Thermal physics

Question 1

What energies do particles of all substances have?

Answer 1

-potential energy
-kinetic energy

Question 2

Why is there energy in substances?

Answer 2

kinetic energy is due to the speed of the particles
potential energy is due to the separation/bonds between the molecules

Question 3

Define what temperature is

Answer 3

the average Ek of a substance

Question 4

How can we convert between degrees and kelvin?

Answer 4

T= θ + 273

where:
T= kelvin
θ= °C

Question 5

What do we mean by absolute zero?

Answer 5

the temperature at which the molecules in a substance have zero kinetic energy (0K or -273°C)

Question 6

What do we mean by the internal energy of something?

Answer 6

the sum of all the kinetic and potential energies of all its particles

U= ∑Ep + ∑Ek

Where all components are measured in joules

Question 7

What factors affect the internal energy for a system? (4)

Answer 7

-temperature ( ↑temp, ↑Ek)
-phase of matter ( ↑U when in gaseous state, low U when in solid state)
-intermolecular forces between particles
-random motion of particles

Question 8

Explain how we can increase the internal energy of a system

Answer 8

-doing work on the system
this transfers energy to it e.g. increases the Ek or Ep

-adding heat to it
this increases the temperature which as a result increases the Ek leading to an overall increase in internal energy

Question 9

Explain how we can decrease the internal energy of a system

Answer 9

-losing heat to the surroundings
-changing state from a solid to a liquid or a liquid to gas

Question 10

Why does the internal energy of a substance change when changing state?

Answer 10

as the substance changes state the potential energy changes due to the changing bonds whilst the kinetic energy is kept constant

Question 11

Draw a graph showing how internal energy varies with temperature. Include the different process that occur

Answer 11

https://img-aws.ehowcdn.com/877x500/s3-us-west-1.amazonaws.com/contentlab.studiod/3/11/c0e9e8124ce64f48987682be4aaea0b1.png

Question 12

How can we work out the amount of energy required to change the temperature of a substance?

Answer 12

Q = mcΔθ

where:
Q= energy required (J)
m= mass (kg)
c= specific heat capacity (Jkg^-1°C^-1)
Δθ= change in temperature (°C)

Question 13

What is specific heat capacity?

Answer 13

the amount of energy required to increase the
temperature of 1 kg of a substance by 1 °C/1 K, without changing its state

Question 14

What are the different processes to change state?

Answer 14

melting = solid to liquid
vaporisation = liquid to gas
sublimation = solid to gas
freezing = liquid to solid
condensation = gas to liquid

Question 15

What is specific latent heat?

Answer 15

the thermal energy required to change the state of 1 kg of mass of a substance without any change of temperature

Question 16

What is the equation involving energy required to change the state of a substance and specific latent heat?

Answer 16

Q=mL

where:
Q= energy required to change state (J)
m= mass (kg)
L= specific latent heat (Jkg^-1)

Question 17

What are the two types of latent heat? When are they used?

Answer 17

latent heat of fusion (melting)
-melting a solid
freezing a liquid

latent heat of vaporising (boiling)
-vaporising a liquid
-condensing a solid

Question 18

Define latent heat of fusion

Answer 18

the thermal energy required to convert 1 kg of solid to liquid with no change in temperature

Question 19

Define latent heat of vaporisation

Answer 19

the thermal energy required to convert 1 kg of liquid to gas with no change in temperature

Question 20

What are some things to know when a substance is changing state?

Answer 20

-there is no change in temperature
-the potential energies of the particles change, but not their kinetic energies

Question 21

What does it mean if bonds are being broken/stronger? Explain why

Answer 21

-the heat absorbed in melting and boiling causes the particles to move further apart by overcoming the intermolecular forces of attraction
-the heat released in freezing and condensation allows the particles to move closer together and the intermolecular forces of attraction become stronger

Ek is proportional to temperature (as temperature is the avg. Ek)
-if there is no change in temperature, there must be no change in kinetic energy either

Question 22

What are the ideal gas laws? What does it mean if they’re empirical?

Answer 22

the experimental relationships between pressure (P), volume (V) and temperature (T) of an ideal gas

meaning they are not based on theory but arose from observation and experimental evidence

Question 23

What is constant for all the ideal gas laws?

Answer 23

mass
number of particles

Question 24

State Boyle’s law

Answer 24

if the temperature T of an ideal gas is constant, then Boyle’s Law is given by:

p ∝ 1/V

where:
p= pressure (Pa)
v= volume (m^3)

Question 25

How else can Boyle's law be shown for situations involving a before and after scenario?

Answer 25

p1V1= p2V2

Question 26

Draw the graph showing Boyle's law

Answer 26

https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/05/6.5.2-Boyles-Law.png

Question 27

State Charles' law

Answer 27

if the pressure P of an ideal gas is constant, then Charles’s law is given by: V ∝ T

Question 28

How else can Charles' law be shown for situations involving a before and after scenario?

Answer 28

V1/T1 = V2/T2

Question 29

Draw the graph showing Charles' law

Answer 29

https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/05/6.5.2-Charles-Law.png

Question 30

State the pressure law

Answer 30

if the volume V of an ideal gas is constant, the Pressure law is given by: p ∝ T

Question 31

How else can the pressure law be shown for situations involving a before and after scenario?

Answer 31

p1/T1 = p2/T2

Question 32

Draw the graph showing the pressure law

Answer 32

https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/05/6.5.2-Pressure-Law.png

Question 33

What is an ideal gas?

Answer 33

an ideal gas is one that obeys the relation: pV ∝ T Where: p = pressure of the gas (Pa V = volume of the gas (m^3) T = temperature (K)

Question 34

How can we find out the internal energy of an ideal gas?

Answer 34

equal to the kinetic energy

Question 35

What are some characteristics of an ideal gas? (5)

Answer 35

-has molecules with negligible volume -collisions which are elastic -cannot be liquified -has no interactions between the molecules (except during collisions) -obeys the (ideal) gas laws (Boyles law, Charles’ law and Pressure law)

Question 36

What are the two equations of state for an ideal gas?

Answer 36

pV = nRT where: p= pressure (Pa) V= volume (m^3) R= molar gas constant (Jkg^-1mol^-1) n= number of moles (mol) pV=NkT where: p= pressure (Pa) V= volume (m^3) k= Boltzmann constant (JK^-1) N= number of molecules T= temperature (K)

Question 37

What are the two constants which can be used in the ideal gas equation?

Answer 37

k (boltzmann constant)- 1.38 x 10^-23JK^-1 R (molar gas constant)- 8.31Jkg^-1mol^-1

Question 38

How can we find the number of molecules (N) from the number of moles?

Answer 38

N = Na x n

Question 39

How can we change the volume of a gas at a constant pressure?

Answer 39

do work on it

Question 40

What is meant when gas expands/compresses

Answer 40

expands- work is done BY the gas compresses- work is done ON the gas

Question 41

What is the equation linking work done, pressure and volume?

Answer 41

W = pΔV where: W= work done (J) p= pressure (Pa) V= volume (m^3)

Question 42

What do we mean by the atomic mass?

Answer 42

the mass of a proton/neutron (1.67 x 10^-27)

Question 43

State avagadro's number

Answer 43

6.0-2 x 10^23

Question 44

What do we mean by relative atomic mass?

Answer 44

the mass number e.g. H20 relative atomic mass (2 x 1) + 16= 18g

Question 45

What do we mean by molar mass?

Answer 45

the mass in one mole g mol-1

Question 46

Give the equation that links moles, molar mass and mass

Answer 46

https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/04/6.5.1-Number-of-Moles-Equation_2.png

Question 47

What is brownian motion?

Answer 47

the random motion of larger particles in a fluid caused by collisions with surrounding particles

Question 48

Compare the three laws to the kinetic theory model

Answer 48

-the three laws are empirical whereas the kinetic theory model arose only from theory -the kinetic theory model has mulitple assumptions

Question 49

What are some assumptions of the kinetic theory model? (4)

Answer 49

-no intermolecular forces act on the molecules -the duration of collisions is negligible in comparison to time between collisions -the motion of molecules in random, and they experience perfectly elastic collisions -the motion of the molecules follows Newton’s laws -the molecules move in straight lines between collisions

Question 50

How can we work out the change in internal energy for an ideal gas?

Answer 50

ΔU= 3/2kΔT where: U= internal energy (J) k= boltzmann constant T= temperature (K)

Question 51

How is internal energy and temperature related for an ideal gas?

Answer 51

ideal gas molecules are assumed to have no intermolecular forces (this means they have no Ep, only Ek) as the avg. Ek is the definition of temperature: ΔU ∝ ΔT

Question 52

Derive the kinetic theory of gases equation (derivation which is needed)

Answer 52

1. determine change in momentum as a single molecule hits the wall perpendicularly Δp = final p – initial p = −mv − (+mv) = −mv − mv = −2mv 2. calculate the number of collisions per second by the molecule on a wall time between collisions= distance/speed t= 2L/v 3. calculate the force exerted by the molecule on the wall f= rate of change of momentum (mv/t) f=2mv / (2L/v) f= 2mv^2/L 4. calculate the total pressure for one molecule p= force/area p= (2mv^2 / L) / L^2 p= mv^2 / L^3