Thermal Energy

Question 1

what is heat

Answer 1

form of energy that flows from a region of higher temperature to a region of lower temeprature

Question 2

SI unit of heat

Answer 2

Joule (J)

Question 3

definition, SI unit

differences between heat and temperature

Answer 3

heat:
1) refers to the amt. of thermal energy being transferred from hotter to colder regions
2) SI unit is Joule (J)

temperature:
1) refers to how hot or cold an object is
2) SI unit is Kelvin (K)

Question 5

what is heat

Answer 5

form of energy that flows from a region of higher temperature to a region of lower temperature

Question 6

SI unit of heat

Answer 6

Joule (J)

Question 7

definition, SI unit

difference between heat and temperature

Answer 7

heat:
1) refers to the amount of thermal energy being transferred from a hotter region to a colder region
2) SI unit: Joule (J)

temperature:
1) refers to how hot or cold an object is
2) SI unit: Kelvin (K)

Question 8

why does the liquid level in the glass tube fall initially before rising when heated? (for that 1 famous diagram from this chapter)

Answer 8

glass of test-tube gets heated first, causing it to expand and increase the volume of test-tube, leading to initial fall

after a while, water in test-tube gains heat from ___, causing water to expand and since water expands more than glass, liquid level rises

Question 9

why does the liquid level in the glass tube rise initially before falling when cooled? (for that 1 famous diagram from this chapter)

Answer 9

glass of test-tube gets cooled first, causing it to contract and decrease the volume of test-tube, leading to initial rise

after a while, water in test-tube loses heat from ___, causing water to contract and since water contracts more than glass, liquid level falls

Question 10

what is the order of magnitude of expansion and intermolecular forces in solids, liquids and gases

Answer 10

magnitude of expansion:
gases (largest), solids (least)

intermolecular forces:
solids (strongest), gases (weakest)

Question 11

why do solids expand the least and gases expand the most when heated

Answer 11

intermolecular forces is strongest in solids, followed by liquids then gases

Question 12

what happens to the volume, mass and density of an object when heated and cooled

Answer 12

when heated, volume increases, mass remains the same, density decreases
when cooled, volume decreases, mass remains the same, density increases

Question 13

3 points

why does ice float on water

Answer 13

volume of water decreases as temperature decreases towards 4°C

volume starts to increase as temperature decreases from 4°C to 0°C

volume is lowest at 4°C and hence, density is highest

Question 14

what is the importance of ice floating on water

Answer 14

allows aquatic life to survive during winter as the pond/lake does not freeze completely

Question 15

negative effects of expansion and contraction + solutions

1) concrete road
2) railway tracks
3) bridges
4) electric cables and telephone wires
5) laboratory glassware
6) water pipes

Answer 15

concrete road:
thermal expansion forces on very hot days will cause roads to crack
provide expansion gaps

railway tracks:
can bend or twist due to expansion during hot weather
provide sliding joints/expansion gaps

bridges:
can buckle under forces of expansion and contraction
build rollers into it/provide expansion gaps

electric cables and telephone wires:
may become taut or snap when contracting in cold weather
string them loosely from pole to pole

laboratory glassware:
when boiling water poured into thick glass, it might crack due to uneven expansion
use pyrex (expands very little when heated)

water pipes:
unusually high pressures and leaks may occur when transporting hot water or steam
build expansion loops or joints into them

Question 16

describe the 2 benefits of expansion and contraction

Answer 16

1) fixing axel into wheels
cool the axel for it to contract before fitting it into the wheel and allowing it to expand

due to the forces of expansion, there is a very tight fit between the axel and wheel such that it will not come loose easily

2) hot riveting
used to join smaller sheets of steel in ships, and shorter steel bars in cranes and bridges

insert hot rivet through holes of 2 metal pieces which are to be fastened
the other end of the rivet is hammered to form a new head

when rivet cools, it contracts and pulls the metal pieces tightly together

Question 17

what are bimetallic strips

Answer 17

formed by riveting or welding 2 different metals that expand at different rates together (e.g. brass and iron; brass expands more than iron)

Question 18

what 3 processes does heat transfer by

Answer 18

conduction
convection
radiation

Question 19

what is conduction

Answer 19

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

Question 20

what are good thermal conductors

Answer 20

materials that allow heat to flow through easily and have high thermal conductivity (e.g. silver, gold, copper)

Question 21

what are thermal insulators (poor thermal conductors)

Answer 21

materials that do not allow heat to flow through easily and have low thermal conductivity (e,g, wood, plastic, glass, vacuum)

Question 22

a piece of paper around a wooden pole chars while a piece of paper aroud a copper pole does not burn; why?

Answer 22

heat is conducted away from the paper quickly by the metal and the paper around the metal rod never gets hot enough to burn

Question 23

materials, speed

difference between molecular vibration and free electron diffusion

Answer 23

molecular vibration:
1) applies to all materials
2) slower process

free electron diffusion:
1) applies only to materials with free electrons (e.g. metals)
2) faster process

*free electron diffusion occurs in metals on top of molecular vibration and thus transfers heat easier

Question 24

3 points

describe molecular vibration

Answer 24

when heated, particles at heated end gain energy and vibrate faster and more vigorously

they collide with neighbouring particles more often, passing some of their energy to them

these neighbouring particles will vibrate faster and more vigorously and the energy is passed along to the next neighbouring particles, and subsequently the rest of the object

Question 25

3 points

why are liquids and gases poorer conductors of thermal energy relative to solids

Answer 25

solids have particles closely packed together compared to liquids and gases as its intermolecular forces are stronger

liquids and gases are poorer conductors of thermal energy relative to solids for the same volume as there are fewer particles to collide against

because the distance between particles in liquids and gases are larger than solids

Question 26

2 points

describe free electron diffusion

Answer 26

metals have many free electrons which are free to move about

free electrons gain kinetic energy and move more rapidly; they collide with atoms in cooler parts of the metal and pass on their energy in the process

Question 27

why is vacuum the best thermal insulator

Answer 27

there are no particles in vacuum and thus no conduction of heat can occur

Question 28

what is convection

Answer 28

transfer of thermal energy by means of convection currents in a fluid due to a difference in density

Question 29

4 points

describe convection currents in gases

Answer 29

the gas above the heat source gets heated and expands

as the hot gas is less dense than the surrounding cooler air, it rises

the cooler, denser gas from the surroundings is drawn in downwards to take the place of the hot gas which escaped, and is then heated by the heat source, becoming less dense and rising

circulation of hot gas and cool gas sets up a convection current

(smouldering paper used to make movement of air current visible)

Question 30

4 points

describe convection currents in liquids

Answer 30

the liquid at the bottom is heated by the heat source and expands

the hot liquid becomes less dense and rises

the cooler, denser surrounding liquid sinks to the bottom and is then heated by the heat source, becoming less dense and rises

circulation of hot liquid and cool air sets up a convection current

(potassium permanganate crystals used to dissolve in water to make movement of water current visible)

Question 31

2 points (day), 3 points (night)

describe land and sea breeze

Answer 31

day (sea to land):
air over the land becomes warm, expands and rises as its density decreases

cooler, denser air from over the sea blows into the land to take the place of the warm air rising above the land

night (land to sea):
land loses heat more quickly than the sea

air over the sea is warmer and less dense and rises

cooler, denser air from above the land blows out into the sea to take the place of the warm air rising above the sea

Question 32

how do you eliminate conduction and convection

Answer 32

create a vacuum

Question 33

3 points

difference between conduction and convection

Answer 33

conduction:
1) occurs mainly in solids
2) heat can flow in any direction
3) energy is transferred through the solid without the solid itself moving

convection:
1) occurs mainly in fluids
2) heat flows upwards
3) hot fluid moves freely and carries energy from one place to another

Question 34

what is radiation

Answer 34

transfer of thermal energy in the form of infrared radiation without the aid of a medium

Question 35

1 point

difference between radiation and conduction or convection

Answer 35

can take place in a vacuum and its speed in vacuum is light speed

Question 36

describe net heat loss (radiation)

Answer 36

when an object is at a higher temperature than its surroundings, it will radiate more infrared radiation than it absorbs, resulting in a net heat loss

Question 37

describe net heat gain (radiation)

Answer 37

when an object is at a lower temperature than its surroundings, it will absorb more infrared radiation than it emits, resulting in a net heat gain

Question 38

explain why there is no net heat loss or gain when an object is of the same temperature as its surroundings

Answer 38

rate of emission of infrared radiation is equal to rate of absorption of infrared radiation

Question 39

what are 3 factors affecting rate of emission/absorption of radiation

Answer 39

1) temperature
higher the temperature, greater the rate of infrared radiation emission

2) surface area
greater the surface area, faster the rate of absorption and emission of infrared radiation

3) nature of the surface
a black and dull surface absorbs infrared radiation very well (good absorber)
a shiny and silvery surface reflects infrared radiation very well (poor absorber)

*if an object is a good absorber of infrared radiation, it is also a good emitter of infrared radiation
*when answering a question, DO NOT describe an object to be BOTH a good emitter and absorber

Question 40

describe heat transfer of a thermal flask

1) plastic stopper/cap
2) trapped air
3) vacuum
4) shiny/light-coloured outer case
5) silvered walls
6) double glass walls

Answer 40

plastic stopper/cap:
reduce heat gain/loss by conduction as plastic is a poor conductor of heat
reduce heat loss/gain by convection as they prevent surrounding air from getting into contact with fluids in flask

trapped air:
reduce heat gain/loss by conduction as air is a poor conductor of heat

vacuum:
prevents heat gain/loss by conduction/convection as both processes cannot take place in a vacuum

shiny/light-coloured outer case:
reduces heat gain/loss by radiation as shiny/light-coloured surfaces are good reflectors but poor absorbers and radiators of heat

silvered walls:
reduces heat gain/loss by radiation as silvered surfaces are good reflectors but poor absorbers and radiators of heat

double glass walls:
reduce heat gain/loss by conduction as glass is a poor conductor of heat