Thermal Physics

What is the absolute zero of temperature, in terms of the ideal gas laws?

(Using) the gas laws it is the temperature at which the volume/pressure of a gas extrapolates to zero.

What are the three gas laws?

• Boyle’s Law

• Charles’ Law

• Pressure Law

What are the three laws?

They are empirical laws.

What are empirical laws?

Based on experimental observations rather than from theory then followed by experiment.

What is Boyle’s law?

When the number of particles & temperature are constant, pressure and volume are inversely proportional.

How can you represent Boyle’s Law mathematically?

• P ∝ 1/V = P ∝ k/V

• ….PV = k (constant)

• Can also use for before & after style:

• pv(1) = pv(2)

Sketch a graph to represent Boyle’s law.

What is Charles’ law?

When the number of particles & pressure are constant, the volume and the absolute temperature of a gas (IN KELVIN) are directly proportional.

How do you represent Charles’ law mathematically?

• V ∝ T

• …. V/T = k

• Or can be used as before & after

• V/T(1) = V/T(2)

Sketch a graph to represent Charles’s law

What is pressure law?

When the number of particles and volume are constant, the pressure and absolute temperature of a gas (In kelvin) are directly proportional.

How can you represent pressure law mathematically?

• P ∝ T

• .. P/T = k

• Or can be used as before & after style:

• P/T(1) = P/T(2)

How can absolute zero be found?

• Extrapolating Charles’s law graph

• Extrapolating the Pressure law graph

Complete a graph to show how absolute zero can be extrapolated from Charles’s law.

• An example of how this can be done is using pressure law (where p ∝ t).

• As p ∝ t, this means p∝ (0o + 0) - where 0o = absolute temp (-273)

• This means p ∝ k x (0o + 0)

• Also, 0o (absolute temp) is the value of 0 when pressure = 0

Complete a graph to show how how absolute zero can be extrapolated from the pressure law & explain how.

• To find absolute temp, you need to aim to find the x - intercept.

• As y = Gx + C, as we don’t have C yet, so we need to find the y -intercept (C)

• … So you first you find the gradient.

• .. Then pick a point on the line of the graph (to sub in for x & y).

• … then you can find C.

• Once you have found C, you then need to find Xo (i.e. the x - intercept when y = 0, aka when pressure = 0)

• … 0 = Gx + C

What is molecular mass?

It is the sum of all the atoms that make up a singe molecule (in kg).

What is relative molecular mass?

It is the sum of the relative atomic masses of each atom within the molecule.

What is relative atomic mass?

It is the number of times heavier an atom is than 1/12th of a carbon - 12 atom.

What is avagadros law?

A gas with a fixed v, p & T would have the same number of gas molecules regardless of the gas.

What is Avagadro’s constant?

• 6.02 × 10²³ mol^-1

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

• The number of molecules in a mole.

How do you find out the number of molecules in a gas?

Number of molecules = number of moles x Avagadro’s constant

How can calculate the total mass of a gas?

• Number of molecules X mass of one molecule(kg)

• Number of moles x mass of one mole of molecules(kgmol-1)

State three key assumptions for an ideal gas?

• Particles obey Newton’s laws.

• Particle collisions are elastic.

• Particles move in straight lines between collisions.