Chapter 14 - Thermal Physics

Question 1

Define temperature

Answer 1

Level of hotness of an object

Question 2

outline the Celsius scale

Answer 2

uses the melting and freezing point of water, under atmospheric pressure

Question 3

outline the thermodynamic scale

Answer 3

triple point of water, 273.16K and absolute zero

Question 4

converting between kelvin and Celsius

Answer 4

K = C + 273

Question 5

what is thermal equilibrium

Answer 5

No net transfer of thermal energy between two objects or substances

Question 6

electrostatic potential energy in solids, liquids and gases

Answer 6

weak in solids, max of 0J in gases

Question 7

Brownian motion

Answer 7

particles exhibit random motion because of the collisions of other molecules, for example smoke particles in air exhibit Brownian motion showing air particles are much smaller than smoke particles and move erratically

Question 8

Internal energy

Answer 8

Sum of randomly distributed kinetic and potential energies associated with the atoms or molecules which make up the substance

Question 9

What happens to the KE and PE of a substance when heated

Answer 9

During state change, so melting, PE increases but KE stays the same, temperature also remains constant, because thermal energy is being used to break bonds
When a substance is heated but doesn’t change state, KE increases and PE is constant

Question 10

Absolute zero

Answer 10

-273C or 0K where the molecules completely stop moving and has minimal internal energy.

Question 11

Specific heat capacity with equation

Answer 11

energy required per unit mass (1kg) to increase the temperature by 1K, E=mcΔθ

Question 12

Specific latent heat with equation

Answer 12

Energy required to change the state of a substance per unit mass at constant temperature, E=mL

Question 13

What is the amount of a substance

Answer 13

mole

Question 14

How to find amount of a substance

Answer 14

n = m/M where m is the mass and M is the molar mass

Question 15

Assumptions for Kinetic theory of gases

Answer 15

VENCT

• Volume of gas atoms is negligible compared to the volume of the gas
• Electrostatic forces between atoms are negligible, except for when the atoms are colliding
• The gas contains a large number of atoms which move with random, rapid motion
• Collisions between atoms and walls are perfectly elastic
• Time taken for atoms to collide is negligible compared to the time between collisions

Question 16

Explain how a gas in a container produces a pressure

Answer 16

• as the collisions between atoms and the walls are elastic, they rebound with the same speed they travel in at.
• this makes their change in momentum mv - (-mv)
• equal to 2mv
• As the change in momentum is equal to Ft
• the average force exerted on the wall is F = 2mv/t
• from newtons third law we know the wall will exert an equal force in an opposite direction
• The total pressure on the wall is equal to the sum of each collision between atom and wall, and the area of the wall

Question 17

Boyle’s Law

Answer 17

For a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume

Question 18

Charles Law

Answer 18

For a fixed mass of gas at a constant volume, the pressure is proportional to temperature

Question 19

Ideal gas equation

Answer 19

pV = nRT

Question 20

how to find root mean square speed

Answer 20

• sum all individual velocities of atoms
• divide by number of atoms
• square root this value

Question 21

r.m.s equation for pressure

Answer 21

pV = ⅓Nmc^2 where c is the r.m.s

Question 22

What does the Maxwell-Boltzmann distribution show

Answer 22

number of molecules with each speed, against speed

Question 23

what happens to the graph when temperature is increased

Answer 23

it gets shallower and wider

Question 24

derive boltzmann constant (k) equation

Answer 24

• if k = R/Na where Na is avagrados number
• pV = NRT/Na
• pV = NkT

Question 25

relation of KE and temperature

Answer 25

KE α Temperature (K)

Question 26

derive equation for mean kinetic energy of gas molecules

Answer 26

• pV = NkT and pV = ⅓Nmc^2
• kt = ⅓mc^2
• 3/2kt = 1/2mc^2
• KE = 3/2kt

Question 27

relation of KE and internal energy

Answer 27

KE α internal energy as we assume electrostatic forces are zero, there is no potential energy.