific latent heat of fusion?
heat needed to change a mass of 1 kg the substance from a solid at its melting point into liquid at the same temperature.
What is specific latent heat of vaporisation?
heat needed to change a mass of 1 kg the substance from a liquid at its boiling point into a gas at the same temperature
What must always be mentioned when referring to temperature?
Kinetic energy of molecules
What must always be mentioned when referring to bonds?
intermolecular forces of attraction
What is Boyle’s gas law?
P ∝ 1/V :
pressure is inversely proportional to volume for a fixed mass of gas at a constant temperature
What is Charles’ gas law?
V ∝ T :
volume is directly proportional to temperature for a fixed mass of gas at a constant pressure
What is Avogadro’s law?
P ∝ T :
pressure is directly proportional to temperature for a fixed mass of gas with constant volume
What is the Ideal Gas Equation?
PV = nRT
or PV = NkT
What assumptions must be made when deriving the kinetic theory equation?
any gas contains a large enough sample of particles for statistics to be used
motion is rapid and random
collisions are perfectly elastic (no loss of energy)
there are no attractive forces between particles
repulsive forces only act during collisions
volume of particles is negligible
Derive the kinetic theory equation.
1. calculate the change in momentum: ΔP = -2mu 2. Calculate force on particle: i. Time between collisions: t = s/u t = 2l/u ii. Calculate number of collisions per second (freq): f = 1/t f =u/2l iii. calculate the rate of change of momentum ΔP/ t = ΔP x f ΔP/ t = -m(u^2) / l iv. use Newton II: F = ΔP / Δt F on particle from wall = -m(u^2) / l v. use Newton III: F on particle = - F on wall F on wall from particle = m(u^2) / l 3. Consider N particles in box: F on wall = F1 + F2 + ... + Fn F on wall = m(u^2) / l + m(u^2) / l + ... + m(u^2) / l F on wall = m/l ( u1^2 + u2^2 + ... + un^2) 4. Use mean of squares F on wall = mN(mean u^2) / l 5. convert force to gas pressure: P = F/A P = mN(mean u^2) / lA P = mN(mean u^2) / V PV = mN(mean u^2)
How do you calculate the kinetic energy of gas molecule?
Ek = 3/2 x k x T
How do you calculate work done?
work = PΔV
How do you convert between moles and molecules?
N = n x avogadros constant
What is brownian motion?
the random motion of gas molecules caused by collisions between molecules
What is brownian motion evidence for?
the existence of atoms
What is the difference in validity of the gas laws and the kinetic theory model?
gas laws are empirical by nature whereas kinetic theory is simply a theory
