5 Thermal Physics

Question 1

the triple point

Answer 1

the triple point of a substance is one specific temperature and pressure where the three phases of matter of that substance can exist in thermal equilibrium, that is, there is no net transfer of thermal energy between the phases

Question 2

triple point of water

Answer 2

0. 01 degrees celsius

0. 61kPa

Question 3

temperature

Answer 3

a measure of the hotness of an object on a chosen scale

if one object is hotter than another there is a net flow of thermal energy from the hotter object to the colder one.

Question 4

net flow of thermal energy

Answer 4

if one object is hotter than another there is a net flow of thermal energy from the hotter object to the colder one.
this increases the temperature of the colder object and lowers the temp it the hotter one

Question 5

thermal equilibrium

Answer 5

no net flow of thermal energy between objects

any objects in thermal equilibrium must be the same temp

Question 6

measuring temperature

Answer 6

in order to measure temp a scale is needed that includes two fixed points at defined temps
the temp of other objects can then be defined as a position in the this scale

Question 7

absolute temp scale

Answer 7

or thermodynamic temp scale
uses the triple point of pure water and absolute zero as its fixed points
SI base unit is Kelvin

Question 8

Kelvin

Answer 8

=celsius + 273

always positive
lowest temp on absolute scale is 0K

Question 9

kinetic model

Answer 9

describes how all substances are made up of atoms or molecules, which are arranged differently depending on the phase of the substance

Question 10

solid kinetic model

Answer 10

atoms or molecules are regularly arranged and packed closely together, with strong electrostatic forces of attraction between then holding them in fixed positions, but they can vibrate and so have kinetic energy

Question 11

liquid kinetic model

Answer 11

atoms and molecules are still very close together, but they have more kinetic energy than in solids, and -unlike solids- they can change position and flow past each other

Question 12

gases kinetic model

Answer 12

atoms and molecules have more kinetic energy again than those in liquids, and they are much further apart. they are free to move past each other as there are negligible electrostatic forces between them, unless they collide with each other or the container walls. they move randomly with different speeds in diff directions

Question 13

density

Answer 13

spacing between particles in a substance in different phases effects the density of the substance
in general a substance is most dense in solid phase and least dense in gaseous

Question 14

water

Answer 14

usually solid water is less dense than liquid water
water freezes into a regular crystalline pattern held together by strong electrostatic forces between the molecules
in this structure the molecules are held slightly further apart than in their random arrangement in liquid water, so ice is slightly less dense

Question 15

internal energy

Answer 15

the sum of the randomly distributed kinetic and potential energies of atoms or molecules within the substance

Question 16

increasing the internal energy of a body

Answer 16

increasing temp of a body
kinetic energy of the particles in the body increase

when substance changes phase the temp doesn’t change nor does the KE or the particles. however, their electrostatic potential energy increases significantly. when a substance reaches its melting or boiling point, while it’s changing phase the energy transferred to the substance doesn’t increase its temp. instead the electrostatic potential energy of the substance increases as the electrical forces between the particles change.

Question 17

gas’ electrostatic potential energy

Answer 17

EPE is zero because there are negligible electrical forces between atoms or molecules

Question 18

liquid’s electrostatic potential energy

Answer 18

the electrostatic forces between atoms or molecules give the EPE a negative value
the negative simply means that energy must be supplied to break the atomic or molecular bonds

Question 19

solid’s electrostatic potential energy

Answer 19

the electrostatic forces between atoms or molecules are very large, so the EPE has a large negative value

Question 20

specific heat capacity

Answer 20

the energy required per unit mass to change the temp by 1K and has units of Jkg-1K-1

E= mc(change in temp)

Question 21

determining specific heat capacity of solid

Answer 21

heater and thermometer in solid
insulation surrounding
c=IVt/m(change in T)

Question 22

determining specific heat capacity of liquid

Answer 22

calorimeter
heater and thermometer in liquid
lid and insulation
c=IVt/m(change in T)

Question 23

temp-time graphs

Answer 23

used to determine shc

c=P/m x gradient

Question 24

method of mixtures

Answer 24

another way of determining the shc
known masses of two substances at diff temps are mixed tog
recording their final temp at the thermal equilibrium allows the shc of one of the substances to be determined if the shc of the other is known
c2= m1 c1 T1/m2 T2

Question 25

constant volume flow heating

Answer 25

technique used to heat a fluid passing over a heated filament
used to heat water in some showers and dishwashers and to transfer energy away from heat sources like car engines or nuclear reactors
kg s-1
E/change in t= (change in m/change in t) c change in T

Question 26

specific latent heat

Answer 26

the energy required to change the phase per unit mass while at constant temp
L=E/m
slh of fusion= Lf
slh of vaporisation= Lv

Question 27

specific latent heat of fusion

Answer 27

when a substance is at its MP it requires energy to change phase from solid to liquid
the energy transferred to the substance increases the internal energy of the substance without increasing its temp

Question 28

determining Lf of water

Answer 28

using electric heater
collecting melted ice
thermometer and heater in funnel w ice
thermometer used to ensure the ice is at its melting point, not at a lower temp, and the ice should be seen to be just starting to melt before the heater in switched on
E=IVt
Lf=IVt/m

Question 29

specific latent heat of vaporisation

Answer 29

energy required to change 1kg of substance from its liquid phase to its gaseous phase at its boiling point
is often considerably more than its Lf, because there is a much larger difference between the internal energy of a gas and a liquid than between a liquid and a solid
Lv > Lf for most substances

Question 30

determining Lv

Answer 30

electrical heater used with condenser to collect and then measure the mass of liquid that changes phase
Lv=IVt/m

Question 31

mole

Answer 31

the amount of substance that contains as many elementary entities as there are atoms in 1/12 of carbon-12
avogadro constant NA
6.02 x 1023

Question 32

one mole of any substance contains…

Answer 32

6.02 x 1023 individual atoms or molecules

Question 33

total number of atoms or molecules in a substance equation

Answer 33

N=NA x n

Question 34

kinetic theory of gases

Answer 34

model used to describe the behaviour of the atoms or molecules in an ideal gas

Question 35

what assumptions are made in the kinetic model for an ideal gas?

Answer 35

• gas contains a very large number of atoms or molecules moving in random directions with random speeds
• atoms or molecules of the gas occupy a negligible volume compared with the volume of gas
• collisions of atoms or molecules with each other and the container walls are perfectly elastic (no KE lost)
• time of collisions between the atoms or molecules is negligible compared to the time between the collisions
• electrostatic forces between atoms or molecules are negligible except during collisions

Question 36

how do atoms or molecules in an ideal gas cause pressure?

Answer 36

the A or M in a gas are always moving, and when they collide with the walls of a container the container exerts a force on them, changing their momentum as they bounce off the wall
when a single atom collides with the container wall elastically, its speed doesnt change, but its velocity changes from +u ms-1 to -u ms-1. the total change in momentum is -2mu
the atom bounces between the container walls, making frequent collisions. according to newtons 2nd law, the force acting on the atom is F=change in p/change in t, where change in p=-2mu and change in t is the time between collisions with the wall. from newtons 3rd law, the atom also exerts an equal but opposite force on the wall
a large number of atoms collide randomly with the walls of the container. if the total force they exert on the wall is F, then the pressure they exert on the wall is given by p=F/A where A is cross sectional area of the wall.

Question 37

boyle’s law

Answer 37

for a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume
p directly prop to 1/V
pV=constant
temp + moles of gas are constant
P1V1 = P2V2

Question 38

charles’s law

Answer 38

for a fixed mass of gas at a constant pressure, the volume is proportional to temperature
V dir prop to T
V/T=constant
pressure + moles of gas are constant
V1 / T1 = V2 / T2

Question 39

Gay-Lussac’s Law

Answer 39

the pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature.
p is dir prop to T
p/T=constant
volume + moles of gas are constant 
p1 / T1 = p2 / T2

Question 40

combined gas law

Answer 40

pV/T=k
P1V1 / T1 = P2V2
The volume of a given amount of gas is proportional to the ratio of its Kelvin temperature and its pressure.

Question 41

the equation of state of an ideal gas

Answer 41

pV = nRT
where p is the pressure of the gas (Pa), V is the volume the gas is contained in (m3), n is the
number of moles of gas (mol), R is the molar gas constant (8.31 Jmol-1K-1), and T is the temperature of the gas (Kelvin)

Question 42

investigating boyle’s law

Answer 42

Boyle’s law states that the pressure exerted by a fixed amount of gas is inversely proportional to
its volume at a constant temperature. To investigate this experimentally, a sealed syringe can be filled with gas and connected to a pressure gauge. The syringe can be used to vary the volume of the container, and the values for volume and pressure recorded. When a graph of pressure against 1/volume is plotted, a straight line should be produced, showing a constant relationship.
To increase accuracy in this experiment, the syringe should be lowered slowly so that no heat is produced from friction.

Question 43

Estimating absolute zero using gas

Answer 43

To determine the value of absolute zero in °C, a sealed container of air, connected to a pressure
gauge, is placed in a water bath. The temperature of the water is varied, and the values of temperature and pressure and recorded. When pressure is plotted against temperature, a linear graph will be produced. At absolute zero, the gas molecules will have no kinetic energy, so there will be no collisions with the container walls, resulting in there being no gas pressure. By extrapolating the graph back the x intercept can be found, and this is equal to absolute 0.

Question 44

root mean squared speed

Answer 44

determined by summing the square of all of the individual velocities of molecules, dividing by the number of molecules, N, and then finding the square root of this value
The pressure exerted by a gas, and the mean kinetic energy of molecules in the gas, are related to the r.m.s. speed of the molecules.

Question 45

finding the pressure of a gas at the microscopic level

Answer 45

pV=1/3 Nm mean c2

Question 46

The Maxwell-Boltzmann distribution

Answer 46

Particles in a gas move with random velocities, in
random directions. This means some of the
molecules are moving very fast, whilst other
molecules are barely moving at all. The Maxwell-Boltzmann distribution shows the number of
molecules with each speed, against speed c. The
area under the graph represents the total number of
molecules. As the temperature of the gas increases,
the peak of the graph shifts to a higher speed, and the distribution becomes more spread out.

Question 47

The Boltzmann constant

Answer 47

k=R/NA
k= 1.38 x 10-23
The equation pV = nRT can be written as pV = NRT/NA
because n is equal to N divided by Avogadro’s
constant.
As k = R/NA, we can then write this equation as
pV=NkT

Question 48

Mean kinetic energy and temperature

Answer 48

The kinetic energy of gas molecules in an ideal gas is proportional to the temperature (which
must be measured in kelvin). At a given temperature, every gas molecule has the same mean
kinetic energy, so more massive molecules have lower r.m.s. speeds.
We can relate the two pressure equations, pV=NkT and pV= 1/3 m mean c2 to produce the equation kT=1/3 m mean c2
The equation for kinetic energy is 1/2mv2, so by adjusting the equation, we can produce
3/2kT= 1/2 m mean c2
This shows that Ek= 3/2 kT

Question 49

particle speeds at diff temps

Answer 49

at a given temp the A or M in diff gases have the same average KE
particles with different masses have diff r.m.s speeds

Question 50

Internal energy of an ideal gas

Answer 50

The internal energy of an ideal gas is the sum of the kinetic and potential energies. Since we
assume there are no electrostatic forces between molecules in an ideal gas, there is no potential
energy. This means that for an ideal gas, the kinetic energy is equal to the total internal energy,
hence the internal energy is proportional to temperature.