Thermal Physics

Question 1

Describe the graph of Boyles law (pressure against volume)

Answer 1

Positive rectangular hyperbola. Higher temp further from origin

Question 2

Define Charles’ law

Answer 2

At constant pressure the volume of a gas is directly proportional to temperature. V/T =constant

Question 3

What does the graph of Charles’ law looks like (volume against temp)

Answer 3

Straight line. Positive gradient. Goes through absolute zero (0K)

Question 4

Give the pressure law

Answer 4

At constant volume the pressure is directly proportional to temperature P/T =constant

Question 5

What does the graph of the pressure law look like (pressure against temp)

Answer 5

Straight line positive gradient goes through absolute zero (0K)

Question 6

Define Avagadro’s constant

Answer 6

The number of atoms in exactly 12g of carbon 12.

Question 7

What does Avagadro’s constant give you?

Answer 7

The number of atoms or molecules in any volume of substance whose mass in grams is the same as its relative atomic mass

Number of molecules(N) =n(number of moles)xNA(Avagadro’s constant)

Question 8

How do you derive the ideal gas equation?

Answer 8

pV/T=constant put in room temp, atmospheric pressure for one mole gives constant as 8.31 (R) pV/T changes depending on amount of gas so constant becomes nR then times up by T to get pV=nRT

Question 9

What is Boltzmanns constant,k, equivalent to?

Answer 9

K=R/Na

Question 10

Give Boyles law

Answer 10

At a constant temperature the pressure and volume of a gas are inversely proportional pv=constant

Question 11

How do you derive the the pressure of an ideal gas?

Cube of lxlxl one face called A

Answer 11

Molecule Q moving to wall A with velocity u. Momentum=mu. Assume collision perfectly elastic, wall pushes back on molecule so rebounds and heads back with -mu momentum. Change momentum =2mu. Assume q suffers no collisions with other molecules time between collisions with A=2l/u number of collisions per sec =u/2l so rate of change of momentum =mu^2/l =force exerted on wall by one molecule. Q is one of many molecules so goal force =m(u1^2+u2^2…)/L. Mean square speed =u1^2+u2^2…/N so f=Nm(mean square speed)/l pressure=f/A so f/l^2 so Nmu’^2/l^3. L^3=V gas molecules move in three dimensions so get speed(c) for pythag treat all N molecules in same way the crms^2 =u’^2+v’^2+w’^2 molecules move randomly so u’^2=v’^2=w’^2 so Crms^2=3u’^2 sub into p equation so p=1/3 NmCrms^2/V

Question 12

Give some assumptions in teh kinetic theory

Answer 12

All molecules in the gas are identical
Gas contains a large number of molecules
Molecules move rapidly and randomly
Motion of molecules follows newtons laws
Collision between molecules and walls are perfectly elastic
No forces between molecules except during collisions
Forces during collisions are instantaneous
Molecules act as point masses

Question 13

What sort of curve is the graph of speed distribution of gas molecules?

Answer 13

Maxwell-Boltzmann

Question 14

Give the definition specific heat capacity

Answer 14

Spc of a substance is the amount of energy needed to raises the temperature of 1kg of substance by 1 kelvin

ΔQ=mcΔT

Question 15

Define specific latent heat

Answer 15

Slh of fusion or vaporisation is the quantity of thermal energy required to change the state of 1kg of a substance

ΔQ=ml