Chap. 3 Thermal physics

Question 1

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

Answer 1

Teacher’s notes:Students should be familiar with the concept of thermal equilibrium.

Thermal energy (heat)

• It is the total kinetic energy of the particles of an object and is measured in joules (J).
• It is absorbed, given off, or transferred from on object to another (the energy can not be created or destroyed but it can be transfered) .
• Thermal energy travels from the body of a higher temperature to that of a lower temperature.

Temperature

• It is a measure of kinetic energy of individual particles in a substance rather than a measure of heat energy.
• It is a scalar quantity
• It defines the direction of thermal energy transfer between two bodies in thermal contact. In other words, it is a measure of how hot or cold an object is, and in what direction heat flow is flowing.
• It is measured in a Kelvin (K), or Celsius (C) scale.

Thermal equilibrium

• Two objects are in thermal contact if they can affect each other’s temperature.
• Thermal equilibrium exists when two objects in thermal contact no longer affect each other’s temperature. The thermal energy from the hotter object may flow towards the colder object. They are at thermal equalibrium when the heat stops flowing and their temperatures are the same.

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Question 2

3.1.2 State the relation between the Kelvin and Celsius scales of temperature.

Answer 2

Teacher’s notes: T/K = t/°C + 273 is sufficient.

Absolute zero: The lowest temperature possible. -273.16°C or zero kelvin (0K). When the molecules stop moving.

• Kelvin is the absolute thermodynamic temperature scale.
• Kelvin scale is based on the properties of gas.

E.g. 0°C = 273K ⇔0K = -273°C

Question 3

3.1.3 State that the internal energy of a substance is the total potential energy and random kinetic energy of the molecules of the substance.

Answer 3

Teachter’s note: 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.

**Internal energy **(thermal energy): The energy contained in an object due to the random KE and PE of the molecules.

In other words, internal energy is the sum of all the individual molecules’ random KE (which includes translational [linear] KE and, rotational KE), and PE.

• *KE: * consists of how fast the particles are moving and vibrating.
• PE: consists of the chemical energy stored in bonds in particles called bond energy, and the intermolecular forces of attraction between the particles.

E.g. We increase internal energy when we do work on an object, it enables the molecules to move faster (increasing KE), and move apart by breaking bonds or decreasing intermolecular force (increasing PE)

In a solid, we can increase the KE and PE of the molecules; in a gas it is just the KE. This is because there are no forces between the molecules of a gas, so it doesn’t require any work to pull them apart.

Question 4

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

Answer 4

Teacher’s notes: Students should understand that the term thermal energy refers to the non-mechanical transfer of energy between a system and its surroundings. In this respect it is just as incorrect to refer to the “thermal energy in a body” as it would be to refer to the “work in a body”.

Macroscopic

• Physical properties are macroscopic. At the macroscopic level, temperature is the degree of hotness or coldness of a body as measured by a thermometer. Thermometers are made using the thermometric properties of a substance.
• Internal energy the energy contained in an object due to the random KE and PE of the molecules.
• Thermal energy is the non-mechanical transfer of energy between a system and its surroundings.

Question 5

3.1.5 Define the mole and molar mass.

Answer 5

Atoms

Atoms are the smallest particle of matter, which cannot be split further. It’s (about 10^-10 m, and the nucleus is 10^-15 in diameter)

Mole: The amount of a substance that contains the same number of particles as there are atoms in 12g of carbon-12.

Molar mass:The mass of 1 mole of a substance. E.g. molar mass of water is 18g

Question 6

3.1.6 Define the Avogadro constant.

Answer 6

Avogadro constant:The number of particles in a mole. A=6.02 x 1023

Question 7

3.2.1 Define Specific heat capacity and thermal capacity

Answer 7

**Specific heat capacity (c): **The amount of thermal energy required to raise the temperature of an object by 1K.

Unit: J kg^-1 ºC^-1

Q=mc∆T

Thermal capacity (C): The amount of thermal energy required to raise the temperature of 1kg of a substance by 1K.

Unit: J ºC-1

Q=c∆T

[This does not only apply when things are given heat, but also when they lose heat.]

Question 8

3.2.2 Solve problems involving specific heat capacities and thermal capacities.

Answer 8

Question 9

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

Answer 9

Question 10

3.2.4 Describe and explain the process of phase changes in terms of molecular behaviour.

Answer 10

Students should be familiar with the terms melting, freezing, evaporating, boiling and condensing, and should be able to describe each in terms of the changes in molecular potential and random kinetic energies of molecules.

• While melting, vibrational kinetic energy increases and particles gain enough thermal energy to break from fixed positions. Potential energy of system increases.
• While freezing, particles lose potential energy until thermal energy of the system is unable to support distance between particles and is overcome by the attraction force between them. Kinetic energy changes form from vibrational, rotational and part translational to merely vibrational. Potential energy decreases.
  •While evaporating, certain particles in the liquid gain enough potential energy to escape the intermolecular bonds as a gas. The escape of the higher-energy particles will lower the average kinetic energy and thus lower the temperature.
  •While boiling, substance gains enough potential energy to break free from inter-particle forces. Similar to evaporation, the only difference being that energy is supplied from external source so there is no decrease in temperature.
  •While condensing, the energy changes are opposite to that of boiling.

Question 11

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

Answer 11

During a phase change, the thermal energy gained or lost will not cause the matter to change tempurature. This is because the energy is going towards increasing or decreasing the potential energy of the particles to either overcome or succumb to the inter-molecular force that pulls particles together (aka, try to overcome the forces in order to evaporate). In the process, the average kinetic energy will not change.

However, the internal energy increases during the process but this is hidden by the temperature remaining constant. This hidden internal energy increase is called latent heat.

Question 12

3.2.6 Distinguish between evaporation and boiling.

Answer 12

These are two different processes by which liquids can change to gases:

• *Boiling:** takes place throughout the liquid and always at the same temperature.
• *Evaporation:** takes place only at the surface of the liquid and can happen at all temperatures.

Question 13

3.2.7 Define specific latent heat.

Answer 13

Specific latent heat (L) : The amount of thermal energy required to change the state of 1kg of a substance without a change in temperature.

Specific latent heat of fusion = solid to liquid

Specific latent heat of vaporisation = liquid to gas

Unit: Jkg^-1

Question 14

3.2.8 Solve problems involving specific latent heats.

Answer 14

Teacher’s notes: Problems may include specific heat calculations.

Question 15

Graphical representation of heating

Answer 15

Question 16

3.2.9. Define pressure.

Answer 16

**Pressure: **The force exerted per unit area. Measured in pascals.

Microcopic description of pressure: Many moving particles exert forces when they collide with container walls give the effect of pressure being exerted by the gas.

Force per unit area

Pressure = Force / Area

Unit: Nm^-2
1atm = 1.013 X 10^5 Pa or 101.3kPa

Question 17

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

Answer 17

**Ideal gas: **A theoretical gas composed of a randomly moving non-interacting particles. It is a useful idea because many real gases behave like an ideal gas unless temperature or pressure are very high or low.

· The molecules obey Newton’s laws
· The intermolecular forces are negligible
· The molecules are spherical with negligible volume
· The motion of the molecules is random.
· The collisions are perfectly elastic.
· The time taken for a collision is negligible.

• The molecules are spheres.
• The molecules are identical.
• There are no forces between the molecules (except when they collide) 2 this means that the molecules move with constant velocity between collisions.
• The molecules are very small, that is, their total volume is much smaller than the volume of the gas.

Question 18

3.2.11. State that the temperature is a measure of the average random kinetic energy of the moleculesof an ideal gas.

Answer 18

· The average kinetic energy of the particles is a measure of the gas’s temperature.

Question 19

3.2.12. Explain the macroscopic behavior of an ideal gas in terms of a molecular model.

Answer 19

**Teacher’s notes: **Only qualitative explanations are required. Students should, for example, be able to explain how a change in volume results in a change in the frequency of particle collisions with the container and how this relates to a change in pressure and/or temperature.

Increase in temperature is equivalent of an increase in average kinetic energy (i.e. a greater average speed of particle movement). This leads to more collisions and collisions with greater impulse. Thus resulting in higher pressure.

Decrease in volume results in a smaller space for gas particles to move, and thus a greater frequency of collisions. This results in an increase in pressure. Also, depending on the speed at which the volume decreases, particles colliding with the moving container wall may bounce back at greater speeds. This would lead to an increase in average kinetic energy and thus an increase intemperature.

An increase in volume would have an opposite effect.