Physics: Kinetic Model of Matter
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Over a particular temperature range, a volume of liquid expands more than the same volume of solid because liquid molecules, compared to solid molecules
A are less dense
B are larger
C move faster
D exert smaller forces on surrounding molecules
D's the answer, however, I do not understand why. Please help to explain.
Next.
The rate of heat transfer between an object and the surroundings does not depend on
Acolour
B mass
C density
D surface area
I chose B but answer is C. I thought C would somehow have an effect(consider rate of heat transfer due to convection currents)
thanks to one and all.
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Over a particular temperature range, a volume of liquid expands more than the same volume of solid because liquid molecules, compared to solid molecules
A are less dense
B are larger
C move faster
D exert smaller forces on surrounding molecules
D's the answer, however, I do not understand why. Please help to explain.
By elimination, we should know A, B and C are wrong already. How can a molecule be less dense than its own molecule? Or larger? Or move faster in the same temperature range?
Intermolecular forces (or van der waal forces) are weak for liquid, and is a reason for the molecules being further apart as compared to solids.
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The rate of heat transfer between an object and the surroundings does not depend on
A colour
B mass
C density
D surface area
I chose B but answer is C. I thought C would somehow have an effect(consider rate of heat transfer due to convection currents)
My original ans not correct. Look Below
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Originally posted by eagle:
at first I also not sure... then I think again w.r.t. to you pointing out of convection currents before I come up with a possible explanation
What is important in heat transfer in convection current is the speed at which the molecules move. And temperature is sort of a measure of the kinetic energies of the molecules. So for a certain temperature, molecules will have the same average kinetic energy. A lighter molecule will thus have a speed larger than a heavier molecule for the same kinetic energy. Thus, because it moves faster, the convection current will be faster.
Density doesn't matter because it also depends on volume as well. Furthermore, kinetic energy formula does not make use of density.
Hi eagle. Just read your reply to bonkysleuth's question. May I know whether the rate f heat transfer between objectand surroundings depend on volume then? If possible, can you simplify the explanation you have just given? I 'd like to understand these questions too. thank you!^^
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is this O levels physics?
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I don't understand.
Density DOES affect the rate of heat transfer in my opinion.
Density affects both conduction and convection, the lower the density, the less likely the molecules will interact, the lower the rate of heat transfer. Hence, air is not a good conductor of heat compared to solids, and therefore does not transfer heat to another substance well.
It is mass that should not affect the rate of heat transfer, looking at the molecular level.
Colour affects radiation.
Surface area, by the means explained aboved.
But mass...besides affecting the heat capacity of the substance at a given temperature should not affect the rate of heat transfer.
Someone please clarify thanks...
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We compare mass and density one by one while holding the rest of the factors constant.
We have two objects, with identical density and colour. However, object A is twice as heavy as object B. We can now say that there are twice as many particles in A than in B, so heat transfer is definitely faster. (on a side note, surface area and volume will also increase because we assumed density constant. so more surface radiation also. but not important).
As for density, I think you are a bit confused. If we say that object A is denser than object B, we actually mean that A's particles are individually heavier than B's particles. Let me make an example. If you cut out a slice of 1 cubic meters worth of uranium, it's gonna be a hell lot heavier than 1 cubic meters worth of carbon-12. Does the weight of the individual atoms in the uranium slice matter in heat transfer?
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Originally posted by anpanman:
Hi eagle. Just read your reply to bonkysleuth's question. May I know whether the rate f heat transfer between objectand surroundings depend on volume then? If possible, can you simplify the explanation you have just given? I 'd like to understand these questions too. thank you!^^
Typically, it is not dependent on volume.
I think TS didn't copy the whole question word for word, or the question wasn't set properly... Got a bit confused initially... Let me add some words:
The rate of heat transfer between an object and the surroundings does not depend on
A colour of object
B mass of object
C density of object
D surface area of object
It is dependent on mass because heat loss = mass * specific heat capacity * change in temperature.
And rate of heat transfer/loss/gain = heat loss/gain divided by time, that's all.
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Originally posted by SBS2601D:
I don't understand.
Density DOES affect the rate of heat transfer in my opinion.
Density affects both conduction and convection, the lower the density, the less likely the molecules will interact, the lower the rate of heat transfer. Hence, air is not a good conductor of heat compared to solids, and therefore does not transfer heat to another substance well.
It is mass that should not affect the rate of heat transfer, looking at the molecular level.
Colour affects radiation.
Surface area, by the means explained aboved.
But mass...besides affecting the heat capacity of the substance at a given temperature should not affect the rate of heat transfer.
Someone please clarify thanks...
Density of object does not affect conduction or convection.
I think the confusing part is that this 2nd question isn't linked to the topic title of Kinetic Model of Matter. It should be under Heat Transfer, Conduction, Convection and Radiation.
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Originally posted by crimson soldier:
We compare mass and density one by one while holding the rest of the factors constant.
We have two objects, with identical density and colour. However, object A is twice as heavy as object B. We can now say that there are twice as many particles in A than in B, so heat transfer is definitely faster. (on a side note, surface area and volume will also increase because we assumed density constant. so more surface radiation also. but not important).
As for density, I think you are a bit confused. If we say that object A is denser than object B, we actually mean that A's particles are individually heavier than B's particles. Let me make an example. If you cut out a slice of 1 cubic meters worth of uranium, it's gonna be a hell lot heavier than 1 cubic meters worth of carbon-12. Does the weight of the individual atoms in the uranium slice matter in heat transfer?
As for density, I think you are a bit confused. If we say that object A is denser than object B, we actually mean that A's particles are individually heavier than B's particles.
Not necessarily...
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Of course not totally true, but just trying to help him see the light
.[just a discussion on density with eagle, not physics kinetic model]
I remember from material science module last sem: Density is explained by atomic packing factor(in 1 cubic meters of volume of space, how much the atoms actually displace). Many elements have the atomic packing factors 0.68 - 0.74. Not really a big difference on the larger scale of things (like when element A has twice the atomic mass of element B, does it matter whether A's APF = 0.68 or B's APF 0.74? If they both have roughly the same atomic mass, then B can have 8.8% higher density at most only.
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I seem to differ, density of the surrounding ambience is very crucial to heat transfer under convection. Just relook at the Grashof's Number, where we relate density of the surrounding fluid to "rate" of natural convection. okok...sorry, read wrong question, its says about the "object", not the environment.
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Originally posted by nightzip:
I seem to differ, density of the surrounding ambience is very crucial to heat transfer under convection. Just relook at the Grashof’s Number, where we relate density of the surrounding fluid to “rate” of natural convection.
I just think, this is an extremely poor question, drafted by someone who may not have though deeply before setting the question.
it's the question
density of the object shouldn't matter much with heat transfer to the surroundings
problem now is the question did not state properly whether it is the density of the object or the surroundings :(
crimson soldier, that's interesting... I agree with u.... will reply a bit after my tuition which is going to start soon :D
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Originally posted by eagle:
Density of object does not affect conduction or convection.
I think the confusing part is that this 2nd question isn't linked to the topic title of Kinetic Model of Matter. It should be under Heat Transfer, Conduction, Convection and Radiation.
There's always wiki or google...Actually density does affect heat transfer. And therefore rate of heat transfer.
http://en.wikipedia.org/wiki/Heat_transfer
A more densely packed set of molecules will allow the molecules greater chance interact and transfer thermal energy as opposed to say....air?
That's my interpretation so far.
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Originally posted by SBS2601D:
There's always wiki or google...Actually density does affect heat transfer. And therefore rate of heat transfer.
http://en.wikipedia.org/wiki/Heat_transfer
A more densely packed set of molecules will allow the molecules greater chance interact and transfer thermal energy as opposed to say....air?
That's my interpretation so far.
erm
I think that will affect transfer of thermal energy by conduction within the object itself, and not between the object and surroundings.
What I meant was conduction of heat between the object and the surroundings.
For this, the main variables affecting heat transfer is temperature gradient and surface area.As for transfer of thermal energy by conduction within the object itself, density will and should affect, as you have pointed out.
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Originally posted by eagle:
erm
I think that will affect transfer of thermal energy by conduction within the object itself, and not between the object and surroundings.
What I meant was conduction of heat between the object and the surroundings.
For this, the main variables affecting heat transfer is temperature gradient and surface area.As for transfer of thermal energy by conduction within the object itself, density will and should affect, as you have pointed out.
Actually I did think about that.
But shouldn't it hold that when the heat is conducted faster throughout the object, then the heat transfered from within the object should also be transfered faster to the environment?
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Originally posted by SBS2601D:
Actually I did think about that.
But shouldn't it hold that when the heat is conducted faster throughout the object, then the heat transfered from within the object should also be transfered faster to the environment?
I think that the only reason why it could be faster is because the temperature gradient is maintained. :D
But the rate at which heat is transferred out (i.e. how many J per second) shouldn't change much because of density...
Actually, I'm also not very sure of the mass part as well in the question... See if I can seek some other expert's opinion outside :D
Interesting...
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Originally posted by eagle:
I think that the only reason why it could be faster is because the temperature gradient is maintained. :D
But the rate at which heat is transferred out (i.e. how many J per second) shouldn't change much because of density...
Actually, I'm also not very sure of the mass part as well in the question... See if I can seek some other expert's opinion outside :D
Interesting...
At a molecular level, the faster the molecules transfer heat, the faster the rate of transfer...
Thus, density should play an important factor in heat transfer and its rate.
But I'm interested to know how mass affects the rate of heat transfer...because larger molecules move slower? That may not hold true.
And as for solids, the molecules are more or less only vibrating in fixed structures, so mass should not be a factor for solids.
And unless I'm mistaken, also at a molecular level, density of the substance also affects convection currents, which clearly meant that density plays an important part in the rate of heat transfer to the environment.
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Originally posted by SBS2601D:
At a molecular level, the faster the molecules transfer heat, the faster the rate of transfer...
Thus, density should play an important factor in heat transfer and its rate.
But I'm interested to know how mass affects the rate of heat transfer...because larger molecules move slower? That may not hold true.
And as for solids, the molecules are more or less only vibrating in fixed structures, so mass should not be a factor for solids.
And unless I'm mistaken, also at a molecular level, density of the substance also affects convection currents, which clearly meant that density plays an important part in the rate of heat transfer to the environment.
Within the object, yes, density should play a part. The question should be heat transfer to the surroundings. At the molecular level, it is how fast heat energy is transferred from the molecules of the object to the gas molecules.
In convection currents, I thought initially that mass should play more. This is because the internal energy of a gas is due to the solely due to the speeds of the gas molecules. And the energy of the molecule is related to temperature by the equation 3/2 k T, where k is the boltzmann's constant.
Also, speed of molecules = 1/2 m v^2
So we can equate 1/2 m v^2 to 3/2 kT, giving us v = sqrt (3KT/m)
* Not sure if I remember all the equations proprely*That's the only reason so far I can think of on why mass is related to convection currents. The reason why convection currents occur is due to density of the different temperatures of the gas, and not because of the density of the gas itself at r.t.p.
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Originally posted by eagle:
Within the object, yes, density should play a part. The question should be heat transfer to the surroundings. At the molecular level, it is how fast heat energy is transferred from the molecules of the object to the gas molecules.
In convection currents, I thought initially that mass should play more. This is because the internal energy of a gas is due to the solely due to the speeds of the gas molecules. And the energy of the molecule is related to temperature by the equation 3/2 k T, where k is the boltzmann's constant.
Also, speed of molecules = 1/2 m v^2
So we can equate 1/2 m v^2 to 3/2 kT, giving us v = sqrt (3KT/m)
* Not sure if I remember all the equations proprely*That's the only reason so far I can think of on why mass is related to convection currents. The reason why convection currents occur is due to density of the different temperatures of the gas, and not because of the density of the gas itself at r.t.p.
Then if mass play a part, given density is also based on the mass at a given volume, then density should play a part as well...
But apparently the density of a substance at rtp/stp does play a part in convection current strength, but I'm not completely sure, so....hopefully someone will enlighten me on that.
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Originally posted by SBS2601D:
Then if mass play a part, given density is also based on the mass at a given volume, then density should play a part as well...
But apparently the density of a substance at rtp/stp does play a part in convection current strength, but I'm not completely sure, so....hopefully someone will enlighten me on that.
How fast a convection current is depends entirely on the different densities of the gas at different temperatures.
The main thing is that hot air rises and cold air sinks :D
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Originally posted by eagle:
How fast a convection current is depends entirely on the different densities of the gas at different temperatures.
The main thing is that hot air rises and cold air sinks :D
Hmm....I go and check it out if possible.But that's not a very straight-forward question thrown at homework forum this time.
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that's what makes it interesting :D
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Originally posted by SBS2601D:
Hmm....I go and check it out if possible.But that's not a very straight-forward question thrown at homework forum this time.
Cause the people here are not paid professionals. Try asking a paid professional... like your teacher.