1. learning goals
2. intro to heat and conduction notes
3. more conduction applications
4. convection notes (coming soon)
5. convection applications abnd questions(coming soon)
6. radiation notes (coming soon)
7. radiation apllications (coming soon)
8. summary tables and common questions (coming soon)
9. answers and extra comments (coming soon)
no 1-9 can all be found in the june archives just read from bottom post to top post^_^ thanks
btw just ignore visitor contents the tagboard will be uploaded at the etcetc so hope the navigation isnt a huge problem ya....
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Name: jaclyn chua Also: in 3a2.
hi, yes yes so this is my physics notes and first aid blog. which hopefully by the end f the holidays would cover most of the topics needed and all the importatnt notes and exercises needed for revision.
the notes are still being updated. the late updates are due to house under renovation and being ill for a long time so please excuse the lack of posts T_T they'll be all up veryvery very soon...
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application of radiation
application of radiation~
1. solar panels are black in order to absorb more radiation
2. vacuum flask
keeps hot liquids hot or cold liquids cold liquids cold
it consists of a double walled glass vessel with a vacuum between the walls
the vacuum minimizes heat loss by condution and convection
the silvered walls reduce heat transfer by radiation
the corkm stopper also reduces heat tranfer by conduction
posted by supermassive on Wednesday, June 24, 2009
radiation
radiation the third mode of heat transfer is radiation. radiation is the means by which heat in the form of infrared rays from the sun reaches the earth. this process does not require a material medium. therefore heat energy can pass through by radiation.
all bodies above absolute zero radiate infra-red radiations when these radiation fall on other objects, they may be reflected or absorbed, thus raising the temperature of the object
good and bad absorbers black and dull surfaces = good absorber and good emitter shiny and polished surfaces = bad absorber and bad emitter
posted by supermassive on
applications of convection
application of convection~
1. electric kettle
the heater is placed at the bottom of the kettle so that as the hot water rises, cold water form the top moves down. in this way, heat is transferred to all parts of the liquid.
2. refrigerator
the cooler box is found usually on the top. this is to constantly cool thehot air which rises and to
3. sea and land breeze
during the day, the land gets heated up faster than the sea, the hot air above the land rises and is replaced by the cooler air from the sea. a sea breeze is thus poduced.
At night the land cools faster than the sea therefore the sea is warner the warm air above the sea rises and cool air form the land move toawrds the sea. hence a land breeze is produced.
~gliding because of the land and sea breeze, hang gliders and birds like seagulls can stay aiborne
4. heaters
the heaters are usually placed on the floor at one corner
this is so that they can warm up the cool dense air at the bottom constantly
ventilation.
cold fresh air enters a house through open doors, windows and airholes at the bottom of the windows, the air in the house is warmed up by heat from the human bodies, lighted bulbs and fire in the kitchen.
warm air rises and escapes through the air holes at the top
posted by supermassive on Tuesday, June 23, 2009
convection
convection
convection is the main mode of heat transfer in fluids such as air and water
when a fluid is heated it expands. the density of the fluid decreases and the hot fluid moves upwards. heat is then tranferred by the movement of the fluid that moves from he bottom to the top. streams of warm moving fluids are called convection currents
in the experiment below you will see that the color will move along the convection current.
molecular account of convection in liquids.
in liquid and gases, molecules are constantly moving form one place to another in a random manner.
when a liquid is heated, liquid molecules gain more energy and move at great speeds upwards. therefore the heat is being transported upwards
when the heated molecules move up, they ar replaced by otehr molecules which in turn are also heated up .
posted by supermassive on
conduction ~ more application
over here ...
the weight prevents the ice form floating to the area in
which the water is heated thus it doesnt melt even if the water above boils.
this shows water is a poor conductor of heat
BUTBUT.
this happens not because its a poor conductor. due to CONVECTION
inwhich the heat rises less heat will be found at the bottom and instead the denser and cooler particles would be found.
WARNING
the explanation in the diagram is WRONG
when a bunsen burner is lighted below a metal gauze, the gas only burns below the gauze and no flame is obtained above it .if the burner is lighter\d above the gauze the flame will be above it only.
therefore metal is a good conductor of heat as it conducts the heat of the flame rapidly away from where the flame is directed at. thus making sure the heat is equally distributed and the wire does not melt.
~ air is an insulato
1. when a match stick is held 3 or 4m away from the bunsen flame it does not light up
2. pockets of air trapped in amaterials such as wood, fibre glass, felt and expanded polysterene reduce heat by conduction. they are therefore used to make
1. woolen clothing and blankets
2.fibre glass for insulation in roofs and walls 3. expanded polysterene in the walls of refrigerator
vacuum is an insulator
1. in a thermal flask there will be a space of vacuum in between the walls of the flask inside. thus keeping the heat inside preventing heat loss due to its insulation properties
(diagram and full question will be shown in the viewer's content section )
** double wall things!!
these products have double walls inwhich reduces the transfer of heat between the inside and the outside of a room.
the air or vacuum between reduces or eliminates heat transfer
double glass --- walls
glazed windows
same thing!
posted by supermassive on Monday, June 22, 2009
intro to heat! + conduction notes
intro to heat-
heat is not a substance but an energy which can be gained or lost via the principle of conservation of energy. thus it cannot be made or destroyed but only transferred from one substance to another.
* note *
students often mix up what is thermal energy and what is heat.
Q. what is the difference of thermal energy and heat?
Thermal energy is the energy portion of a system that increases with its temperature while heat is the TRANSFER of thermal energy.
Q.What causes transfer if thermal energy ?
It is the difference in temperatures between objects that allows thermal energy to flow from a region of higher temperature to a region of lower temperature (***the greater the difference of temperature the faster the heat transfer)
Q. what is the difference between temperature and thermal energy? temperature is the measure of thermal energy whereas thermal energy is an energy.
as the video stated, there are mainly 3 common methods of heat transfer.
1. conduction 2. convection 3. radiation
application notes~ ** the greater the difference of temperature the faster the heat transfer
when you put an iron to your hand (which i expect you to be more intellectual to decide wether you should or not) the heat travels much faster due to the graeter difference of temperature and you feel the heat very quickly
a cold spoon put in a hot glass of water feels hot after awhile.
OK
so conduction
is the main mode of heat transfer in solids
1. heat is transferred form hot to cold
in this diagram it shows that the vibration of a particle due to heat causes its neighbouring particles to vibrate more. the particles actually move futher from each other this is the actual reason why heated objects expand
Process
1. When a part is supplied with thermal energy, the molecules at this area gain kinetic energy. They vibrate faster and collide with the neighbouring particles
2. As molecules collide, there is kinetic energy transfer. The less energetic molecules gain kinetic energy, vibrate faster and collide with other less energetic molecules in the colder part of the object
3. Process continues until heat energy from the hotter part spreads throughout the colder area
Solids are better than liquids and gases because the molecules are much closer together. Since the atoms are closer together, solids conduct heat better than liquids or gasses. This means that two solid materials in contact would transfer heat from one to the other better than a solid in contact with a gas or a gas with a liquid
the expansion varies
in which solids expand the most as their particles are close together thus increasing the capacity in which the particles move further form each other liquids expand less as they are alraedy spaced apart alittle bit and of course gas expands the least as they are already far apart though heating will infact increase pressure due to more kinetic energy and the higher chance of particles hitting against the inner surface of a container.
os here is a common diagram proving how heat is tranferred from hot to cold ...
** metals are good conducters as they have free electrons unlike non metals which are poorer conductors
application ~
All metals are good conductors of heat. cooking pans, kettles, electric iron are made of copper, aluminium or steel. Plastic and wood are poor conductors but good insulators, therefore handles of cooking utensils are made of plastic or wood.
so what are good conductors and what are bad?
the good - metals
listing from 1. iron 2. steel 3.tungsten (application note- thats why these are used in light bulbs) 4. platinum 5.aluminium 6. gold 7.silver 8. copper (notice the betetr ones are the ones with more number of free electrons)
the bad 1.vacuum (the worst ever...) ~ reason being - there are no particles in the first palce XD therefore there is no conduction as conduction requires particles to move and vibrate
2. air 3. water 4. non metals
posted by supermassive on
learning goals
these are the mostly the questions that will come out in practise sheets and exams and they will be answered in the future posts.
how is thermal heat usually transferred?
how does energy tranfer occur in solids?
Describe convection in fluids in terms of density changes.
Explain why energy transfer of a body by radiation does not require a material medium
Explain how rate of energy transfer is affected
Apply a concept of thermal energy to an everyday application
the difference between tempetrature and thermal energy
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