Unit 10: Transfer of Thermal Energy and its Effects

Question 1

Transmission of Heat (Main points)

Answer 1

• Temperature, Heat and Transfer of Thermal Energy
• Expansion and Contraction
• Radiation
• Convection
• Conduction

Question 2

Heat (3pt)

Answer 2

• Heat is the transfer of thermal energy from a region of higher temperature to a region of lower temperature until thermal equilibrium is achieved between the two regions.
• When two objects of different temperatures are placed in contact, thermal energy will flow from the hotter region of the two objects to the colder region until they both have the same temperature.
• When they reach this state, we say they are in thermal equilibrium.
• Thermal energy is transferred ONLY when there is a difference in temperature.

Question 3

Temperature (5pt)

Answer 3

• Temperature, T (in Kelvin, K):
• The temperature of an object is a measure of the average kinetic energy of all the particles in an object/substance. (microscopic understanding)
• Measure of how hot or cold a substance is (macroscopic understanding)
• A temperature of 0K is the lowest temperature possible anywhere. For this reason, it is called absolute zero.
• 0K = - 273°C (lowest possible temperature)
• 1K = -272°C
• 1 K of temperature change is equal to 1 °C of temperature change.

Question 4

Thermal Energy (2pt)

Answer 4

• Thermal energy, Q (in joules, J):
  Amount of energy in object due to its temperature. (macroscopic understanding)
• Temperature of an object determines the sensation of warmth or coldness felt from contact with it.

Question 5

Expansion and Contraction (3 impt pt)

Answer 5

• When objects are heated, they gain thermal energy and generally increase in size. This increase in size is called expansion.
• When objects are cooled, they lose thermal energy and generally decrease in size. This decrease in size is called contraction.
• Gases expand much more than solids and liquids. (Expansion of gases > liquids > solids)
• As a substance expands, its volume increases but its mass does not change.
• Density is mass divided by volume. Hence, as the volume increases, the density decreases / substance contracts, its density increases.
• Sink/Float = Increase/Decrease in density

Question 6

Radiation (2pt)

Answer 6

• Radiation is the transfer of thermal energy by electromagnetic waves which does not require a medium.
• Heat transfer by radiation is possible with solids, liquids, gases and in vacuum (empty space).

Question 7

infra-red radiation (2pt)

Answer 7

• In radiation, the source of thermal energy transmits energy in the form of waves.
• These waves constitute a part of the electromagnetic spectrum and are known as infra-red radiation.
• When absorbed, the energy of the infra-red radiation transforms into thermal energy of the receiving object.

Question 8

Factors affecting the Emission of infra-red radiation?

Answer 8

• The emission of infra-red radiation depends on:
  1. The surface temperature of the object
  → The hotter an object, the higher the rate of energy transfer.

2. The colour and texture of the object
   → dull, rough, black surfaces are better emitters of infra-red radiation than white, smooth, shiny (bright) surfaces.
3. The surface area of the object
   → The larger the surface area, the higher the rate of energy transfer.

Question 9

Absorbers/Radiators of infra-red radiation (3pt)

Answer 9

• Not all surfaces emit infra-red radiation equally well.
• Good absorbers of infra-red radiation are also good radiators of infra-red radiation.
• Poor absorbers of infra-red radiation are also poor radiators of infra-red radiation.
• In general, a good emitter is also a good absorber of infra-red radiation.
• A shiny surface is a good reflector of infra-red radiation.

Question 10

Convection (3pt)

Answer 10

• Convection is the transfer of thermal energy due to the movement of heated particles of fluids (liquids and gases).
• Convection can occur in fluids (liquids and gases) only.
• This is because convection currents are caused by the change in the density of the fluid that allows it to rise/sink (hence forming convection currents).
• The particles in solids are arranged in fixed positions and cannot move. For convection to occur, the particles must be free to move (e.g., in liquids and gases).

Question 11

Conduction (5pt)

Answer 11

• Conduction is the transfer of thermal energy through a material without the obvious (net) movement of the medium itself.
• Conduction can occur in solids, liquids, and gases, though the rate of conduction is different.
• Conduction of solids > liquids > gases (as the solids are packed very closely than liquids (closely) than gas (far apart).
• Conductions of metals > non-metals (as the metals atoms are packed more closely than non-metals)
• Thermal energy is conducted quickly/slowly by free electrons (Metal/Non-Metal) – Good/Poor Conductor of Thermal Energy
• A poor conductor of heat is also called an insulator.
• A vacuum (empty space) is the perfect insulator.
• As matter is needed to conduct thermal energy, no heat transfer occurs through a vacuum by conduction at all.
• e.g. Thermal energy is transferred from the hot end of the rod to the cool end.
• The metal rod is said to be a conductor of heat.

Question 12

• How to answer open-ended questions (4pt)

Answer 12

• 1m question – state concept (with brief statement of effect)
• 2m question – explain concept (which is the cause – half sentence, 1m), then explain the effect (half sentence, 1m)
• 3m question – explain cause (1m), then concept (1m) then effect (1m).
• Must have keywords!! (e.g. poor radiator/absorber of thermal energy, how convection current is formed due to difference in density, gain/lose thermal energy and expand/contract)