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T2_06thermoprop
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by johnlittlephysics 14 years, 2 months ago
Thermal Properties of Matter
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Comments (Show all 41)
BrittanyNg said
at 3:49 pm on Jun 12, 2011
and for gases, y are the forces negligible (only act during the moments of collision)?
johnlittlephysics said
at 10:51 pm on Jun 13, 2011
yes.
BrittanyNg said
at 3:54 pm on Jun 14, 2011
erm mr ang...i asked y..><
BrittanyNg said
at 3:52 pm on Jun 12, 2011
mr ang, regarding latent heat of fusion and vaporization, i dun rly c the difference between the 2 of em. the notes mentioned tt latent heat of fusion= heat absorbed without change in temperature while latent heat of vaporization= heat tt is gained or released without rise in temp.
so latent heat of fusion n vaporization= melting/ boiling?
Goh Ying Ting said
at 4:02 pm on Jun 13, 2011
i think latent heat of fusion applies for melting and vaporisation applies for boiling
johnlittlephysics said
at 10:53 pm on Jun 13, 2011
That's correct, different names for two processes of change of state, both of which occurring at constant temperature (melting point and boiling point respectively for a substance).
BrittanyNg said
at 4:03 pm on Jun 14, 2011
thx
BrittanyNg said
at 10:59 am on Jun 13, 2011
mr ang, will a piece of ice of a large volume with eg. 60cm2 of exposed area melt faster or a piece of ice with a smaller volume but eg.40cm2 of exposed area melt first? issit possible to tell? thx
BrittanyNg said
at 11:16 am on Jun 13, 2011
this is sth under pressure??? i suppose
johnlittlephysics said
at 11:00 pm on Jun 13, 2011
the ice is exposed to standard atmospheric pressure, assuming to be constant.
If external pressure increases, it may melt faster.
johnlittlephysics said
at 10:59 pm on Jun 13, 2011
I can't tell. Need to consider rate of thermal energy gain due to the various processes:
1. rate of heat gain by conduction from surrounding air, and from surface on which ice is placed
2. rate of heat gain by convection of surrounding air
3. rate of heat gain by radiation from surroundings
4. any other source of heat from environment, effect of melted ice
BrittanyNg said
at 4:04 pm on Jun 14, 2011
oh..
BrittanyNg said
at 11:18 am on Jun 13, 2011
mr ang, for the effect of pressure on the boiling point of water, went the pressure applied opposes the expansion of steam (when liq state changes to solid state), y it help the water to boil at a higher temp?
johnlittlephysics said
at 11:02 pm on Jun 13, 2011
That's right. So increase in external air pressure would increase boiling point of water (like in a pressure cooker).
Also refer to notes.
BrittanyNg said
at 4:05 pm on Jun 14, 2011
i was asking Y mr ang...as in how would it increase the boiling point?
johnlittlephysics said
at 12:58 am on Jun 15, 2011
You've kind of answered it yourself. An increase in external air pressure (air pressing down on the water surface), it becomes harder for the water to expand (to increase its volume) to turn into steam. So more heating is needed to provide more kinetic energy to its molecules to escape into the air, so that water turn into steam at a higher temperature, hence higher boiling point.
BrittanyNg said
at 1:37 pm on Jun 15, 2011
as in i was wondering, y would the water taking a longer time to boil INCREASE the boiling point in the process..as in, the pressure applied results in the longer time needed to boil the water, BUT isnt it lyk, the water juz takes a longer time to boil thats all? Y will it RESULT in a HIGHER boiling point. in this case, im asking how does the time taken to boil the water affect the boiling point. i noe tt the pressure applied causes the time taken to b longer, but i Dun get how it actually increases the boiling point.
and does it min tt if less energy is needed to melt/ boil the substance, the melting point adn boiling points will decrease? vice versa?
thx ><
johnlittlephysics said
at 10:41 am on Jun 16, 2011
To your question below:
Heating the water a longer time means supplying more thermal energy to the molecules, and increasing the average k.e. of its molecules.
We've learnt that higher average k.e. means a higher temperature, hence higher boiling point.
Without the higher average k.e., the molecules would not be able to overcome the increase in air pressure to escape into the air, so the water can't boil (change into steam).
Yes, the melting or boiling point is associated with how much energy is needed for the process to happen.
BrittanyNg said
at 1:19 pm on Jun 16, 2011
okays...ty
BrittanyNg said
at 11:20 am on Jun 13, 2011
According to the notes, when pressure is applied on substances(other than water), the melting point will increase. y is this so? thx
johnlittlephysics said
at 11:05 pm on Jun 13, 2011
Use argument similar to that for effect of pressure on boiling.
Most substances (other than water) expand when they melt, their volume increases (from solid to liquid state). When external pressure is applied, this opposes the expansion of the substance, so it more energy is needed to melt (to expand), and melting point increases.
BrittanyNg said
at 4:19 pm on Jun 14, 2011
so does it min tt if less energy is needed to melt/ boil the substance, the melting and boiling point will decrease? n vice versa?
johnlittlephysics said
at 10:18 pm on Jun 19, 2011
As far as you've learnt, 2 factors directly affect actual melting and boiling point of substances:
1. impurities
2. external pressure
This is classified as enrichment, as indicated in your notes.
Goh Ying Ting said
at 4:35 pm on Jun 13, 2011
Mr Ang, do you think you can explain everything abt the effect of pressure on the melting and boiling points of substances?thx!
johnlittlephysics said
at 11:06 pm on Jun 13, 2011
See the discussions above.
Goh Ying Ting said
at 4:46 pm on Jun 13, 2011
Mr Ang, does the temp of a sub. (eg.ice) continue to decrease after it has frozen (to completely solid)?
johnlittlephysics said
at 11:09 pm on Jun 13, 2011
The temperature of a substance in solid state will continue to decrease if there is continous heat loss to the surrounding.
E.g. ice in freezer at -10 degC would continue a drop in temperature from 0 degC to -10 degC.
BrittanyNg said
at 4:07 pm on Jun 14, 2011
so tt oso applies to boiling?
johnlittlephysics said
at 1:15 am on Jun 15, 2011
Yes. After all the water has become steam at 100 degC, if heating of the steam is continued (assuming you can trap the steam), its temperature would rise about 100 degC
johnlittlephysics said
at 1:16 am on Jun 15, 2011
This further drop in temperature occurs only after ALL the water has frozen into ice.
johnlittlephysics said
at 10:50 pm on Jun 13, 2011
We mean the forces are still strong in liquid (while forces between molecules are very weak in a gas).
Yes, you may say the forces between molecules in a solid state are generally stronger than those in the liquid state.
BrittanyNg said
at 3:53 pm on Jun 14, 2011
oh ic...thx
BrittanyNg said
at 1:42 pm on Jun 15, 2011
regarding latent heat, is there a specific term for condensation?
because latent heat of fusion= melting (solid to liquid)
latent heat of vaporization= boilng (liquid to gas)
so wad abt for gas to liquid and liquid to solid? issit the same? iguess its the same but juz wana make sure ><
johnlittlephysics said
at 10:31 am on Jun 16, 2011
1. Changes between solid and liquid states: use latent heat of fusion
When solid melts into a liquid, latent heat of fusion is absorbed by the solid (from surroundings)
When liquid freezes into a solid, latent heat of fusion is lost by the liquid (to the surroundings)
2. Changes between liquid and gaeseous states: use latent heat of vaporisation
When liquid changes into a gas, latent heat of vaporisation is absorbed by the liquid (from surroundings)
When gas condenses into a liquid, latent heat of vaporisation is lost by the gas (to the surroundings)
Don't worry, we would not be using these terms in assessing Sec 1 physics course. Discussion and calculations involving these would probably be covered in Sec 4.
BrittanyNg said
at 1:21 pm on Jun 16, 2011
ok, thx
BrittanyNg said
at 10:35 am on Jun 19, 2011
m ang, when a question asks you: e.g. will the boiling point increase or decrease with decreased atmospheric pressure and y, is this an acceptable ans?
A: When the atmospheric pressure decreases, the boiling point of the water would decrease. When water boils (changes to steam), it would expand. decreased atmospheric pressure would not opppose the expansion frce as much. Thus, the water would take a shorter time to boil,resulting in a lower boiling point.
johnlittlephysics said
at 10:21 pm on Jun 19, 2011
Last sentence: water will boil at lower boiling point - we are not talking about the time taken.
BrittanyNg said
at 5:34 pm on Jun 21, 2011
oh, ok thx. den oni when they toking abt time then u tok abt the shorter/ longer thing rite? i noe the ans is yes so no nid reply.
johnlittlephysics said
at 3:04 pm on Jun 22, 2011
The last sentence "take a shorter time to boil, resulting in a lower boiling point" - You are saying the shorter time caused the lower boiling point, which is inaccurate. The lower atmospheric pressure caused the lower boiling point, regardless of time taken.
BrittanyNg said
at 5:55 pm on Jun 22, 2011
oh! ok ty
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