Thermal Physics

Question 1

Why do solids have a fixed shape and a fixed volume?

Answer 1

• Particles vibrate about fixed positions
• Particles packed very closely together with little spaces in between in an orderly manner

Question 2

Why do liquids have a not fixed shape but a fixed volume?

Answer 2

• Particles slide past one another randomly
• Particles are packed closely together with little spaces in between in a random manner

Question 3

Why do gas have not a fixed shape and volume?

Answer 3

• Particles move randomly at high speeds constantly
• Particles are far apart from one another

Question 4

Where can you see brownian motion?

Answer 4

• Microbeads in water that seem to move randomly
• Smoke particles in air moving randomly.

Question 5

How does heating affect the expansion of solid, liquid and gas?

Answer 5

When heat applied, the temperature increases. Particles gain kinetic energy and move faster. Particles will be further from one another hence leading to the expansion of solid/liquid/gas

Question 6

How does gas in an enclosed container create a pressure?

Answer 6

Gas particles are moving randomly at high speeds and are always colliding ,exerting a force, with the walls of the container. A large number of this gas particles colliding with the walls of the container at a high rate creates a large force creates a pressure.

Question 7

Explain why gas in an enclosed container kept at a constant volume with increasing temperature results in an increase in pressure?

Answer 7

An increase in the gas temperature results in an increase in the kinetic energy of the gas particles. The air particles will move faster. The air molecules will then bombard the walls of their container more forcefully and more frequently. By P=F/A, with a larger magnitude of force and same area, The gas pressure increases.

Question 8

Explain why gas in an enclosed container kept at a constant temperature with increasing volume results in an decrease in pressure?

Answer 8

An increase in the gas volume results in** decrease in the number of air molecules per unit volume.** The air molecules will therefore bombard the walls of the container less frequently. There is a lesser average force per unit area .This result in a decrease in the gas pressure.

Question 9

How does gas in an enclosed container kept at constant pressure with increasing temperature result in a increase in volume?

Answer 9

An increase in the gas temperature results in an increase in the gas particles’ kinetic energy. The air molecules will move faster. The air molecules will then bombard the walls of their container more forcefully and more frequently. To maintain a constant pressure, the air expands and the air molecules move farther apart. As the air molecules occupy a larger volume, they collide with the walls less frequently.

Question 10

Define internal energy.

Answer 10

Internal energyis an energy store that is made up of the totalkinetic energyassociated with the motion of the particles and the totalpotential energybetweenintermolecular forces

Question 10

Define temperature.

Answer 10

Thetemperatureof a body is related to the average kinetic energyof all the particles in the body.

Question 11

Define conduction.

Answer 11

Conduction is the transfer of thermal energy due to collisions/vibrations of particles.

Question 12

Define convection.

Answer 12

Convection is the transfer of thermal energy by means of convection currents of a fluid, due to a difference in density.

Question 13

Define radiation

Answer 13

Radiation is the transfer of thermal energy by infrared radiation without the need for a medium.

Question 14

How is energy transferred through conduction?

Answer 14

Particles closer to the heat source vibrate more vigorously about their fixed positions. They collide with neighbouring particles, transferring energy and neighbouring particles vibrate more vigorously. This process continues

Question 15

Why are metals good conductors of heat?

Answer 15

Metal conducts heat faster as it consists of **fast-moving free electrons. **The small size and high speed of the electrons allow them to move easily and quickly to the cooler end and **transfer energy to the particles there. **

Question 16

Explain why solids are better thermal conductors than liquids and gases

Answer 16

Particles in solids are very closely packed as compared to particles in liquids and gases. Hence, kinetic energy is able to be **passed on to neighbouring particles faster by collisions. **

Question 17

How is a convection current set up?

Answer 17

When a liquid is heated, it expands and becomes less dense. The warmer liquid ,being less dense than surrounding, rises. The colder fluid, being denser, sinks to the bottom to be heated again.

Question 18

What are some applications of convection?

Answer 18

• mines are ventilated by vertical shafts
• household ventilation (air-con unit or heater)
• location of heater coil in kettle and boiler
• heat loss in car radiator and refrigerator

Question 19

What makes an object a good emitter of radiant heat?

Answer 19

• colour of the surface (black)
• texture of the surface (rough)
• surface area (large surface area)

Question 20

Define heat capacity

Answer 20

Heat capacity (C) is the amount of thermal energy required to raise the temperature of a substance by 1 K or °C.

Question 21

Define specific heat capacity

Answer 21

Specific heat capacity (c) the amount of thermal energy required to raise the temperature of a unit mass (e.g. 1 kg) of a substance by 1 K (or 1°C).

Question 22

Define latent heat of fusion

Answer 22

Latent heat of fusion is thermal energy required to change the state to solid to liquid or vice versa, at a constant temperature.

Question 23

Define latent heat of vaporisation

Answer 23

Latent heat of vaporisation is the thermal energy required to change the state from liquid to gas or vice versa, at a constant temperature.

Question 24

How does cooling take place in evaporation?

Answer 24

The particles in the liquid move at different speeds and are in constant and random motion (different average kinetic energy). Evaporation occurs when the ** faster particles at the surface of the liquid** have enough energy to break their intermolecular forces to escape into the air. After the faster particles have escaped into the air, the particles left behind have a slightly lower average kinetic energy. As temperature decreases with the average kinetic energy of the particles in a body, the liquid is slightly cooler than the surroundings.

Question 25

What are some factors that affect evaporation?

Answer 25

• exposed surface area (evaporation increases as area increases),
• temperature difference (evaporation increases as temperature difference increases),
• wind speed (evaporation increases as wind speed increases),
• humidity (evaporation decreases as humidity increases)

Question 26

What is the difference between Boiling and Evaporation?

Answer 26

Boiling
- Takes place within liquid
- Constant Temperature
- Produces bubbles
- Occurs at fixed temperature
Evaporation
- Takes place only at the surface of the liquid
- Cooling occurs
- Does not produce bubbles
- Occurs at any temperature

Question 27

What occurs in melting/boiling in terms of molecules and heat?

Answer 27

• During melting/boiling, latent heat is absorbed to overcome the intermolecular forces of attraction between the molecules.
• The average separation of the particles increases and so the potential energy increases.
• Average kinetic energy of the molecules remain constant therefore temperature is constant.
• Potential energy of the molecules increases as molecules move apart.

Question 28

What occurs in solidification/condensation in terms of molecules and heat?

Answer 28

• During solidification/condensation, latent heat is released as intermolecular forces are strengthened (this latent heat is equal to the heat lost to the surroundings).
• The average separation of the particles decreases and so the potential energy decreases.
• Average kinetic energy of the molecules remain constant therefore temperature is constant.
• Potential energy of the molecules decrease as molecules move closer.

Question 29

What occurs during evaporation?

Answer 29

Evaporation occurs when the faster particles at the surface of the liquid have enough energy to break their intermolecular forces to escape into the atmosphere.