Chapter 10. Transfer of heat

Question 1

What is heat transfer

Answer 1

The transfer of that from a region of higher temperature to lower temperature (temperature change)

Question 2

Heat = ?

Answer 2

Thermal energy

Question 3

How does matter expand

Answer 3

• substance is heated
• particles gain energy and vibrate vigorously about their fixed positions
• Distance between each particle increases
• causing volume to increase while density to decrease
• Substance expands

Question 4

How does matter contract

Answer 4

• Substance is cooled
• particles lose energy and vibrate less vigorously about their fixed positions
• distance between each particle decreases
• causing volume to decrease while density increases
• substance contracts

Question 5

Law of conservation of mass

Answer 5

Mass cannot be created or destroyed through chemical reactions or physical changes

Question 6

The ____ and _____ of particles in a substance does not change during expansion or contraction. Thus there is conservation of mass

Answer 6

number, size

Question 7

Mass. After being cooled and after being heated

Answer 7

After being cooled: Remains the same (no change in size and number of particles)

After being heated: Remains the same (no change in size and number of particles)

Question 8

Volume. After being cooled and after being heated

Answer 8

After being cooled: Decreases (particles move closer together)

After being heated: Increases (particle moved further away from each other)

Question 9

Density: After being cooled and after being heated

Answer 9

After being cooled: Increases (More particles per volume)

After being heated: Decreases (Less particles per volume)

Question 10

Examples of problems caused by the effects of heat ad how they can be overcome

Answer 10

1. Concrete pathways crack under sun
   Solution: Expansion joints/ Leave gaps to provide space for expansion without causing cracks
2. MRT tracks bend in extremely hot weather
   Solution: Expansion gaps between tracks allow space for expansion without causing tracks to bend
3. Overhead cables snap during cold nights
   Solution: Allow the cable to slightly droop (instead of taut) to allow for contraction without snapping. Or let the cable sag by using a longer cable

Question 11

Bimetallic strips

Answer 11

A strip with two different types of metal securely fastened to each other

Question 12

Two combinations of metal for bimetallic strips

Answer 12

• Brass and steel
• Copper and iron

Question 13

When heated, brass/copper expands _______ steel. So the strip bends towards the shorter metal side (_____)

Answer 13

more than, steel

Question 14

When cooled, brass/copper contracts ________ steel. So the strip bends towards the shorter metal side (______)

Answer 14

more than, brass/copper

Question 15

Application of bimetallic strips

Answer 15

1. Thermostat in electric iron
2. control temperature in an oven or refrigerator
3. fire alarm

Question 16

Bimetallic strips in fire alarms

Answer 16

When temperature is too high, the brass strip will expand more than the steel strip and bend towards the screw. This will make a closed circuit, allowing the current to flow through and will cause the fire alarm to ring.

Question 17

Ways heat can be transferred

Answer 17

1. Conduction
2. Convection
3. Radiation

Question 18

Conduction

Answer 18

The transfer of heat from one substance to another through direct contact. Particles transfer heat to each other through collisions.

Question 19

describe, using particles, how heat is transferred in metal pipes by conduction

Answer 19

When heated, atoms in the metal pipe are more energised and start vibrating more vigorously on the spot. This causes them to collide into other atoms and transfer heat to each other. This process continues until the cooler end of the pipe. and the particles vibrate faster and more vigorously.

Question 20

Solids are better heat conductors because their particles:

Answer 20

Are more closely packed than liquids and gases
Are arranged in fixed position so its easier to transfer energy

Question 21

Metals are better conductors of heats than non-metals because

Answer 21

They have electrons to help transfer energy quickly

Question 22

Convection

Answer 22

Transfer of heat through a fluid caused by molecule motion due to density differences in the medium

Question 23

It is the main form of heat transfer in _____________ because they can ________

Answer 23

liquids and gases, flow

Question 24

Applications of convection

Answer 24

• Heaters are placed at the bottom of an electric kettle to set up convection currents in water to heat up all the water efficiently
• Air conditioners / coolers are placed near the ceiling of a room to set up convection currents in air to cool down all the air in the room efficiently

Question 25

Sea breeze

Answer 25

in the daytime, the land heats up faster than the sea. The warm air above the land becomes less dense and rises, while the cool air above the sea which is denser flows towards the land to replace the rising hot air. This sets up a convection current known as sea breeze.

Question 26

Land breeze

Answer 26

in the night-time, the sea retains more heat than the land. The warm air above the sea becomes less dense and rises, while the cool air above the land which is denser, sinks and flows towards the sea. This sets up a convection current known as land breeze.

Question 27

Conduction vs convection

Answer 27

- in conduction, thermal energy is transferred from one particle to another without any flow of the medium. - In convection, thermal energy is transferred through the actual movement of the particles in the medium due to differences in density

Question 28

Conduction and convection of thermal energy cannot occur in space because

Answer 28

- Space is a vacuum - There are no particles for convection and conduction to take place

Question 29

Radiation

Answer 29

The transfer of energy the form of rays, waves or particles without the need of a medium

Question 30

Factors affecting radiation

Answer 30

Nature of the surface - Darker the surface, the faster radiation is absorbed or emitted Surface area of the object - The larger the surface area, the larger the rate of radiation absorbed or emitted Surface temperature - The higher the surface temperature, the higher the rate of radiation emitted *Note: Black, dull and rough surfaces are better absorbers of radiation compared to white, shiny and smooth surfaces.

Question 31

What if we want to stay hot/cold for a longer period of time?

Answer 31

If we want to stay hot/cold for a longer period of time, we should wear white/silver and smooth surfaces - Light coloured clothing are poor emitters of radiation so they do not become as hot as dark coloured clothing in the sun - Kettles have shiny, smooth surfaces so they are poor emitters of heat. This allows the water inside to lose heat slowly

Question 32

What if we want to gain/lose heat in a shorter period of time?

Answer 32

We should use black, dull and rough surfaces - Solar heaters are painted black so they are good absorbers of radiation when placed under the sun. This allows them to get hot quickly as they trap more solar radiation. - Cooling fins (for engines and computer chips) have black, rough surfaces so that they are good emitters of radiation > Both solar heaters and cooling fins have large surface areas so that they can transfer heat by radiation at a faster rate *Note: Heat gain by radiation can only be reduced; it cannot be stopped

Question 33

How do particles (atoms and molecules) transfer energy to each other?

Answer 33

- Collisions - One molecule collides with another molecule and passes on kinetic energy - Molecule with greater KE passes energy to the molecule with less KE

Question 34

sort the states of matter in increasing order based on which one expands the most when subjected to the same change in thermal energy?

Answer 34

gas>liquid>solid

Question 35

what is a convection current?

Answer 35

when the heated liquid/gas expands and rises due to the decrease in density, then cools off, becoming more dense and sinks.

Question 36

Why does hot air rise

Answer 36

When heated, air becomes less dense and thus rises

Question 37

Why does cold air sink

Answer 37

When cooled, air becomes denser and thus sinks