2. Thermal Physics

Question 1

State the properties between solid, liquids and gases

Answer 1

S-L-G
1. Fluidity: No-shape of the container-shape of the container
2. Compressibility: No-No-High
3. Volume: Definite-Not definite-Not definite
4. Shape: Definite-Not definite-Not definite

Question 2

Name the terms of changes of state between solid , liquid and gas

Answer 2

1. Melting: Solid→Liquid
2. Boiling/evaporation: Liquid→Gas
3. Condensation: Gas→Liquid
4. Freezing: Liquid→Solid

Question 3

List the particle structure of soid, liquid and gas

Answer 3

S-L-G
1. Arrangement: Regular-Random-Random
2. Separation: Tightly packed-Loosely packed-Far apart
3. Motion: Vibrate at a fixed position-Move around each other-Move quickly in all directions

Question 4

How does the forces and distances between particles (atoms, molecules, ions and electrons) and the motion of the particles affects the properties of solids, liquids and gases

Answer 4

The forces between molecules and matter (atoms, ions and electrons) affect that state of matter because the magnitude of the forces affect the relative distances and motion of the particles

Question 5

Describe the relationship between the motion of particles and temperature

Answer 5

As the temperature increases, the motion of particles also increase due to an increase of average speed in particles as they’ve more kinetic energy (directly proportional)

Question 6

Describe the idea that there is a lowest possible temperature

Answer 6

• The amount of pressure a gas exerts in a container is dependent on the temperature as they gain kinetic energy
• Must be a lowest possible temperature where particles are stationary as they cannot travel slower than 0m/s; known absolute zero, −273°C

Question 7

Describe the pressure and the changes in pressure of a gas in terms of the motion of its particles and their collisions with a surface

Answer 7

• Molecules in a gas are in constant random motion at high speeds; so a sudden change in motion when they collide
• Collisions with the walls of its container creates pressure

Question 8

Describe the pressure and the changes in pressure of a gas in terms of the forces exerted by particles colliding with surfaces, creating a force per unit area

Answer 8

• Collisions of gas particles with the walls of its container, produces a force
• Pressure is force per unit area so the force exerts a pressure
• Particles travel faster, collisions occur more frequently, exerting a greater force; if the force exerted (per unit area) is greater and more frequent, the pressure exerted also increases

Question 9

Describe and explain this motion (sometimes known as Brownian motion) in terms of random collisions between the microscopic particles in a suspension and the particles of the gas or liquid

Answer 9

• Brownian motion: The random movement of particles in a liquid or a gas produced by large numbers of collisions with smaller particles which are often too small to see (‘microscopic’ particles)

Question 10

Describe qualitatively, in terms of particles, the effect on the pressure of a fixed mass of gas of: a change of temperature at constant volume

Answer 10

• At a constant volume: If Temperature ↑, then Pressure ↑ (directly proportional, so in a graph, they form a straight line)
• An increase of temperature in a gas, increases the average speed of gas molecules causing it to travel faster. Molecules collide with the walls more frequently at a greater force, increasing the pressure

Question 11

Describe qualitatively, in terms of particles, the effect on the pressure of a fixed mass of gas of: a change of volume at constant temperature

Answer 11

• At a constant temperature: If Volume ↑ (Expansion), then Pressure ↓; and if Volume ↓ (Compression), then Pressure ↑ (inversely proportional, so in a graph, they form an inward curve)
• Molecules collide with the walls more frequently when there’s a decrease in volume (lesser space). Increasing the pressure as the overall net force on the walls is larger

Question 12

Convert temperatures between kelvin and degrees Celsius; recall the equation

Answer 12

• Equation: T (in K) = θ (in °C) + 273

Question 13

State the equation of Boyle’s law for a fixed mass of gas at constant temperature, including a graphical representation of this relationship

Answer 13

• Boyle’s Law: pV = constant / p ∝ 1/V OR p₁V₁ = p₂V₂
• Units: Pressure (Pa/MPa) and Volume (m³/cm³)
• In a graph: An inward curve

Question 14

Describe, qualitatively, the thermal expansion of solids, liquids and gases at constant pressure

Answer 14

• Increase in temperature at constant pressure, increases the overall volume
• Thermal expansion happens when molecules gain kinetic energy, hence they vibrate faster. As a result, collisions between them are more frequent, pushing each other apart

Question 15

Explain, in terms of the motion and arrangement of particles, the relative order of magnitudes of the expansion of solids, liquids and gases as their temperatures rise

Answer 15

• Solid: The particles are tightly packed and vibrate in place. As a result, it expands slightly as low energy molecules cannot overcome the intermolecular forces
• Liquid: The particles are loosely packed and slide past each other, so they expand more than solids. As molecules have enough energy to partially overcome the intermolecular forces
• Gas: The particles are far apart and move freely in every direction, leading to a significant expansion. For the high energy molecules have enough energy to completely overcome the intermolecular forces

Question 16

Describe some of the everyday applications of thermal expansion

Answer 16

a) Liquid-in-glass thermometer: Relies on the expansion of liquids to measure temperature
b) Temperature-activated switches: Utilises a bimetallic strip that expands at different rates and bends by a predictable amount at a given temperature (upwards when heated)

Question 17

Describe the consequences of thermal expansion

Answer 17

• When solid materials expand, they can buckle if it gets too hot.
• Objects prone to this have gaps to create space for the expansion to happen without causing damage

Question 18

(not in the syllabus b.p, js for understanding) Explain thermal expansion

Answer 18

As heat is added, the increase in temperature leads to an increase in kinetic energy, causing the molecules and atoms to move more quickly and move apart (separation).

Question 19

If temperature increase, what happens to the internal energy of the object

Answer 19

Increase

Question 20

Describe an increase in temperature of an object in terms of an increase in the average kinetic energies of all of the particles in the object

Answer 20

• Heating a system increases the kinetic energy of a substance. Henceforth, changing a substance’s internal energy
• Therefore the relationship of temperature, average kinetic energy and the internal energy (sum of all kinetic (motion) and potential energies (positions)) is directly proportional

Question 21

Define specific heat capacity

Answer 21

The energy required per unit mass per unit temperature increase

Question 22

Describe melting and boiling in terms of energy input without a change in temperature

Answer 22

At the M.P and B.P, the internal energy doesn’t rise as temperature doesn’t increase even if more thermal energy is added

Question 23

State the melting and boiling temperatures for water at standard atmospheric pressure

Answer 23

M.P for water is 0°C and B.P for water is 100°C

Question 24

Describe condensation in terms of particles

Answer 24

Condensation (gas → liquid): At B.P, energy has been transferred away from the potential store of the particles, reducing its potential energy. They don’t have enough energy to overcome intermolecular forces, but to only flow (liquid state)

Question 25

Describe solidification in terms of particles

Answer 25

Solidification (liquid → solid): At M.P, energy has been transferred away from the system, reducing its potential energy. They don’t have enough energy to overcome intermolecular forces, but to only vibrate around a fixed point

Question 26

Describe evaporation in terms of the escape of more-energetic particles from the surface of a liquid

Answer 26

Evaporation (liquid → gas): Occurs when more energetic molecules moving near the surface of the liquid have enough energy to escape. Reducing the average temperature, as particles with the most energy leaves

Question 27

Describe how temperature, surface area and air movement over a surface affect evaporation

Answer 27

a) Temperature ↑, E↑: Increases the kinetic energy of molecules. So more energy molecules able to overcome the intermolecular forces and escape the surface b) Surface Area ↑, E↑: Since molecules only escape the intermolecular forces at the surface of the liquid c) Air Movement ↑, E↑: Carries away evaporated water vapour, replacing the air with a drier air which accepts water vapour easily

Question 28

Explain the cooling of an object in contact with an evaporating liquid

Answer 28

When an object is placed in contact with an evaporating liquid, the object cools as the cooler liquid absorbs thermal energy from the object

Question 29

Describe the differences between boiling and evaporation

Answer 29

-Boiling: At the B.P, throughout the liquid -Evaporation: At any temperature between the B.P & M.P, from the surface of the liquid

Question 30

Describe thermal conduction in all solids in terms of atomic or molecular lattice vibrations and also in terms of the movement of free (delocalized) electrons in metallic conductors

Answer 30

- Heating causes the atoms/ions to vibrate more. Atoms at the source of heat vibrate the most, bumping into each other; transferring internal energy until thermal equilibrium - Free electrons can move freely, therefore they transfer thermal energy through collisions (against the atoms)

Question 31

Describe, in terms of particles, why thermal conduction is bad in gases and most liquids

Answer 31

Thermal conduction uses the vibration of particles to transfer thermal energy (heat), they require the particles to be closed together. The arrangement of gases and liquids aren’t close.

Question 32

Explain convection in liquids and gases in terms of density changes

Answer 32

1. Heated molecules vibrate and push each other apart, causing the liquid/gas to expand 2. Hot liquid/gas becomes less dense than the surroundings 3. Causing them to rise, and the cooler liquid/gas falls (so that less dense liquid/gas move into the space created) 4. Hot liquid/gas that was raised above, eventually cools and contracts as molecules lose energy and collide less 5. Their density increases and so they sink down again

Question 33

Brief introduction of radiation

Answer 33

Thermal radiation is infrared radiation, which all objects emit. They don’t require a medium (like conduction and convection)

Question 34

Describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission, absorption and reflection of infrared radiation

Answer 34

- Black: Good absorber and emitter - White: Poor absorber and emitter - Dull: Reasonable absorber and emitter - Shiny: Very poor absorber and emitter (Reflects)

Question 35

Explain how an object can be at a constant temperature

Answer 35

For an object to be at a constant temperature it needs to transfer energy away from the object at the same rate that it receives energy - Needs to reach thermal equilibrium

Question 36

What happens to an object if the rate at which it receives energy is less or more than the rate at which it transfers energy away from the object

Answer 36

- If the rate at which an object receives energy is greater than the rate at which it transfers energy away: the object's temperature will increase - If the rate at which an object transfers energy away is greater than the rate at which it receives energy: the object's temperature will decrease

Question 37

How is the temperature of the Earth is affected by factors controlling the balance between incoming radiation and radiation emitted from the Earth’s surface

Answer 37

- The Earth receives heat from the thermal radiation of the sun, and emits its own thermal radiation (longer wavelength) - Greenhouse gases in the atmosphere absorbs the radiation and emits (reflects) it back to the surface of the Earth – called the greenhouse effect which makes the Earth warmer

Question 38

Describe how the rate of emission of radiation depends on the surface temperature and surface area of an object

Answer 38

- Surface Temperature ↑, E ↑: Particles on the surface are more energetic, leading to more emission of radiation - Surface Area ↑, E ↑: Radiation is emitted on the surface and more particles can vibrate and release energy as radiation

Question 39

Explain some of the basic everyday applications and consequences of conduction, convection and radiation, including: heating a room by convection

Answer 39

1. Metal radiator transfers thermal energy to air nearby 2. Hot air particles spread out, decreasing the density 3. Becomes less dense than the surroundings, rising above 4. Radiator heats the cold air, replacing the hot air 5. Newly heated air rises, cools and sinks

Question 40

Explain some of the complex applications and consequences of conduction, convection and radiation where more than one type of thermal energy transfer is significant, including: (a) a fire burning wood or coal

Answer 40

1. A lot of thermal energy is transferred to the room by radiation as the fuel is so hot 2. Greater amount of thermal energy is transferred to nearby objects by radiation 3. Air surrounding the fire is heated and rises, forming a convection current

Question 41

Explain some of the complex applications and consequences of conduction, convection and radiation where more than one type of thermal energy transfer is significant, including: (b) a radiator in a car

Answer 41

1. A liquid passes over the engine, absorbing energy through conduction 2. Travels back to the radiator, transferring heat to the radiator by conduction 3. Radiator transfers thermal energy to the surrounding by radiation 4. Cycle repeats as the liquid cools after the radiator has absorbed its thermal energy