Transfer of Thermal Energy

Question 1

when does net transfer of thermal energy occur?

Answer 1

when there is a difference in temperatures

Question 2

which region to which region does net thermal energy flow from?

Answer 2

from a region of higher temperature to a region of lower temperature

Question 3

what does thermal equilibrium mean?

Answer 3

it means that there is no net transfer of heat. objects in contact have the same temperature.

Question 4

define conduction

Answer 4

the transfer of thermal energy through a medium without any flow of the medium.

Question 5

in what state does conduction typically take place in?

Answer 5

solids

Question 6

how does conduction work?

Answer 6

particles at heated end vibrate vigorously; collide with neighbouring particles and transfer their energy; particles at cooler end are also set into vigorous vibration.

Question 7

why are metals better conductors of heat?

Answer 7

metals are better conductors than non-metals as metals have free mobile ions.

Question 8

how is thermal energy transferred in non-metals?

Answer 8

through vibration and collision of particles only.

Question 9

how is thermal energy transferred in metals?

Answer 9

through vibration and collision of particles; through electron diffusion.

electrons gain kinetic energy and move more rapidly, colliding with atoms in the cooler parts of the metal and passing their energy on in the process.

Question 10

why are liquids and gases the worser conductors of heat?

Answer 10

liquid particles further apart and collisions between particles less frequent. frequency of collisions even lower in gases.

therefore, transfer of kinetic energy from fast moving particles to neighbouring particles generally slower in liquids and slowest in gases.

Question 11

in what state does convection typically take place in?

Answer 11

liquids

Question 12

define convection

Answer 12

transfer of thermal energy by means of convection currents in a fluid (liquid or gas), due to difference in density.

Question 13

what is the difference between conduction and convection?

Answer 13

thermal energy transferred from one particle to another by vibration and collision in conduction. particles do not move from fixed positions.

thermal energy transferred through actual movement of heated particles from warmer to cooler parts of fluid in convection. hotter part of fluid flows to cooler part of fluid.

therefore, convection cannot take place in solids because solids cannot flow.

Question 14

how does convection work?

Answer 14

water at bottom of flask is heated–> expands.

expanded water less dense than surrounding water–> rises

since upper region cooler, it is denser and therefore sinks

difference in densities of water in different regions sets up a convection current.

(if any arrows in the diagram) this is shown by arrows rising from bottom and sinking at the sides.

Question 15

sea breeze convection example

Answer 15

in the day, the land heats up faster than sea. air above land that is heated, expands and rises.

cool air above sea is denser and moves in to replace warmer air.

this sets up a sea breeze.

Question 16

land breeze convection example

Answer 16

at night, the land cools faster than the sea.

air above land now cooler and denser than air above sea. it moves out to replace warmer air rising above sea.

this sets up a land breeze.

Question 17

define radiation

Answer 17

the transfer of thermal energy in the form of electromagnetic waves such as infrared radiation without the aid of a medium

Question 18

what is infrared radiation?

Answer 18

all bodies emit infrared radiation. infrared radiation does not require a medium to be transmitted. it can travel through vacuum.

Question 19

how is the earth warmed by the sun?

Answer 19

through infrared radiation

Question 20

a good emitter of radiation is also

Answer 20

a good absorber of radiation

Question 21

what factors affect the rate of emission or absorption of radiation by an object?

Answer 21

colour and texture of surface, difference btw surface temp. and surroundings’ temp. and its surface area

Question 22

to obtain a higher rate of radiation, what is needed for the factor: colour and texture of surface?

Answer 22

dull, black, rough

Question 23

to obtain a higher rate of radiation, what is needed for the factor: difference btw object’s surface temp. and surroundings’ temp.

Answer 23

larger temp. diff.

Question 24

to obtain a higher rate of radiation, what is needed for the factor: surface area?

Answer 24

larger surface area

Question 25

to obtain a lower rate of radiation, what is needed for the factor: colour and texture of surface?

Answer 25

shiny, white, smooth

Question 26

to obtain a lower rate of radiation, what is needed for the factor: difference btw object’s surface temperature and surroundings’ temperature?

Answer 26

smaller temp. diff.

Question 27

to obtain a lower rate of radiation, what is needed for the factor: surface area?

Answer 27

smaller surface area

Question 28

applications of good conductors

Answer 28

• cooking pots and pans generally made of metal to allow faster conduction of heat to cook food
• tips of soldering iron rods are made of copper to conduct heat quickly to melt solder
• copper tubes in heat exchangers conduct heat quickly from the hot water to cold water

Question 29

applications of bad conductors

Answer 29

• handles of pots and pans made of insulating plastic or wood to prevent injury
• woollen clothes for cold weather help trap an insulating layer of air to keep us warm
• styrofoam boxes are used to keep food warm because of trapped air spaces inside styrofoam. make styrofoam good insulators.

Question 30

applications of convection

Answer 30

• air conditioners usually placed high up near ceiling so that convection currents can be set up to cool room
• heaters placed near floor so that convection currents can be set up to heat room
  -heating elements are placed near bottom of kettles and cookers for same reason

Question 31

applications of radiation

Answer 31

• in hot countries, exteriors of buildings usually painted with light and shiny colours so as to reflect radiant heat from sun
• tendency to wear light-coloured clothes on hot days to absorb less radiation
• kettles and other heating appliances have reflective surfaces on their insides so as to keep heat in

Question 32

excellent application of heat insulation

Answer 32

vacuum flask

Question 33

plastic stopper in vacuum flask

Answer 33

an insulator that traps air. reduces heat loss or heat gain of stored liquid via conduction. stopper stops heat loss or heat gain via convection.

Question 34

vacuum between double walls in vacuum flask

Answer 34

prevents heat loss or heat gain via convection and conduction

Question 35

silver lining between double walls in vacuum flask

Answer 35

reduces heat transfer via radiation

Question 36

cork in vacuum flask

Answer 36

poor conductor of heat, reduces heat loss or gain by conduction