Thermal physics

Question 1

Thermal equilibrium

Answer 1

when the rate at which 2 objects exchange thermal energy is zero which is when they are at the same temperature

Question 2

2 fixed points on the Celsius scales are

Answer 2

melting point of ice: 0 degrees celsius
Boiling point of water: 100 degrees celsius

Question 3

1 fixed point on the kelvin scale is

Answer 3

Absolute zero: 0K

Question 4

Conversion between Celsius and Kelvin

Answer 4

Kelvin = Celsius + 273

Question 5

Describe solids in terms of spacing ordering and motion

Answer 5

spacing: atoms are very closely packed
ordering: regular
motion: very slow and only vibration

Question 6

describe liquids in terms of spacing ordering and motion

Answer 6

spacing: atoms are closely packed
ordering: random
motion: slow but atoms are free to move

Question 7

Describe gases in terms of spacing ordering and motion

Answer 7

spacing: atoms are very far apart
ordering: random
motion: fast and atoms are free to move

Question 8

describe the simple kinetic model of matter

Answer 8

the idea is that all materials can be modelled by considering the kinetic energy of the individual atoms and their collisions

Question 9

describe how Brownian motion provides evidence for the kinetic model

Answer 9

smoke particles are inserted into a container of gas under a microscope
the smoke particles are seen to randomly change direction, due to the collisions with the invisible, small gas atoms

Question 10

define internal energy

Answer 10

the sum of a systems kinetic energy and potential energy

Question 11

relate internal energy to absolute zero

Answer 11

at absolute zero, a system is at minimum internal enegry

Question 12

describe the relationship between internal energy and temperature

Answer 12

if temperature incresases, kinetic energy increases and therefore internal energy always increases.
however, if internal energy increases, that does not mean that temperature increases

Question 13

Describe the energy transfers occurring during phase change and temperature change

Answer 13

before a phase change, thermal energy and electric potential energy are exchanged, resulting in a change in internal energy but no temperature change as kinetic energy is constant

during a temperature change, thermal energy and kinetic energy + electric potential energy are exchanged (materials expand/contracts as well as particle changing KE)

Question 14

Define and calculate specific heat capacity

Answer 14

the thermal energy required to change the temperature of 1 kilogram of a substance by 1 degree.

c = Q / m∆T

Question 15

describe an experiment to use an electrical method to determine specific heat capacity

Answer 15

(Set up a series circuit with a cell, voltmeter, heater and ammeter, with the voltmeter in parallel with the heater and the ammeter after.)

Measure the mass of the object using a mass balance
Make sure the material is insulated to ensure minimal heat is exchanged with the surroundings and measure the initial temperature with a thermometer. Turn on the heater, record the potential difference and current and measure the temperature every 30s. Plot a graph of temperature against time and find the gradient.

gradient = VI / mc

c = VI / m x gradient

Question 16

define specific latent heat of fusion

Answer 16

the thermal energy required to change 1kg of a substance between a solid and a liquid

Question 17

define specific latent heat of vaporisation

Answer 17

the thermal energy required to change 1kg of a substance between a liquid and a gas

Question 18

calculate specific latent heat of vaporisation

Answer 18

L = Q / m

Question 19

describe an experiment to use an electrical method to determine specific latent of a material

Answer 19

(Set up a series circuit with cell, voltmeter, heater, and ammeter, voltmeter being in parallel with the heater, and the ammeter after.)

Measure the mass of the material using a mass balance. Turn on the heater, measure the current and the potential difference and record the temperature every 30s until the material has completely changed state and the temperature has started to rise. calculate the time for which the material was at constant temperature and so was changing phase.

L = Q / m = VI / m

Question 20

define Avogadro’s constant

Answer 20

the number of molecules of a substance in 1 mole. this is the same for every substance: 6.02 x 10^23

Question 21

describe the assumptions made to model a gas using the kinetic model

Answer 21

the mass of the molecule is negligible compare to the container to allow the collision to be elastic and the molecule to keep all the kinetic energy . the volume of the molecule is negligible compare to the container so that the collision time is negligible compared to the time between collision. the molecules travel in a completely random way, obey the laws of Newtonian physics and the forces between molecules are negligible.

F = ∆mv / ∆t = 2mv / (2L/v) = (mv^2) / L
=> P = F / A = mv^2 / AL

Question 22

explain how increasing the temperature of a gas at constant volume increases pressure

Answer 22

increasing the temperature increases the average kinetic energy of the molecules which means their r.m.s speed increases which increases the momentum change during the collision. since the cross-sectional area and length are constant, the increases in momentum change will cause an increase in pressure

Question 23

explain how decreasing the volume of a container at constant temperature increases pressure

Answer 23

since temperature is constant, molecules r.m.s speed is constant and therefore the momentum change during the collision is the same. decreasing the volume will decrease both the area and also the time between collisions, both of which will act to increase the pressure.

Question 24

state the ideal gas equation

Answer 24

pV = nRT

p: pressure
v: volume
n: number of moles
r: molar gas constant
t: temperature in kelvin

Question 25

describe an experiment to investigate Boyle’s Law

Answer 25

measure the cross-sectional area of the syringe by using a micro meter to measure diameter and also measure the atmospheric pressure using a barometer and the initial gas volume. seal the end of the syringe by melting it and add a known masses to the block and record the new volume. Plot a graph of pressure against the reciprocal of volume which, if Boyle’s law is obeyed, will be a straight line through the origin.

Question 26

describe an experiment to determine absolute zero

Answer 26

completely submerge a containers of gas in a water bath and connect a barometer to measure the pressure. measure the initial temperature with a thermometer and also the initial pressure. adjust the water bath to a higher temperature, allow time for the system to reach thermal equilibrium and record the new temperature and pressure. repeat the process at 5 more temperature and plot a graph of pressure vs temperature in Celsius.

Question 27

state the equation of state and show how it is derived from the ideal gas equation

Answer 27

pV = NkT

k = R/Na
Nk = N(R / Na) = nR
=> pV = nRT

Question 28

define mean square speed and relate it to pressure

Answer 28

the sum of the speed of individual molecules squared divided by the number of molecules. (Average speed squared)

Question 29

describe the impact of temperature of root-mean-speed and the range of speeds of gas particles.

Answer 29

increasing the temperature increases the average kinetic energy/ r.m.s speed. increasing the temperature also increases the range of possible kinetic energies/ speeds the molecule can have.