# Help with A2 physics coursework (thermodynamics)

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#1
1, Everyone knows that if u hv a piece of red hot iron and use forced convection (air), its not as effective as pouring water. But why?

2, can anyone tell me why Newton's Law of cooling works? and also what did he say about his law of cooling, because i can't find where he say that.

3, can someone please tell me how convection actually works in terms of fluid mechanics and thermodynamics (simplify version please.. I read something about buoyancy forces and the velocity, flow stuff but i dun quite get it)

thank you!
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#2
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15 years ago
#3
1, Everyone knows that if u hv a piece of red hot iron and use forced convection (air), its not as effective as pouring water. But why?
err... water has a larger thermal conductivity than air (particles are closer together in liquids)... it conducts faster... so more heat is absorbed
water also has a larger thermal capacity than air... so more heat can be absorbed without a great rise in temperature.. and note that temperature difference is proportional to the rate of conduction...

2, can anyone tell me why Newton's Law of cooling works? and also what did he say about his law of cooling, because i can't find where he say that.
Well... its obvious... the rate of cooling is proportional to the difference in temperature as heat transfer is PROPORTIONAL to the temperature... so a hotter object transmit more heat to a colder one whereas a colder one transmit less... overall the greater the difference... the faster the rate of transfer as the more heat is transfered...

3, can someone please tell me how convection actually works in terms of fluid mechanics and thermodynamics (simplify version please.. I read something about buoyancy forces and the velocity, flow stuff but i dun quite get it)
I'm sorry... never done fluid mechanics before.... but I guess.. by bernoillis principle.. the FASTER the fluid.. the less pressure.... thats all I know... sorry about that
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