heat (wa2)

Question 1

SI unit of temperature

Answer 1

Kelvin (K)

Question 2

describe expansion

Answer 2

refers to the increase in volume an object when thermal energy is supplied

Question 3

describe contraction

Answer 3

refers to decrease in the volume of an object when thermal energy is lost

Question 4

for a given increase/decrease in temp,

Answer 4

gas > liquid > solid

Question 5

railway tracks

Answer 5

• hot weather can cause railway tracks to expand
• the large forces causes distortion or even cracking of tracks
• thus gaps must be deliberately placed to allow for expansion

Question 6

metal structures

Answer 6

• can be damaged by forces of expansion and contraction
• one end of the structure is fixed, while the other end rests on rollers to allow for the expansion and contraction of the structure

Question 7

water/gas pipes

Answer 7

• pipes carrying hot or cold liquids and gases often have large bends in them
• allows the pipes to expand or contract without cracking

Question 8

rivets

Answer 8

• used to join steel plates
• axel is first immersed into a cold mixture so that it contracts
• when red-hot, a rivet is placed through the holes in the plates and its pointed end is hammered flat
• as the rivet cools, large forces of contraction pull the plates tightly together

Question 9

hot air balloon

Answer 9

• the fire of a hot air balloon heats up the air in the balloon, causing the air to expand
• expansion of the air causes its density to be lower than the density of air outside the balloon
• difference between density of air inside and outside produces an upward force that enables the hot air balloon to rise

Question 10

bimetallic strip

Answer 10

two different metals have different rates of expansion and contraction
application: the thermostat, which helps to maintain a steady temperature

Question 11

conduction

Answer 11

the transfer of heat energy without the physical movement of a medium

Question 12

factors affecting conduction

Answer 12

thermal conduction of different objects

Question 13

why solid better conductor

Answer 13

• solids are generally better conductors of heat than fluids
• the particles in solids are arranged more closesly to one another than the particles in liquids and gases
• the close arrangement of particles in solids enables the transfer of heat energy to occur more quickly

Question 14

convection

Answer 14

convection is the transfer of thermal energy from one place to another by the bulk movement of a medium due to density differences

Question 15

why solid cnt

Answer 15

since particles in a solid can only vibrate about their fixed positions and are unable to move about freely, convection cannot take place in solids

Question 16

convection in liquids

Answer 16

1. identify the heat source: water near heat source gains heat.
2. As the heated water expands, its volume increases
3. Since mass of the water remains constant, the increase in its volume results in a decrease in its density
4. the hotter and less dense water rises to the top of the beaker
5. the cooler and denser water in turn sinks to the bottom of the beaker
6. the hotter and less dense water rises and the cooler denser water sinks continuously
7. convection currents are formed in the water and heat energy is transferred throughout the beaker of water
8. after some time, thermal energy is evenly distributed in the water

Question 17

convection in gases

Answer 17

1. the air above heat soruce heats up and expands
2. becomes less dense and rises
3. cooler air, sinks and takes the place of the hotter air that has risen
4. the cooler air is heated by heat source, becomes less dense and rises
5. a cycle of convection currents are formed

Question 18

radiation

Answer 18

the transfer of thermal energy by eletromagentic waves
- does not require medium
- take place in vaccum

Question 19

absorber of radiation

Answer 19

good: dark colours, rough textures
poor: light colours, smooth

Question 20

emitter of radiation

Answer 20

dull, rough and dark coloured
shiny, smooth, light coloured

Question 21

emittor and absorber relationship

Answer 21

good emitter of radiation is a good absorber of radiation

Question 22

factors affecting radiation

Answer 22

• higher difference in surface temperature will increase the rate of heat energy transfer by radiation
• larger surface area, higher rate of radiation
• colour/texture

Question 23

conditions required for fast heat energy transfer by conduction

Answer 23

a large temperature difference between the objects in contact, high thermal conductivity of the material, and a large surface area in contact.

Question 24

conditions required for fast heat energy transfer by convection

Answer 24

larger surface area, higher temperature difference

Question 25

conditions required for fast heat energy transfer by radiation

Answer 25

large surface area, high temperature, dul, dark coloured, rough

Question 26

shiny outer layer water tank

Answer 26

shiny outer layer, as it is a poor emittor of radiation, thus the rate of transfer of thermal energy is reduced.

Question 27

stryofoam

Answer 27

it traps air which is a poor conductor of heat energy

Question 28

explain how the heating coil at the bottom allows water to be heated up quickly

Answer 28

the water nearest to the heating coil is heated and expands, decreases in density and rises to the top. the cooler denser water sinks. this forms convection currents where thermal energy is transferred throughout, heating the water quickly

Question 29

discuss whether shiny metal/ non-shiny plastic used to make each kettle is suitable for reducing heat loss when kettle is used to boil water

Answer 29

for shiny metal, thermal energy will be conducted away to surroundings quickly as metal is a good conductor of heat however, a is made of shiny material which helps to reduce heat transfer to surroundings by radiation as shiny surfaces are poor emittors of radiation for non-shiny plastic, plastic is a poor conductor, hence it will minimise heat transfer to the surroundings

Question 30

explain how water in kettle becomes warm.

Answer 30

water near the coil gets heated, expands, then rises. it pushes the cold, denser water to the bottom. the cold water then gets heated by the coil and the process is repeated, setting up a convection curremt

Question 31

explain why water at the bottom of kettle remains cold when heating coil is at the top

Answer 31

water near the heating coil gets heated, expands and rises. cold denser water sinks. however, as water is a poor conductor of heat, water at the bottom which is not near coil will not get heated quickly

Question 32

in a cooler box, suggest why the ice pack is placed at the top of the box rather than at the bottom

Answer 32

near the ice pack, the air loses heat, contracts and becomes denser. the denser and colder air then sinks to the bottom of the box. the warmer and denser air rises and gets nearer to the ice pack. then it is cooled by the ice pack and the convection currents are formed to cool the whole box and keep food cold

Question 33

explain why the ice cream does not melt even though the batter is dipped in hot oil and fried

Answer 33

the air space contains air, which is a poor conductor of thermal energy. thus, thermal energy is conducted slower to the ice cream FROM the hot oil, so the ice cream does not melt.

Question 34

aluminium

Answer 34

Aluminum is a good conductor of heat andcan lose heat energy quickly. [

Question 35

fin-like protrusions

Answer 35

Thefin-link protrusions increase the surface area in contact with the air. Hence,more heat energy can be radiated away.

Question 36

vaccum in flask

Answer 36

Conduction and convection cannot take place through the layer of vacuum, thus reducing heat loss from theliquid in the flask since there is no medium.