Mr Ang, is it alright if you go through the notes again because I cannot understand some of them, especially the part about internal energy. I have searched the Internet but there is nothing about decrease in internal energy.
The energy inside any substance (or object or body) is described as internal energy. It consists of two types:
1. kinetic energy (or k.e.) of all molecules in it: either due to vibration or motion of the molecules - this is associated with temperature. The higher the temperature of the substance, the greater the k.e. of its molecules
2. potential energy (or p.e.) of its molecules: this is associated with the distance (or spacing) between the molecules (or state of the substance). The larger the distance between the molecules, the higher the p.e.
E.g. 1 in a normal substance, the molecules in the solid state has less p.e. while those in the liquid state has more p.e. (further apart). E.g. 2 when a substance expands, its p.e. increases, since spacing increases.
-- internal energy = k.e. + p.e. of all molecules in substance.
Mr Ang, can you explain what is the meaning of this answer in ws6.1, 3c?
In section BC, naphthalene is changing from liquid to solid state. The molecules
move closer together, losing potential energy. This is released as latent heat (of
fusion) to the surroundings. The kinetic energy of the molecules remains
unchanged, so the temperature is constant.
As mentioned above (in reply to Mandy) and in the given notes, molecules have two types of energy: kinetic energy and potential energy.
When changing from liquid to solid state, there was no temperature change (temperature is constant), this means that kinetic energy of molecules remain constant.
The molecules would move closer together (from liquid to solid state), the decrease in spacing between molecules mean that potential energy of molecules decreases. This loss in potential energy is actually given out to surroundings (called latent heat of fusion).
So chim o.o I put "kinetic enery from the particles is beong converted to potential energy as they slow down and come closer and tighter together. The heat lost is latent heat of fusion." I thought as the particle move closer together they get more kinetic energy? Why does the decrease in spacing mean less potential energy?
You have to separate the meanings of p.e. (potential energy) and k.e. (kinetic energy).
1. When the temperature of a substance is high, the molecules in it move faster and have higher k.e.
2. Molecules have forces between them. When molecules move further apart, we say we have to do work to pull the molecules further apart (to overcome the attractive forces), and this work becomes an increase in p.e. The p.e. of molecules is associated with spacing between them.
When a substance expands (e.g. from solid to liquid state), its volume increases, the spacing between molecules increases, so the p.e. of the molecules increases.
The word "particles" is a general term to describe the parts that generally make up matter (solid, liquid, or gas).
Examples of particles are molecules, atoms, ions, etc.
We can use "particles" instead of "molecules" to describe the inner structure of matter.
By observing its change in temperature using a thermometer - refer to the cooling curve (graph). When it is a mixture, its temperature will remain constant, as the liquid is changing into the solid state. Once the entire substance is in the solid state, its temperature will continue to fall (cooling) till it reaches room temperature.
Water has a structure (different form normal liquids) that increases its volume when it freezes into ice (solid).
If high pressure is applied onto ice, the effect is to compress it (reduce its volume), or reverse the process, such that ice becomes water again (melting).
You can read an interesting article about why ice is slippery at:
Ref: "Explaining ice: the answers are slippery" http://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=1&_r=1
Mr Ang, why do the particles in a liquid rotate and slide around each other?( worksheet 6.1, question 1bii) And is convection current when heated molecules rise and the cooler molecules go to the heating point, become heated and then also rise? Thank you very much- Jing Ying :) 109 =)
1. In liquid state, molecules can move around each other, indirect evidence provided by experimental observations of properties such like liquid takes the shape of its container.
See animation of water molecules at http://www.visionlearning.com/library/flash_viewer.php?oid=1380&mid=57
(but need flash player on your PC)
2. In the part of the liquid (or gas) which is heated, molecules have more kinetic energy and move further apart, the liquid (or gas) in this part occupy a bigger volume (same mass), so density becomes lower. Less dense substance will rise (float upwards). The cooler part has higher density, so it sinks.
Water molecules are held together by hydrogen bonds. The special and complex properties of these bonds and arrangement of the molecules cause volume of ice to water to decrease (not rise in volume), unlike other substances which expand from solid to liquid state.
Ref: http://www.chem1.com/acad/sci/aboutwater.html
Hi, I don't really understand he answer for question 2b of ws 6.1, especially the 2nd alternative answer. And what does the 'amplitude 'refer to? Is it the speed? Thank you!!!
2b: When solid is heated, molecules gain more kinetic energy, so they vibrate faster (higher frequency), and move to and fro through a larger distance (larger amplitude). Compare a vibrating molecule with the swinging pendulum bob and relate the motion to frequencey and amplitude.
sorry for posting so many times but what is "latent heat (of fusion)" and in the answer it also says that the molecules move closer together, but after that it says that there is no kinetic energy, but I thought kinetic energy is produced when an object is moving? Oh, forgot to tell you, this is question 3c of ws 6.1, and thank you very much~
During solidification, thermal energy is lost to the surroundings from the surface area of the liquid.
If the mass is doubled, volume is doubled, the increase in area is usually less than double. So the rate of heat loss is slower, and a longer time is needed.
3c: Latent heat (of fusion) just refers to the thermal energy associated with melting (this energy is absorbed by substance), or freezing (this energy is given out by substance).
Molecules move closer together - we are looking at the decreasing in spacing (distance) between molecules which would affect the potential energy (not how fast they are moving).
There is hot water in a slightly rigid plastic bottle. After a while, the bottle is deformed due to the effects of convection as stated in the PowerPoint slide of thermal energy (convection ). The diagram shows that the convection current is rising and after a while the bottle contracts?
If there is increase in kinetic energy during melting when heated, the molecules vibrate more , so kinetic energy increases. When k.e increases , shouldn't the temperature increase too?? Why is the temperature constant instead?
No. During melting, temperature remains constant and the average k.e. of the molecules remain constant.
The change of state causes a change in volume hence spacing between molecules, so the p.e. of molecules change.
Oh and when looking at a cooling curve of something which is presented as a graph , how can we determine the melting point of that substance, I thought we only could determine the freezing point of that substance?
A liquid cools and freezes into solid. We talk about a freezing point.
A solid is heated and melts into a liquid. We talk about a melting point. This is just the reverse process.
So melting point is the same as freezing point in general.
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Comments (29)
Mandy.Foong said
at 9:34 pm on Mar 7, 2011
Mr Ang, is it alright if you go through the notes again because I cannot understand some of them, especially the part about internal energy. I have searched the Internet but there is nothing about decrease in internal energy.
johnlittlephysics said
at 9:59 pm on Mar 8, 2011
The energy inside any substance (or object or body) is described as internal energy. It consists of two types:
1. kinetic energy (or k.e.) of all molecules in it: either due to vibration or motion of the molecules - this is associated with temperature. The higher the temperature of the substance, the greater the k.e. of its molecules
2. potential energy (or p.e.) of its molecules: this is associated with the distance (or spacing) between the molecules (or state of the substance). The larger the distance between the molecules, the higher the p.e.
E.g. 1 in a normal substance, the molecules in the solid state has less p.e. while those in the liquid state has more p.e. (further apart). E.g. 2 when a substance expands, its p.e. increases, since spacing increases.
-- internal energy = k.e. + p.e. of all molecules in substance.
Ref: your notes on Topic 06, or http://www.physicsclassroom.com/Class/thermalP/u18l2a.cfm
Jennifer Yao said
at 5:06 pm on Mar 12, 2011
Mr Ang, can you explain what is the meaning of this answer in ws6.1, 3c?
In section BC, naphthalene is changing from liquid to solid state. The molecules
move closer together, losing potential energy. This is released as latent heat (of
fusion) to the surroundings. The kinetic energy of the molecules remains
unchanged, so the temperature is constant.
johnlittlephysics said
at 9:38 pm on Mar 12, 2011
As mentioned above (in reply to Mandy) and in the given notes, molecules have two types of energy: kinetic energy and potential energy.
When changing from liquid to solid state, there was no temperature change (temperature is constant), this means that kinetic energy of molecules remain constant.
The molecules would move closer together (from liquid to solid state), the decrease in spacing between molecules mean that potential energy of molecules decreases. This loss in potential energy is actually given out to surroundings (called latent heat of fusion).
The Walking Chocolate said
at 2:23 pm on Mar 13, 2011
So chim o.o I put "kinetic enery from the particles is beong converted to potential energy as they slow down and come closer and tighter together. The heat lost is latent heat of fusion." I thought as the particle move closer together they get more kinetic energy? Why does the decrease in spacing mean less potential energy?
johnlittlephysics said
at 11:47 am on Mar 19, 2011
You have to separate the meanings of p.e. (potential energy) and k.e. (kinetic energy).
1. When the temperature of a substance is high, the molecules in it move faster and have higher k.e.
2. Molecules have forces between them. When molecules move further apart, we say we have to do work to pull the molecules further apart (to overcome the attractive forces), and this work becomes an increase in p.e. The p.e. of molecules is associated with spacing between them.
When a substance expands (e.g. from solid to liquid state), its volume increases, the spacing between molecules increases, so the p.e. of the molecules increases.
The Walking Chocolate said
at 11:40 am on Mar 13, 2011
Mr ang are particles the same as molecules?
johnlittlephysics said
at 11:38 am on Mar 19, 2011
The word "particles" is a general term to describe the parts that generally make up matter (solid, liquid, or gas).
Examples of particles are molecules, atoms, ions, etc.
We can use "particles" instead of "molecules" to describe the inner structure of matter.
The Walking Chocolate said
at 12:01 pm on Mar 13, 2011
Hello mr ang, how to you tell when the naphthalene is purely solid and a mixture of solid snd liquid? I mean, I guessed but I got it correct :P
johnlittlephysics said
at 11:40 am on Mar 19, 2011
By observing its change in temperature using a thermometer - refer to the cooling curve (graph). When it is a mixture, its temperature will remain constant, as the liquid is changing into the solid state. Once the entire substance is in the solid state, its temperature will continue to fall (cooling) till it reaches room temperature.
Lee Tsinrong said
at 10:08 pm on Mar 15, 2011
Hello Mr Ang! Why does high pressure applied to ice cause the volume to decrease and helps the ice to melt? :)
johnlittlephysics said
at 11:58 am on Mar 19, 2011
Water has a structure (different form normal liquids) that increases its volume when it freezes into ice (solid).
If high pressure is applied onto ice, the effect is to compress it (reduce its volume), or reverse the process, such that ice becomes water again (melting).
You can read an interesting article about why ice is slippery at:
Ref: "Explaining ice: the answers are slippery" http://www.nytimes.com/2006/02/21/science/21ice.html?pagewanted=1&_r=1
See Jing Ying said
at 1:39 pm on Mar 26, 2011
Mr Ang, why do the particles in a liquid rotate and slide around each other?( worksheet 6.1, question 1bii) And is convection current when heated molecules rise and the cooler molecules go to the heating point, become heated and then also rise? Thank you very much- Jing Ying :) 109 =)
johnlittlephysics said
at 6:33 pm on Mar 26, 2011
1. In liquid state, molecules can move around each other, indirect evidence provided by experimental observations of properties such like liquid takes the shape of its container.
See animation of water molecules at http://www.visionlearning.com/library/flash_viewer.php?oid=1380&mid=57
(but need flash player on your PC)
2. In the part of the liquid (or gas) which is heated, molecules have more kinetic energy and move further apart, the liquid (or gas) in this part occupy a bigger volume (same mass), so density becomes lower. Less dense substance will rise (float upwards). The cooler part has higher density, so it sinks.
See Jing Ying said
at 1:41 pm on Mar 26, 2011
Mr ang, what is the structure of water which causes it to rise in volume when heated? Thank you again ;)
johnlittlephysics said
at 2:58 pm on Mar 27, 2011
Water molecules are held together by hydrogen bonds. The special and complex properties of these bonds and arrangement of the molecules cause volume of ice to water to decrease (not rise in volume), unlike other substances which expand from solid to liquid state.
Ref: http://www.chem1.com/acad/sci/aboutwater.html
See Jing Ying said
at 1:49 pm on Mar 26, 2011
Hi, I don't really understand he answer for question 2b of ws 6.1, especially the 2nd alternative answer. And what does the 'amplitude 'refer to? Is it the speed? Thank you!!!
johnlittlephysics said
at 3:02 pm on Mar 27, 2011
2b: When solid is heated, molecules gain more kinetic energy, so they vibrate faster (higher frequency), and move to and fro through a larger distance (larger amplitude). Compare a vibrating molecule with the swinging pendulum bob and relate the motion to frequencey and amplitude.
See Jing Ying said
at 1:57 pm on Mar 26, 2011
sorry for posting so many times but what is "latent heat (of fusion)" and in the answer it also says that the molecules move closer together, but after that it says that there is no kinetic energy, but I thought kinetic energy is produced when an object is moving? Oh, forgot to tell you, this is question 3c of ws 6.1, and thank you very much~
See Jing Ying said
at 2:06 pm on Mar 26, 2011
oh um and why is a longer time needed for solidification of liquids with a greater massthan liquids with a smaller mass?
johnlittlephysics said
at 4:07 pm on Mar 29, 2011
During solidification, thermal energy is lost to the surroundings from the surface area of the liquid.
If the mass is doubled, volume is doubled, the increase in area is usually less than double. So the rate of heat loss is slower, and a longer time is needed.
johnlittlephysics said
at 3:07 pm on Mar 27, 2011
3c: Latent heat (of fusion) just refers to the thermal energy associated with melting (this energy is absorbed by substance), or freezing (this energy is given out by substance).
Molecules move closer together - we are looking at the decreasing in spacing (distance) between molecules which would affect the potential energy (not how fast they are moving).
Lee Tsinrong said
at 9:16 pm on Mar 28, 2011
Why and how does the effects of convection cause a plastic bottle to be deformed? :)
johnlittlephysics said
at 4:08 pm on Mar 29, 2011
Could u give a specific example or context?
Lee Tsinrong said
at 4:44 pm on Mar 29, 2011
There is hot water in a slightly rigid plastic bottle. After a while, the bottle is deformed due to the effects of convection as stated in the PowerPoint slide of thermal energy (convection ). The diagram shows that the convection current is rising and after a while the bottle contracts?
Gan Ming Yi said
at 6:45 pm on Mar 29, 2011
If there is increase in kinetic energy during melting when heated, the molecules vibrate more , so kinetic energy increases. When k.e increases , shouldn't the temperature increase too?? Why is the temperature constant instead?
johnlittlephysics said
at 10:16 pm on Mar 29, 2011
No. During melting, temperature remains constant and the average k.e. of the molecules remain constant.
The change of state causes a change in volume hence spacing between molecules, so the p.e. of molecules change.
Gan Ming Yi said
at 6:48 pm on Mar 29, 2011
Oh and when looking at a cooling curve of something which is presented as a graph , how can we determine the melting point of that substance, I thought we only could determine the freezing point of that substance?
johnlittlephysics said
at 10:18 pm on Mar 29, 2011
A liquid cools and freezes into solid. We talk about a freezing point.
A solid is heated and melts into a liquid. We talk about a melting point. This is just the reverse process.
So melting point is the same as freezing point in general.
You don't have permission to comment on this page.