5.1 Thermal physics

Question 1

What does it mean for a systems of objects to be in thermal equilibrium?

Answer 1

There is no net flow of energy between the objects, they are all at the same temperature.

Question 2

If two objects or materials are in thermal contact, what will happen?

Answer 2

Thermal energy will transfew from the hotter to the cooler, until they are the same temperature.

Question 3

How do you convert from Celsius to Kelvin?

Answer 3

Add 273.
Eg. 10 degrees C = 10 + 273 = 283 K.

Question 4

Why is the absolute scale of temperature (also known as the thermodynamic scale) used to measure temperature?

Answer 4

Because it does not depend on the properties of any particular substance?

Question 5

Describe the properties of solids.

Answer 5

Particles vibrate about fixed positions in a regular lattice. They’re held in position by strong forces of attraction.

Question 6

Describe the properties of liquids.

Answer 6

Particles are constantly moving around and are free to move past one other, but are attracted to one another.

Question 7

Describe the properties of gases.

Answer 7

Particles are free to move around with constant random motion. There are no forces of attraction between particles in an ideal gas.

Question 8

What is Brownian motion?

Answer 8

The random movement of small particles suspended in a fluid.

Question 9

How does Brownian motion give evidence for the particle model of matter?

Answer 9

Smoke particles suspended in air can be seen (with a microscope) to move randomly in all directions. This must be as a result of random collisions with particles making up the air.

Question 10

How can the internal energy of a system be calculated?

Answer 10

Internal energy is the sum of the potential and kinetic energies of a system.

Question 11

What is the temperature of absolute zero in Kelvin?

Answer 11

0 Kelvin.

Question 12

What is the temperature at which a
substance has minimum internal energy known as?

Answer 12

Absolute zero.

Question 13

As the internal energy of a body rises, what happens to its temperature?

Answer 13

It rises too.

Question 14

When a substance changes state, what happens to its temperature and why?

Answer 14

The temperature of the substance will also stay the same whilst it changes phase, because the thermal energy is being used to overcome electrostatic bonds between molecules.

Question 15

When a substance changes state, what happens to its potential energy?

Answer 15

It will increase.

Question 16

When a substance changes state, what happens to its kinetic energy and why?

Answer 16

The kinetic energy of the substance remains the same, meaning that the temperature will remain the same, even though the substance is still being heated.

Question 17

How does the internal energy of a substance change during a phase change from solid -> liquid -> gas?

Answer 17

When a substance changes state (from solid -> liquid -> gas) the potential energy increases, but the kinetic energy remains the same.

Question 18

What is the specific heat capacity of substance?

Answer 18

The energy required to raise the temperature of 1kg of a substance by 1K.

Question 19

What equation can be used to determine the energy required to change the temperature of a substance, and what do the symbols in it mean?

Answer 19

E = mcΔ𝜭
Where E = energy, m = mass, c = specific heat capacity, Δ𝜭 = temperature change.

Question 20

What is the specific latent heat of fusion?

Answer 20

The amount of energy needed to change the state of 1 kg of a substance without changing its temperature.

Question 21

What does Avogadro’s constant represent?

Answer 21

The number of atoms there are in one mole of a substance.

Question 22

How can the number of moles in a substance be calculated?

Answer 22

Number of moles = mass / relative formula mass

Question 23

What is the kinetic theory of gases?

Answer 23

A model which can be used to describe how particles within in ideal gas behave.

Question 24

What is an ideal gas?

Answer 24

A hypothetical gas which follows all of the gas laws exactly.

Question 25

What assumptions are made for ideal gases?

Answer 25

• There are a large number of molecules in rapid, random motion.
• Particles (atoms or molecules) occupy negligible volume compared to the volume of gas.
• All collisions are perfectly elastic and the time of the collisions is negligible compared to the time
  between collisions.
• The molecules exert negligible force on each other, except during interactions (there are no intermolecular forces).

Question 26

Why do gases exert a pressure on the container they’re in?

Answer 26

• Gas particles collide with the surfaces of the container.
• The container exerts a force on the particles to change their direction. The particles exert an equal and opposite force on the container.
• Pressure is force applied (in total, by all particles) per unit area.

Question 27

What is the change in momentum of an atom as it collides with and rebounds from the wall of a container?

Answer 27

The collisions between atoms and the wall of the container are perfectly elastic, so the atoms rebound from the wall at the same speed they travel in at. This makes their change in momentum equal to 2mv.

Question 28

How can the average force excerpted by an atom on a container wall be calculated?

Answer 28

As the change in momentum is equal to force multiplied by time, the average force exerted on the atom by the wall is given by F = 2mv / t

Question 29

What is Boyle’s law?

Answer 29

Pressure is inversely proportional to volume, providing temperature is constant.
i.e. pV = constant.

Question 30

What does Charles’ law state?

Answer 30

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

Question 31

What is the ideal gas equation, and what do the symbols in it represent?

Answer 31

pV = nRT
where p is the pressure of the gas (Pa), V is the volume the gas is contained in (m^3), n is the number of moles of gas (mol), R is the molar gas constant (8.31 J mol^-1 K^-1), and T is the temperature of the gas (Kelvin).

Question 32

Assuming constant volume, how are the pressure and temperature of a gas related?

Answer 32

They’re directly proportional.

ie. P/T = constant.

Question 33

Use the kinetic theory of gases to explain why a temperature increase leads to an increase in pressure.

Answer 33

• A temperature increase means the particles have more kinetic energy.
• More kinetic energy means a greater change in momentum during collisions with the container. There are also more frequent collisions.
• Change in momentum is proportional to force applied, and therefore to pressure as well.

Question 34

What equation links N, V, p, m and c?

Answer 34

pV = 1/3 N m C^2

Where p = pressure, V = volume, N = number of particles, m = mass of a particle, ‘c’ = mean square speed.

Question 35

What is meant by the root mean square speed?

Answer 35

The square root of the mean of the squares of the speeds of the molecules.

Question 36

What does the Maxwell-Boltzmann distribution show?

Answer 36

the number of molecules with each speed, against speed c.

Question 37

What does the area beneath a Maxwell-Boltzmann distribution show?

Answer 37

The total number of molecules.

Question 38

As the temperature of a gas increases, how does its Maxwell-Boltzmann distribution change?

Answer 38

The peak of the graph shifts to a higher speed, and the distribution becomes more spread out.

Question 39

What is the Boltzmann constant?

Question 40

True or false: ‘The internal energy of an ideal gas is proportional to absolute temperature’

Answer 40

True.
In an ideal gas there is no ‘potential energy’ component in the internal energy. This means the internal energy is proportional to the kinetic energy (which is, in turn, dependent on temperature).