20. Thermal Physics

Question 1

What does thermal energy transfer from, to?

Answer 1

Thermal energy will transfer from object w higher temperature to object w lower temperature

Question 2

How can thermal energy be transferred?

Answer 2

Conduction, convection and radiation

Question 3

What is the rate of transfer larger for for all types of transfer?

Answer 3

• larger temperature difference
• larger surface area

Question 4

What is conduction?

Answer 4

• transfer of thermal energy when particles collide
• usually in solids

Question 5

Why are metals particularly good conductors?

Answer 5

• contain delocalised electrons
• delocalised electrons can move quickly throughout material, colliding + passing on energy to ions

Question 6

What is convection?

Answer 6

Convection is transfer of thermal energy in liquids + gases :
- hotter parts of gas/liquid expand and become less dense, then rise
- cooler parts of gas/liquid contracts and become more dense, sink

Question 7

What is radiation?

Answer 7

• thermal radiation = transfer of thermal energy by infrared waves

Question 8

Which objects emit the least/most infrared radiation?

Answer 8

• hotter objects w larger surface area emit more infrared radiation
• dark + matt objects are good at absorbing + emitting infrared radiation but poor at reflecting it
• light + shiny objects are good at reflecting but poor at absorbing + emitting

Question 9

How can thermal energy transfer be reduced?

Answer 9

• conduction -> vacuum, insulating material
• convection -> vacuum, trap liquid/gas
• radiation -> shiny material will reflect it

Question 10

How does a radiator heat an entire room?

Answer 10

1. Radiator heats air by conduction - hot air expands, decreases in density + rises
2. Cooler parts of air sink and become heated by radiator forming convection current

Question 11

How have polar bears evolved to reduce heat loss?

Answer 11

• thick fur, traps air + prevents convection
• air is poor conductor, so prevents conduction