Thermal Physics

Question 1

Temperature Definition

Answer 1

is a measure of the average random kinetic energy of the molecules in the object

Question 2

Internal Energy

Answer 2

is both of:

the total intermolecular potential energy of the molecules and the total random kinetic energy of the molecules

Question 3

Thermal Energy

Answer 3

is the energy that is being transferred between two objects of different temperatures

Question 4

What happens during boiling

Answer 4

it takes place throughout a liquid

it always occurs at the same temperature (for a specific liquid)

Question 5

What happens during evaporation?

Answer 5

takes place at the surface of a liquid (a greater surface area will have a greater rate of evaporation)
can happen at all temperatures

Question 6

Thermal Capacity (C)

Answer 6

the amount of energy required to raise the temperature of an object by 1K (J/K)
Q =C change in T

Question 7

Specific Heat Capacity (c)

Answer 7

the amount of energy required to raise the temperature of the unit mass of a substance by 1K (J/kg/K)
different substances have different SHC because:
- of different molecular structures
- forces between molecules and/or atoms
- densities

Question 8

Specific Latent Heat (L)

Answer 8

the amount of energy per unit mass of a substance absorbed or released during a change of phase without a change in temperature (J/kg)

Question 9

Mole

Answer 9

the amount of a substance that contains the same number of elementary entities as the number of atoms in 0.012 kg of C12

Question 10

Avogadros Constant (NA)

Answer 10

the number of atoms in 0.012kg of C12 (6.02 X 10 ^23)

Question 11

Molar Mass

Answer 11

the mass of one mole of a substance

Question 12

Assumptions about Ideal Gases

Answer 12

the molecules:

• undergo perfectly elastic collisions between themselves and the walls of their container (no Ek lost in collisions)
• have no intermolecular forces between them (the internal energy of an ideal gas is therefore the sum of the kinetic energy of the molecules as Ep=0)
• also follows the three gas laws for all values of p, V and T

Question 13

What is needed to change the pressure the gas exerts on a container?

Answer 13

• the number of collisions per unit area
• the force per collision
• the number of collisions per unit time

Question 14

Pressure Law

Answer 14

“Pressure is proportional to temperature is volume is constant” (isovolumetric)

increase in temp of gas
increase in average Ek of molecules of gas
increase in the number of collisions per unit time and the average force per collision
increase of total force on the container
increase in pressure
P/T = constant

Question 15

Charles Law

Answer 15

“volume is proportional to temperature if pressure is constant” (isobaric)

increase in temp
increase in average Ek of molecules
but if the volume of the gas also increases
there is a decrease in the number of collisions per unit time on a unit area of the container BUT a greater average force per collision
so the average force per unit area of the container remains the same
pressure is constant
V/T = constant

Question 16

Boyle’s Law

Answer 16

“Pressure is inversely proportional to volume if the temperature is constant” (isothermic)

constant temp
no change in Ek
but if the volume of the gas increases
average force per collision is unchanged BUT decrease in the number of collisions per unit time on a unit area
decrease in total force on the container
decrease in pressure 
pV=constant

Question 17

Work Done by an Expanding Gas

Answer 17

when a gas expands it does work on its surroundings
if the pressure is constant then the force the gas exerts is constant
work done = pressure x change in volume

Question 18

Using Graphs to Calculate Work done

Answer 18

for gas expanding:
the arrow is down
the area below is the work done by the gas on its surroundings

for gas contracting:
the arrow is up
the area below is the work done by the surrounding on the gas

Question 19

What factors are pressure and volume of an ideal gas dependent on?

Answer 19

• the number of gas molecules present (N)
• the average random kinetic energy per molecule
  the total internal energy of a sample of an ideal gas = N x Ek per molecule