Thermal Physics

Question 1

What is internal energy?

Answer 1

The sum of randomly distributed kinetic energy and potential energy of all the particles in a body of mass

Question 2

What are the two ways of increasing the internal energy of a system?

Answer 2

Heating - involves energy transfer caused by temperature difference, called a thermal energy transfer
Doing work - involves an energy transfer as a result of a force moving

Question 3

Give an example of when a system does work on an external force? What happens to the system?

Answer 3

If CO2 is contained at high pressure and some gas is allowed to escape, the gas expands rapidly, doing work as it pushes back the atmosphere. This results in the gas cooling as the work done is at the expense of its own internal energy

Question 4

What is the first law of thermodynamics?

Answer 4

• The increase in internal energy of a system is equal to the sum of the energy transferred to the system by heating and by work done on it by an external force
• The decrease in internal energy of a system is equal to the sum of the energy transferred away from the system by cooling and as a result of the system doing work against an external force

Question 5

Why is the specific heat capacity for water so high?

Answer 5

When water is heated most of the energy supplied increases the random potential energy of the water molecules and this doesn’t affect temperature

Question 6

What is specific heat capacity?

Answer 6

The energy needed to raise 1kg of a material by 1°C without any change of state

Question 7

What 3 methods can be used to determine specific heat capacity?

Answer 7

Electrical method - two cavities, immersion heater and thermometer
Method of mixtures - mixing solid with liquid (water) temp. measured from 100°C until thermal equilibrium
Calculating work done - inversion tube method, initial mgh = energy transferred

Question 8

What is specific latent heat of vaporisation?

Answer 8

Energy required to change 1kg of liquid into 1kg of gas with no temperature change, measured in J per kg

Question 9

What is specific latent heat of fusion?

Answer 9

Energy required to change 1kg of solid into 1kg of liquid with no temperature change, measured in J per kg

Question 10

Why doesn’t heating an object always lead to an increase in temperature?

Answer 10

When changing state energy is required to overcome the attractive forces holding the solid or liquid together. Potential energy increase but average kinetic energy does not. Energy is also required to do work against the atmospheric pressure when it changes state (and increases in volume)

Question 11

Why is dissipation a thermal energy transfer that cannot be easily reversed?

Answer 11

The energy has been randomly spread out

Question 12

What is the pressure of a gas?

Answer 12

The force it exerts per unit area

Question 13

Boyle’s law

Answer 13

The volume of a fixed mass of gas at constant temperature is inversely proportional to its pressure

Question 14

What is an isotherm?

Answer 14

A line representing the behaviour of a gas at one specific temperature

Question 15

Charles’ law

Answer 15

The volume of a fixed mass at constant pressure is directly proportional to its kelvin temperature

Question 16

Pressure law

Answer 16

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

Question 17

Avogadro’s law

Answer 17

Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

Question 18

Why are the gas laws much less successful at higher pressures and densities & at lower temperatures?

Answer 18

The average separation between the molecules is much reduced and the van dear waals forces become significant and affect the large-scale properties of the gas

Question 19

What is 1 mole?

Answer 19

Number of atoms in 12 grams or carbon-12, equal to 6.02x10^23

Question 20

During the 1860s and 1870s, James Maxwell, Ludwig Boltzmann and others were developing what theory?

Answer 20

1) The kinetic theory in which liquids and gases were considered to be made up of small particles (atoms or molecules) that are in constant random motion
2) It attempted to explain the gas properties of pressure and temperature in terms of the movement of these particles

Question 21

What is Brownian motion?

Answer 21

The random movements of particles, suspended in a liquid or gas, caused by the high-speed thermal motion of the liquid or gas molecules

Question 22

Why do smoke particles exhibit Brownian motion?

Answer 22

An imbalance of the number of air molecules hitting a smoke particle on one side gives the smoke particle a push in a particular direction. Air molecules must be moving very fast to have sufficient momentum to cause the smoke particles to do this

Question 23

How do we know the gas particle exerts a force on the wall when it bounces off it?

Answer 23

1) It changes direction and therefore momentum (being a vector quantity)
2) Newtons 2nd law states ‘resultant force = rate of change of momentum’, therefore momentum change of the molecule shows that the wall exerted a force on the molecule
3) Newtons 3rd law - molecule must exert equal but opposite force on the wall

Question 24

What does an increase in temperature of a gas result in?

Answer 24

An increase in the average speed of the gas molecules

Question 25

What are the three types of random kinetic energy in gas molecules?

Answer 25

1) Translational - majority of random KE of gas molecules
2) Vibrational - none in a monatomic gas e.g. Helicopters
3) Rotational - very little in monatomic gases

Question 26

What is a monatomic gas?

Answer 26

A gas made up of single atoms

Question 27

What experimental investigation suggested that the temperature of a gas may be linked to the average rather than just speed?

Answer 27

The distribution of molecular speeds of different gases at the same temperature. It showed that the average speed of the molecules depended on the particular gas. On average heavier molecules e.g. Oxygen travelled slower that light atoms such as helium.

Question 28

What is the root mean square speed of the gas molecules in a container?

Answer 28

The sum of the squares of the speeds of all the molecules of gas divided by the number of molecules

Question 29

What is the equation for the average translational kinetic energy per molecule in a gas? Why is this the equation for the average molecular kinetic energy in a monatomic gas?

Answer 29

1) 1/2 x m x (Crms)^2

2) For a monatomic gas the vibrational and rotational kinetic energies are negligible

Question 30

The molecular kinetic theory model of a gas is a theoretical approach to the study of gases. What does it aim to do?

Answer 30

It aims to link the macroscopic quantities of pressure, volume and temperature with microscopic quantities such as the mass, speed and KE of the gas molecules

Question 31

On the basis of the molecular kinetic theory model, what assumptions about an ideal gas are made?

Answer 31

1) The gas molecules move in random directions
2) the volume of the gas molecules is negligible compared to the volume of the gas - molecules relatively far apart
3) The gas molecules do not exert any force on each other except during collisions, there are no IM forces of attraction
4) The time that a gas molecule spends travelling in-between collisions is significantly longer than the duration of a collision
5) The gas molecules have elastic collisions. There is no loss of KE when they collide with the container
6) Newtons laws of motion can be applied to the gas molecules

Question 32

Why are the assumptions about an ideal gas not unreasonable?

Answer 32

1) Brownian motion provides evidence of the random motion of gas molecules
2) Gases can be compressed so the atoms are, on average, far apart
3) Gas molecules in a container at the same temperature are not observed to lose KE or slow down, so their collisions must be elastic

Question 33

What is the equation for the force that the wall of a container exerts on a gas or vice versa?

Answer 33

F = mu^^2/L
m - mass
u - x component of velocity
L - length of container

Question 34

What is the equation for the total force on one wall of a container exerted by all the gases molecules?

Answer 34

F = (Nm x (Urms)^2)/L
N = number of molecules
m = mass
Urms = mean square speed in the x direction
L = length of container

Question 35

What is the kinetic theory equation?

Answer 35

pV = 1/3 x Nm(crms)^2

Question 36

Can the kinetic theory equation be applied to real gases?

Answer 36

Yes with a reasonable degree of accuracy, this is provided the pressures and densities are moderately low

Question 37

What does equating the kinetic theory equation and the ideal gas equation predict? What is this equation?

Answer 37

1) An equation showing the relationship between temperature and the average molecular translational kinetic energy of an ideal gas
2) 1/2 x m(crms) ^2 = 3kT/2 or 3RT/2Na
Where Na = the Avogadro constant