Chapter 10. Transfer of heat Flashcards
What is heat transfer
The transfer of that from a region of higher temperature to lower temperature (temperature change)
Heat = ?
Thermal energy
How does matter expand
- 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
How does matter contract
- 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
Law of conservation of mass
Mass cannot be created or destroyed through chemical reactions or physical changes
The ____ and _____ of particles in a substance does not change during expansion or contraction. Thus there is conservation of mass
number, size
Mass. After being cooled and after being heated
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)
Volume. After being cooled and after being heated
After being cooled: Decreases (particles move closer together)
After being heated: Increases (particle moved further away from each other)
Density: After being cooled and after being heated
After being cooled: Increases (More particles per volume)
After being heated: Decreases (Less particles per volume)
Examples of problems caused by the effects of heat ad how they can be overcome
- Concrete pathways crack under sun
Solution: Expansion joints/ Leave gaps to provide space for expansion without causing cracks - MRT tracks bend in extremely hot weather
Solution: Expansion gaps between tracks allow space for expansion without causing tracks to bend - 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
Bimetallic strips
A strip with two different types of metal securely fastened to each other
Two combinations of metal for bimetallic strips
- Brass and steel
- Copper and iron
When heated, brass/copper expands _______ steel. So the strip bends towards the shorter metal side (_____)
more than, steel
When cooled, brass/copper contracts ________ steel. So the strip bends towards the shorter metal side (______)
more than, brass/copper
Application of bimetallic strips
- Thermostat in electric iron
- control temperature in an oven or refrigerator
- fire alarm
Bimetallic strips in fire alarms
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.
Ways heat can be transferred
- Conduction
- Convection
- Radiation
Conduction
The transfer of heat from one substance to another through direct contact. Particles transfer heat to each other through collisions.
describe, using particles, how heat is transferred in metal pipes by conduction
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.
Solids are better heat conductors because their particles:
Are more closely packed than liquids and gases
Are arranged in fixed position so its easier to transfer energy
Metals are better conductors of heats than non-metals because
They have electrons to help transfer energy quickly
Convection
Transfer of heat through a fluid caused by molecule motion due to density differences in the medium
It is the main form of heat transfer in _____________ because they can ________
liquids and gases, flow
Applications of convection
- 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
Sea breeze
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.
Land breeze
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.
Conduction vs convection
- 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
Conduction and convection of thermal energy cannot occur in space because
- Space is a vacuum
- There are no particles for convection and conduction to take place
Radiation
The transfer of energy the form of rays, waves or particles without the need of a medium
Factors affecting radiation
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.
What if we want to stay hot/cold for a longer period of time?
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
What if we want to gain/lose heat in a shorter period of time?
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
How do particles (atoms and molecules) transfer energy to each other?
- Collisions
- One molecule collides with another molecule and passes on kinetic energy
- Molecule with greater KE passes energy to the molecule with less KE
sort the states of matter in increasing order based on which one expands the most when subjected to the same change in thermal energy?
gas>liquid>solid
what is a convection current?
when the heated liquid/gas expands and rises due to the decrease in density, then cools off, becoming more dense and sinks.
Why does hot air rise
When heated, air becomes less dense and thus rises
Why does cold air sink
When cooled, air becomes denser and thus sinks
