Chapter 3: Thermal physics

Question 1

What determines the direction of the thermal energy transfer?

Answer 1

temperature determines the direction of thermal energy transfer between two objects.

Question 2

the relation between the Kelvin and Celsius scales of temperature.

Answer 2

K = °C + 273

Question 3

What is the internal energy of a substance?

Answer 3

the internal energy of a substance is the total potential energy and random kinetic energy of the molecules of the substance. (Students should know that the kinetic energy of the molecules arises from their random/translational/rotational motion and that the potential energy of the molecules arises from the forces between the molecules.)

Question 4

Explain and distinguish between the macroscopic concepts of temperature, internal energy and thermal energy (heat)

Answer 4

The heat content of an object, sometimes called the internal energy, is the total amount of heat energy in the object. The thermal energy is the total random kinetic energy of all the individual atoms or molecules in the object. As the temperature increases the heat content also increases by an amount dependent on the specific object

Question 5

Define the mole and molar mass.

Answer 5

Mole is the amount of a substance that contains as many atoms, molecules, ions, or other elementary units as the number of atoms in 0.012 kilogram of carbon 12. The number is 6.0225 × 1023, or Avogadro’s number. Molar Mass is the mass of one mole of a substance, usually expressed in grams or kilograms.

Question 6

Avogadro constant

Answer 6

the number of particles in a mole (for 1 mole, there must be 6.02*10^23 particles).

Question 7

specific heat capacity

Answer 7

The specific heat, c, of a substance is equal to the quantity of heat which must be supplied to unit mass of the material to increase its temperature by 1 degree with no change of state.

Question 8

thermal capacity

Answer 8

Thermal capacity of an object is the quantity of heat which must be supplied to it to raise its temperature by 1 degree. Its units are J K-1.

Question 9

Explain the physical differences between the solid, liquid and gaseous phases in terms of molecular structure and particle motion

Answer 9

♠Solids: Particles vibrate only around a fixed position and maintain a structural shape due to relative closeness of the particles that attract each other strongly.
♠Liquids: Particles flow and slide past each other with some mobility and are relatively close together but do not maintain a structural shape as solids due to their ability to move.
♠In order of ascending entropy: solid -> liquid -> gas.
♠Gas: Particles have fast and free random motion with very high entropy, occupying all the space available to them. The distance between particles that are approximately 10 times greater than in solids and liquids

Question 10

Explain in terms of molecular behaviour why temperature does not change during a phase change.

Answer 10

Temperature does not change during a phase change because the energy supplied goes into increasing the total potential energy stored in the bonds between molecules rather than the kinetic energy which according to the kinetic theory increases temperature when it is increased

Question 11

Distinguish between evaporation and boiling

Answer 11

Boiling occurs when the vapor pressure of the liquid equals at least that of the atmosphere. It may occur throughout the liquid. Evaporation however occurs only on the surface of the liquid.

Question 12

specific latent heat

Answer 12

♪ The quantity of energy needed to change the state of 1kg of a substance without a change in temperature. (J/Kg)

Question 13

pressure

Answer 13

force per unit area

Question 14

State the assumptions of the kinetic model of an ideal gas.

Answer 14

1) All gases consists of particles in constant, random motion that have no intermolecular forces (no attraction/repulsion between particles of the gas) and have no volume .
2) As the temperature increases, the average speed of movement increases.The Kinetic Energy of a gas increases with increasing Temperature.

Question 15

in what is temperature of an ideal gas measured

Answer 15

temperature is a measure of the average random kinetic energy of the molecules of an ideal gas

Question 16

Explain the macroscopic behaviour of an ideal gas in terms of a molecular model

Answer 16

Pressure law:
•Macroscopically, at a constant volume the pressure of a gas is proportional to its temperature in kelvin
•If the temperature of a gas goes up, the molecules have more average KE – they are moving faster on average
•Faster moving molecules will have a greater change of momentum when they hit the walls of the container.
•Thus the microscopic force form each molecule will be greater
•The molecules are moving faster so they hit the walls more often
•For both these reasons, the total force on the wall goes up
•Thus the pressure goes up

Question 17

the equation of state for an ideal gas.

Answer 17

PV=nRT

Question 18

Describe the concept of the absolute zero of temperature and the Kelvin scale of temperature.

Answer 18

• Absolute zero is -273°C

* It is the minimum temperature where no molecules are able to move

Question 19

Deduce an expression for the work involved in a volume change of a gas at constant pressure.

Answer 19

Work is done when there is a change in volume

Question 20

first law of thermodynamics

Answer 20

The internal energy U is the sum of the heat Q and the negative work -W done by the substance. U = Q - W. A positive U means that the substance has increased in temperature, positive W means that the object is doing work by expanding and negative W means that work is done on the object.

Question 21

Isobaric

Answer 21

• Isobaric – the pressure is constant
• The volume increases => work is done by the gas
• The temperature increases => ∆V > 0
• Heat is gained

Question 22

Isochoric

Answer 22

• Isochoric – the volume is constant
• The volume does not change so no work is done W=0
• The pressure increases => temperature increases ∆U > 0
• Heat is added

Question 23

Isotherma

Answer 23

• Isotherma – the temperature is constant
• The temperature is constant => ∆U = 0
• The volume increase => Work is done
• Heat is added

Question 24

Adiabatic

Answer 24

-Adiabatic – the heat is 0
-The temperature increase ∆T > 0 => ∆V > 0
-The volume decreases, so work is done on the gas
Q=∆U+W → 0=∆U-W

Question 25

entropy

Answer 25

entropy is a system property that expresses the degree of disorder in the system.

Question 26

State the second law of thermodynamics in terms of entropy changes.

Answer 26

(A statement that the overall entropy of the universe is increasing will suffice or that all natural processes increase the entropy of the universe.)

Question 27

Discuss examples of natural processes in terms of entropy changes.

Answer 27

(Students should understand that, although local entropy may decrease, any process will increase the total entropy of the system and surroundings, that is, the universe.)

• A refrigerator is an example of a heat pump
• Thermal energy taken from ice box and ejected to surroundings
• Source of work is the electric energy supply.