Thermal Physics

Question 1

What happens when a substance changes state?

Answer 1

Temp stays constant, K.E of particles does not change, internal energy changes as bonds are made/broken

Question 2

What is the energy supplied from a heater?

Answer 2

E = IVt

Question 3

What is the energy transfer with continuous flow heating?

Answer 3

Energy per second = power of heater = mass flow x specific heat cap x change in temp.

Question 4

What is the number of molecules, N, in a substance of n moles?

Answer 4

N = n x Na (Na is Avogadros)

Question 5

What is the number of moles given the mass of the sample and the molar mass?

Answer 5

n = mass of sample / molar mass

Question 6

What is the number of molecules N given molar mass, mass of sample?

Answer 6

N = (mass of sample)(Na) / molar mass

Question 7

What is 0K in celcius?

Answer 7

-273.15*C

Question 8

What is Boyle’s law?

Answer 8

For a gas at a constant temp, pressure and volume are inversely prop. So P prop to 1/ V. p1v1 = p2v2

Question 9

What is the pressure law?

Answer 9

For a gas at a constant volume the pressure and absolute temperature are directly proportional. p1/t1 = p2/t2; p/t = c

Question 10

What is Charles’ law?

Answer 10

For gas at a constant pressure the volume and absolute temp are directly proportional; v1/t1 = v2/t2; v/t = c

Question 11

What are the units of the parts of the ideal gas equation?

Answer 11

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

Question 12

What is the ideal gas equation for n moles of gas?

Answer 12

pV = nRT; R is a constant given on data sheet.

Question 13

What is the ideal gas equation for N molecules of gas?

Answer 13

pV = NkT, k is a constant given on data sheet.

Question 14

What is the work done by an expanding gas?

Answer 14

Work done to increase / decrease volume at a constant pressure; W = p deltaV

Question 15

What is the theoretical explanation for Boyle’s law?

Answer 15

That reducing the volume increases the pressure as there are more collisions per second between gases and the chamber walls

Question 16

What is the molecular kinetic theory of gases?

Answer 16

Pressure is causes by particles colliding with container walls/surfaces
Temperature is proportional to the average k.e of the molecules

Question 17

What is the theoretical explanation for the pressure law?

Answer 17

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.

Question 18

What is the theoretical explanation of Charles’ law?

Answer 18

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.

Question 19

What are the parts of the kinetic theory equation?

Answer 19

N, molecules of gas; m, mass of each molecule; V, volume of container, and crms, room mean square speed.

Question 20

How do you calculate crms?

Answer 20

crms^2 = (c1^2 + c2^2 + c3^2….. cN^2) / N

Question 21

What is brownian motion?

Answer 21

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.

Question 22

What are the assumptions used to derive the kinetic theory equation?

Answer 22

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

Question 23

What do the kinetic energy equations show for ideal gases?

Answer 23

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

Question 24

Derive the kinetic theory equation (good luck!!)

Answer 24

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