Heat Transfers

Question 1

What are the three types of heat transfers in detail?

Answer 1

Conduction, convection, radiation

Question 2

What temperature of objects does thermal energy travel from and to?

Answer 2

Thermal energy will always transfer from the hotter object to the colder object

Question 3

What is convection?

Answer 3

• Liquids and gases are fluids because they can be made to flow. -The particles in these fluids can move from place to place. -Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy.
• When particles in fluids are heated, particles gain more energy and therefore move faster. As they mover faster, the gaps between particles expand and therefore take up more volume. The heated particles become less dense and therefore, they rise.
• Cooler particles sink, taking the place of where the hotter particles previously were.

Question 4

What are examples where we can reduce convection?

Answer 4

• putting a lid on a saucepan
• getting insulation on your roof
• wrapping yourself in a space blanket

Question 5

How do we reduce convection?

Answer 5

-by trapping particles so that they cannot move or escape

Question 6

In which states does convection occur and why?

Answer 6

• only in liquids and gases
• liquids and gases are fluids, so their particles are free to move about. Convection cannot happen in solids because particles are not free to move.

Question 7

In which states does conduction occur and why?

Answer 7

• mainly in solids
• in solids, particles are held tightly together, so when one particle vibrates, it collides with other particles nearby and vibrations quickly pass from particle to particle through the structure.

Question 8

What is conduction?

Answer 8

• thermal conduction is the process where vibrating particles transfer energy from their kinetic energy store to the kinetic energy store of the neighbouring particles.
• Heat energy is conducted from the hot end of an object to the cold end.

Question 9

Why are metals particularly good conductors of heat?

Answer 9

• The electrons in a piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now positively charged metal ions.
• metals are solids so the particles are closely bonded together. This means that when one metal ion vibrates, it causes surrounding ions to vibrate and eventually most ions in the solid will vibrate.
• metals have delocalised electrons so when an electron has a larger energy store, it moves quickly through the structure and bumps into other particles, transferring the energy.

Question 10

How can conduction be reduced?

Answer 10

• conduction can be reduced by insulators.
• insulators trap hot particles so that they do not escape
• materials which are silver are the best colour to insulate. White is also a good colour to insulate. These colours are good for insulation because they are poor absorbers and good reflectors.

Question 11

Which colour is the best absorber and worst reflector?

Answer 11

Black

Question 12

Which colour is a poor absorber and reasonable reflector?

Answer 12

White

Question 13

Which colour is the worst absorber and best reflector?

Answer 13

Silver

Question 14

How good is the colour black at absorbing and reflecting infrared radiation?

Answer 14

It is the best absorber and worst reflector

Question 15

How good is the colour white at absorbing and reflecting infrared radiation?

Answer 15

It is a poor absorber and reasonable reflector

Question 16

How good is the colour silver at absorbing and reflecting infrared radiation?

Answer 16

It is the worst absorber and best reflector

Question 17

What is infra red radiation?

Answer 17

• it can also be called Thermal Radiation
• all objects are continually absorbing and emmitting infrared radiation, hot objects emit more radiation than they absorb.
• larger surfaces + rough surfaces emit and absorb more because they have a larger surface area.

Question 18

What are features of the vacuum flask design to prevent energy transfer?

Answer 18

• stopper to prevent convection and conduction
• vacuum so that there are no particles which could conduct or convect heat
• silver surfaces to reflect heat and prevent the radiation of heat

Question 19

How do convection currents occur?

Answer 19

• particles are heated, this means that the particles gain more energy, so they move faster and spread further apart.
• the hotter particles become less dense and therefore they rise
• the warm particles rise until they reach a colder area and they cool.
• As they cool, the particles become closer together and the water becomes denser so it sinks
• Warm particles move to fill where the colder particles were previously, forming a convection current

Question 20

How does plastic foam containing small bubbles of trapped air reduce heat loss?

Answer 20

• the small bubbles of trapped air reduce conduction because air is a poor conductor
• the trapped air also reduces convection because the trapped air cannot move a lot.