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 CSS Forums Every Day science Paper 2010

#41
Saturday, April 24, 2010
 Senior Member Join Date: Dec 2009 Location: Islamabad Posts: 351 Thanks: 219 Thanked 222 Times in 143 Posts

Quote:
 Originally Posted by silent.volcano 1. Water is heated in a kettle. The inside water is heated by convection. A person sitting near the fire receives heat by : a) Conduction b) Convection c) Radiation d) Reflection e) None of these its answer should be radiation not conduction because conduction is the heat transferred with in the body. when heat is radiated by the kettle the person sitting near the fire receives the heat
While radiation is transfer of heat through empty space (without any molecules, electrons or any other material)
Convection is the transfer of heat through movement of molecules, in this case, the molecules of air (gases) transfer the heat, so correct answer is Convection. Read http://en.wikipedia.org/wiki/Heat_transfer for more clearance of concept.

Well I personally believe that the transfer of heat is at a time by all methods but almost always one method is predominant. So we consider that as correct answer.
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#42
Saturday, April 24, 2010
 42nd CTP (PAS) Join Date: Oct 2009 Posts: 873 Thanks: 586 Thanked 954 Times in 390 Posts

i am still not convinced.

as far as i know convection is basically vertical movement of heat.
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#43
Saturday, April 24, 2010
 Senior Member Join Date: Sep 2009 Location: R.I.P(Rest In Peace) Posts: 743 Thanks: 0 Thanked 1,143 Times in 374 Posts

@silent volcano
i agree with you

convection heats molecules ,hot and light molecule move up,person is not sitting up.cool molecules move downward so it seems that convection is not possible.
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#44
Saturday, April 24, 2010
 Senior Member Join Date: Dec 2009 Location: Islamabad Posts: 351 Thanks: 219 Thanked 222 Times in 143 Posts

Quote:
firstly radiations travel through empty space, yeah you are right air does not stop traveling radiations. Okay lets do an experiment.

Point A (A highSpeed fan)........Point B (sitting person)................point C( Source of heat).
now if fan blows the air at about speed of 200 Km per hour or more. The person will feel almost no heat.
Why? We know the moving air does not stop heat transfer while in this case it has almost stopped.
How? because the transfer of heat was through a medium, gas molecules which are not not coming from the direction of source of heat.
So what was the medium for transfer of heat? Had it been radiation, not even 600 km/hour blown air not been able to stop this transfer. because radiation is independent of movement of air. while convection needs movement of air molecules.
So the main transfer of heat was by air and not radiation.
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#45
Saturday, April 24, 2010
 Senior Member Join Date: Sep 2009 Location: R.I.P(Rest In Peace) Posts: 743 Thanks: 0 Thanked 1,143 Times in 374 Posts

Conduction- energy is transferred by the direct contact of molecules, not by the movement of the material

Example: putting your hand on a stove burner. The amount of energy transferred depends on how conductive the material is. Metals are good conductors, so they are used to transfer energy from the stove to the food in pots and pans. Air is the best insulator, so good insulating products try to trap air and not allow it to move.
Convection- energy is transferred by the mass motion of groups of molecules resulting in transport and mixing of properties

Example: holding your hand over a stove burner. In meteorology, we speak of convection predominantly as that caused by rising currents of warm air. We refer to all other mass motions of air as advection.

Example: heat felt when standing away from a large fire on a calm night. Everything that has a temperature above absolute zero radiates energy. Radiation is not "felt" until it is absorbed by a substance. It does not require a medium to transfer energy through as do conduction and convection.

question say"A person sitting near the fire receives heat by " so answer seems to be radiation.

i hope its clear now
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#46
Saturday, April 24, 2010
 Senior Member Join Date: Dec 2009 Location: Islamabad Posts: 351 Thanks: 219 Thanked 222 Times in 143 Posts

Convection is not solely vertical or upward movement, yeah it easily occurs in upward direction.
prove my hypotheses wrong, as radiation is through electromagnetic radiations, so why did heat stop transferring when air in opposite direction was blown because Electromagnetic radiation don't depend on movement of air. So it must be something else not radiation.
Okay brother, it seems I am not going to be convinced by you, and neither you are going to be convinced by me. contact some good physicist for right answer and also tell him about my argument and hypotheses.
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#47
Saturday, April 24, 2010
 Senior Member Join Date: Sep 2009 Location: R.I.P(Rest In Peace) Posts: 743 Thanks: 0 Thanked 1,143 Times in 374 Posts

@ arbab
as far your hypothesis or experiment is concerned:
heat radiation is transferred in the form of small packets of energy and when these packets are absorbed by a body it heats up. if air blows against source then these packets of energy will not reach body and one will not feel heat.
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#48
Tuesday, May 04, 2010
 Junior Member Join Date: May 2010 Location: Islamabad Posts: 9 Thanks: 11 Thanked 3 Times in 3 Posts
2. A time can come when we will be able to design a machine which can go on working f

you see the question asked is WITHOUT ENERGY and the solor energy is again an ENERGY, so the answer is NO
#49
Thursday, May 06, 2010
 42nd CTP (PSP) Join Date: Feb 2009 Location: Multan Posts: 670 Thanks: 24 Thanked 627 Times in 325 Posts

to put it in a layman's terms:

conduction is the heat transfer through contact...convection is the heat transfered through mixing and radiation is the heat emitted by a hot body...so the answer was radiation...any science student should have no doubts about it..:P

If we were to believe in your hypothesis then the following scenario must be true as well:

it is a sunny day and a cool breeze from the sea is blowing inland...you would not feel as much heat as you would in case it was a sunny day and there was no breeze...so do you suggest that the sun suddenly radiated lesser amount of energy?...When you have a fire in the room, and you turn on a fan, radiation remains there, but its effect is lessened because of (assumingly) cold air that is being thrown at you by the Fan....

secondly energy transfer is not through a single process alone...in most cases there are many modes of transfer...in a room, with a heater on, you receive energy through both convection and radiation and probably conduction too in case you have a fall on the fire .....in a scenario where you would be directly in front of the fire, the major chunk of energy transfer would be radiation, if you are behind some wall or anything it will be convection...
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Last edited by Princess Royal; Thursday, May 06, 2010 at 01:53 PM. Reason: posts merged
#50
Monday, May 17, 2010
 Junior Member Join Date: May 2010 Location: Lahore Posts: 2 Thanks: 4 Thanked 0 Times in 0 Posts

Assalam-o-Alaikum,

Convection is the movement ov molecules within fluid (Liquids, gases & Rheids), Radiation should be da answer!

Only Hermaphroditism iz a form ov Sexual reproduction. Rest belong to Asexual..dat iz for sure!

Last edited by Princess Royal; Monday, May 17, 2010 at 06:25 PM.