Topic 3 Thermal Physics

Question 1

temperature definition

Answer 1

a measure of the average random Ek of the molecules in the object (ºC/K)

Question 2

what happens at absolute zero (0K)

Answer 2

• molecules of a substance have stopped moving
• Ek=0
• absolute temperature therefore cannot be negative value

Question 3

what does internal energy consist of

Answer 3

• the total intermolecular potential energy of the molecules - due to force between molecules. (heating increases the intermolecular potential energy of the molecules by moving them further apart)
• the total random kinetic energy of the molecules - due to their random motion. hearing will increase the kinetic energy of the molecules

The molecules have two types of Ek:

• transitional (whole molecules move in a certain direction)
• rotational (molecules rotating about an axis)

Question 4

energy will always flow from “____” to “_____” objects

Answer 4

“hot” to “cold”

Question 5

what is thermal equilibrium

Answer 5

when two objects are in thermal contact, a transfer of energy will occur until the objects have reached the same temperature (thermal equilibrium)

Question 6

what is thermal energy

Answer 6

when “hot” object loses internal energy to the “cold” object, the energy that is being transferred is known as thermal energy of heat (Q)

Question 7

boiling definition

Answer 7

• takes place throughout a liquid
• always occurs at the same temperature (for a specific liquid)

Question 8

evaporating definition

Answer 8

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

only molecules with sufficient Ek leave the liquid, the liquid left behind will become cooler as it is losing the molecules with the highest Ek. the average Ek of the molecules of the liquid will decrease and so the temperature of the liquid decreases during evaporation

Question 9

name of process from solid to gas

Answer 9

sublime

Question 10

name of process from gas to solid

Answer 10

deposition

Question 11

forces between molecules for solid, liquid and gas

Answer 11

solid: strong
liquid: strong
gas: very weak

Question 12

volume of solid, liquid, gas

Answer 12

solid: fixed
liquid: fixed
gas: fills container

Question 13

shape of solid, liquid, gas

Answer 13

solid: fixed
liquid: matches shape of container
gas: fills container

Question 14

motion of molecules for solid, liquid, gas

Answer 14

solid: vibrating about a mean position
liquid: vibrating but no longer about a fixed position
gas: independent

Question 15

thermal capacity definition

Answer 15

the amount of energy required to erase the temperature of an object by 1K (J K^–1 or J ºC^–1)
Used for OBJECTS
Remember this formula, it is not given:
Q = CΔT

Question 16

specific capacity definition

Answer 16

the amount of energy required to raise the temperature of the unit mass of a substance by 1K (J Kg^–1 K^–1)
Used for substances
Change of temperature

Question 17

why do substances have different specific heat capacities?

Answer 17

They have different:
- molecular structures
- forces between molecules and/or atoms
- densities

Question 18

specific latent heat definition

Answer 18

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^–1)
(fusion = solid <––> liquid, vaporisation = liquid <––> gas)
Change of phase

Question 19

why when a substance changes phase does its temperature not change?

Answer 19

• the molecules move further apart as bonds between molecules are broken (or closer together, as bonds are formed) but their speed does not change
• the molecules therefore gain or lose potential energy but not kinetic energy
• as temperature is dependent on the average kinetic energy of the molecules, and this does not change, temperature does not change

Question 20

mole (mol) definition

Answer 20

the amount of a substance that contains the same number of elementary entities as the number of atoms in 0.012kg of 12C

Question 21

Avogadro’s constant (Na) definition

Answer 21

the number of arms in 0.012kg of 12C (6.02 x 10^23)

Question 22

molar mass definition

Answer 22

the mass of one mole of a substance (=atomic mass number in g) eg.
- one mole of Helium-4 has mass of 4g
- one mole of Uranium-238 has mass of 238g

Question 23

ideal gases molecule assumptions

Answer 23

molecules:
- *undergo perfectly elastic collisions between themselves and the walls of their container (no kinetic energy is lost in collisions)
- are spheres
- are in random motion
- *have no intermolecular forces between them (the internal every of an ideal gas is therefore the sum of the kinetic energy of the molecules as Ep=0)
- have negligible volume (treated as points) compared to the volume of the gas as a whole
- spend negligible time in collisions compared to the time spent between collisions

Question 24

under what conditions do real gases approximate to ideal gases?

Answer 24

low pressure, moderator temperature, low density

Question 25

how can pressure change with gases

Answer 25

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

Question 26

pressure law

Answer 26

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

at a constant volume, pressure is proportional to temperature (in K)

↑P=↑F/A

increase in number of collusions per unit time AND average force per collision

Question 27

Charles’ law

Answer 27

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

at a constant pressure, volume is proportional to temperature (in K)

P=↑F/↑A

decrease in number of collisions per unit time on a unit area of the container BUT greater average force per collision between gas molecules and container

Question 28

Boyle’s law

Answer 28

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

at a constant temperature, pressure is inversely proportional to volume

↓P=F/↑A

average force per collision between gas molecules and container is unchanged BUT decrease in number of collisions per unit time on a unit area of the container

Question 29

work done by an expanding gas (constant p)

Answer 29

work done = force x distance
work done = FΔx
as p = F/A, F = pA
so work done = pAΔx
but AΔx = change in volume (V)
therefore, work done = pressure x change in volume
W=PV

see pg 21 topic 3 booklet

Question 30

gas diagrams

Answer 30

see pg 22 topic 3 booklet

Question 31

equation of state for ideal gases rule

Answer 31

an ideal gas is one that follows the three gas laws for all values of p, V and T. This means that an ideal gas cannot be liquified

pV = nRT

Question 32

Kinetic energy of gas molecules:
the pressure and volume of an ideal gas are dependant on two factors

Answer 32

• the number of gas molecules present (N)
• the average random kinetic energy per molecule (average Ek)

Question 33

total internal energy of a sample of ideal gas =

Answer 33

N x average Ek

ideal gas - no potential energy, only Ek