Thermal Physics Flashcards
What happens when a substance changes state?
Temp stays constant, K.E of particles does not change, internal energy changes as bonds are made/broken
What is the energy supplied from a heater?
E = IVt
What is the energy transfer with continuous flow heating?
Energy per second = power of heater = mass flow x specific heat cap x change in temp.
What is the number of molecules, N, in a substance of n moles?
N = n x Na (Na is Avogadros)
What is the number of moles given the mass of the sample and the molar mass?
n = mass of sample / molar mass
What is the number of molecules N given molar mass, mass of sample?
N = (mass of sample)(Na) / molar mass
What is 0K in celcius?
-273.15*C
What is Boyle’s law?
For a gas at a constant temp, pressure and volume are inversely prop. So P prop to 1/ V. p1v1 = p2v2
What is the pressure law?
For a gas at a constant volume the pressure and absolute temperature are directly proportional. p1/t1 = p2/t2; p/t = c
What is Charles’ law?
For gas at a constant pressure the volume and absolute temp are directly proportional; v1/t1 = v2/t2; v/t = c
What are the units of the parts of the ideal gas equation?
Volume V in m^3
Pressure P in N m^-2
Temperature T in K
Number of molecules / atoms N (no unit)
Amount of gas n in mol
What is the ideal gas equation for n moles of gas?
pV = nRT; R is a constant given on data sheet.
What is the ideal gas equation for N molecules of gas?
pV = NkT, k is a constant given on data sheet.
What is the work done by an expanding gas?
Work done to increase / decrease volume at a constant pressure; W = p deltaV
What is the theoretical explanation for Boyle’s law?
That reducing the volume increases the pressure as there are more collisions per second between gases and the chamber walls
What is the molecular kinetic theory of gases?
Pressure is causes by particles colliding with container walls/surfaces
Temperature is proportional to the average k.e of the molecules
What is the theoretical explanation for the pressure law?
That increasing the temp. increases the average k.e of the molecules, thus making collisions with walls harder and more frequent and so increasing the pressure.
What is the theoretical explanation of Charles’ law?
That increasing temperature will make the particles in the gas collide more often and harder with the walls, so the volume must increase - making collisions less frequent - for a constant pressure to be maintained.
What are the parts of the kinetic theory equation?
N, molecules of gas; m, mass of each molecule; V, volume of container, and crms, room mean square speed.
How do you calculate crms?
crms^2 = (c1^2 + c2^2 + c3^2….. cN^2) / N
What is brownian motion?
Evidence for the existence of atoms, as molecules randomly jiggle in a fluid - likely caused by collisions with atoms and molecules in random constant motion.
What are the assumptions used to derive the kinetic theory equation?
All molecules are identical and the volume of each is negligible compared to volume of gas
Molecules are in continuous random motion
Newton’s laws can be applied to molecules, and there are enough that averages can be taken
Collisions between particles and walls are perfectly elastic
Molecules exert no force on each other except during collisions
Time between collisions is much longer than the duration of each collision
What do the kinetic energy equations show for ideal gases?
Average kinetic energy is directly prop. to temperature
Mean k.e of a molecule = 3/2 kT
Total k.e of a mol = 3/2 RT
Total ke of N moles = 3/2 nRT
Derive the kinetic theory equation (good luck!!)
Consider molecule of mass m, velocity c, inside cube with sides length L.
Split velocity into vectors, c^2 = cx^2 + cy^2 + cz^2
Change in momentum when hitting a right-hand wall is -2mcx, as -mcx - mcx = -2mcx
Time between collisions is distance to opposite wall and back divided by v, t = 2L/cx
Collisions per second is 1/t = cx / 2L
Force is momentum x collisions per second = -mcx^2 / L
Pressure = F/A, pressure on wall = mcx^2 / L^3
Volume = L^3 so pressure is mcx^2 / V
Ptotal is sum of pressures, Ptotal = m/V (c1x^2 + c2x^2 … cNx^2)
Ptotal = Nm(sum of cx^2) / V
So pressure = Nm(cxrms)^2 / V
But (cxrms)^2 = 1/3 (crms)^2
So p = 1/3 Nm(Crms)^2 / V
