P2 Thermal Physics (2.3)

Question 1

What are thermal conductors and give examples of good thermal conductors

Answer 1

Solids which transfer thermal energy easily
+ All metals are good thermal conductors
e.g metal pan

Question 2

What are bad thermal conductors (thermal insulators) and give examples

Answer 2

Solids which do not transfer thermal energy very well
+ Non-metals/organic materials (from plants/animals) are poor thermal conductors
e.g a wool blanket, plastic, wood

Question 3

What is conduction ?

Answer 3

The transfer of heat in solids from one region to another through atomic/molecular lattice vibrations and the movement of free electrons in metallic conductors

Question 4

What is the main method of thermal energy transfer in solids ?

Answer 4

Conduction

Question 5

How conduction occurs (its mechanism) in metals ?

Answer 5

• When metal is heated, the ions start to move around (vibrate) more
• Atoms at hotter end vibrate more than atoms at cooler end
  => bump into each other transferring energy from atom to atom until thermal equilibrium is achieved
• Metal contains free electrons can travel through material freely & transfer thermal energy through collisions

Question 6

What is the main method of thermal energy transfer in liquids and gases ?

Answer 6

Convection

Question 7

Describe and explain convection in liquids and gases ?

Answer 7

• When a liquid/gas is heated (near the floor), heated molecules vibrate more & push each other apart => liquid/gas expands
• Hot liquid/gas becomes less dense as volume increases and rises up
• Cooler, denser liquid/gas sinks & replace the hot fluid
• Eventually, the hot liquid/gas cools & contracts, becomes more dense & sinks back down
  => creating a convection current

Question 8

What is thermal radiation ?

Answer 8

A method of thermal energy transfer happen mainly due to infrared radiation (EM waves) that is emitted by all objects with temperature above absolute zero & does not need medium to travel through

Question 9

What is the method for thermal energy transfer where heat can travels through the vacuum ?

Answer 9

Thermal radiation
=> does not need a medium to transfer thermal energy (different to convection and conduction)

Question 10

Describe the effect of surface colour and texture on the emission, absorption and reflection of thermal energy

Answer 10

• Black and dull objects are good absorber and emitter of thermal energy
• White and shiny objects are poor absorber and emitter of thermal energy; they are good reflector

Question 11

Describe experiment to distinguish between good and bad emitters of thermal radiation

Answer 11

1. Set up 4 identical flasks painted in different colours: black, dull grey, white, silver
2. Fill the flasks with hot water, ensuring the measurement start from same initial temperature
3. Record starting temperature, then measure the temperature at regular time intervals e.g every 30 seconds for about 5 minutes
   => The black flasks has the greatest rate of cooling as black is good emitter of thermal radiation

Question 12

Describe experiment to distinguish between good and bad absorbers of thermal radiation

Answer 12

1. Place a black metal plate on a heatproof mat with a thermometer in contact with the plate’s surface
2. Record the starting temperature
3. Position the lamp at a fixed distance above the plate; switch on lamp & start stopwatch at the same time
4. Record the temperature every 30 seconds for about 5 minutes
5. Repeat the steps for the shiny plate keep other factors the same
   => Black plate will heat up faster as black surface is good absorber of radiation (shiny is good reflector)

Question 13

What is the temperature of the Earth affected by ?

Answer 13

• Radiation from the sun which is absorbed by the Earth
• Radiation emitted by the Earth
• Greenhouse gases (CO2, water vapour) absorb & re-emit radiation emitted by the Earth => surface temperature increases
• Surface reflectivity

Question 14

Consequences of thermal energy transfer by giving examples of conduction in real life

Answer 14

• Metal pans heat food quickly
• Plastic handles on saucepans to reduce thermal energy transferred to hands
• Air spaces in walls/windows of some houses to help retain heat as air is poor conductor (double glazed window)

Question 15

Consequences of thermal energy transfer by giving examples of convection in real life

Answer 15

• Heating a room with a radiator
• Cooling a room with air conditioner
• Steam rises from hot coffee through convection => cools the hot coffee

Question 16

Consequences of thermal energy transfer by giving examples of thermal radiation in real life

Answer 16

• A black car becoming hot to touch on a sunny day (black is good absorber of radiation)
• Fire becomes hotter as you approach it as amount of thermal energy transferred by flame increases as distance decreases (closer => absorb more radiation)

Question 17

Explain, in terms of motion and energy of water molecules, how sweating cools the athlete’s skin ?

Answer 17

• Thermal energy is transferred from blood in capillaries to water molecules on skin surface
• The most energetic molecules escape from the surface of the skin
• Average kinetic energy of the remaining molecules decreases lowering the temperature