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latent heat is related to potential energy ??


tony lee

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Hello Tony, before I answer this can you tell me if you have done a cooling or melting curve experiment?

 

That is plotted the Temperture v Time graph of some ice or wax as it melts and then warms up or some water or oil as it cools and solidifies?

Edited by studiot
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yes i hv done the graph . i dont understand when changing stage , the temperature remind unchange but it release latent heat ?? what kind of energy does latent heat refers to ? thanks for anwsering my questions studiot .


Hello Tony, before I answer this can you tell me if you have done a cooling or melting curve experiment?

 

That is plotted the Temperture v Time graph of some ice or wax as it melts and then warms up or some water or oil as it cools and solidifies?

yes i hv done the graph . i dont understand when changing stage , the temperature remind unchange but it release latent heat ?? what kind of energy does latent heat refers to ? thanks for anwsering my questions studiot .

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solid to liquid and liquid to gas each involve a change in some kind of attraction/bonding between the atoms or molecules, and forming or breaking those bonds involves a release or absorption of energy that's unrelated to a change in the temperature of the material. That's the latent heat — the energy you have to put in or take out to go from one phase to another, because of the level of "stickiness" the molecules have for each other.

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I wouldn't call it potential energy (as that is defined as Ep = m*g*h), but internal energy. This energy is present in liquid water of 0°C and has to be removed to form ice of 0°C. Same for all other phase transitions.

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OK, I'm glad you have done a cooling curve. This makes things easier.

 

I am going to talk about a heating curve because it makes the explanation flow better.

Heating curves are just the reverse of cooling ones. You can go up and down them as many times as you want.

So if you heated some water (ice) up in a beaker and measured temperature v time and then cooled it down again you would get the similar curves.

The heating one would be quicker simply because you are supplying heat with a burner so can change the temperature more quickly.

 

OK so if we set the burner so it heats the beaker constantly.

That is we are supplying a constant amount of heat per second to first the ice and then the water.

We ensure this constancy by adjusting the burner then leaving it.

Understanding this is quite important and it is easier to do with heating than cooling.

 

So looking at my curve, We see that the temperature remains constant from A to B, then rises steadily from B to C then remains constant from C to D and if we are able to measure in the steam it rises again more steeply from D to E.

 

Looking first at the section B to C (I'll come back to AB)

 

We have ensured that the amount of heat input per second is constant.

(We could easily measure this with an electric heater instead of the burner)

 

Now I have drawn BC as a straight line and in a real experiment it would be very nearly so.

 

This means that the temperature rise is proportional to time and therefore to the total heat input.

 

The temperature rise = a constant times the total heat input.

 

If we turn this round we can also say that the total heat input = a constant times the temperature rise. (Where the second constant is the reciprocal of the first)

 

If we divide through by the mass of water we are heating we get an important constant called the specific heat.

The specific heat and tells us the amount of heat needed to raise the 1 kilogramme of water temperature 1 degree.

 

Now along the line AB we note that the temperature is not rising.

 

Yet we are inputting heat, which we can calculate in the same way as we did for the specific heat.

 

We should also notice that the temperature does not start rising until all the ice has melted.

 

This tells us that the heat we are putting in is doing something different from the specific heat.

 

Indeed it is the heat we have to put in to turn a solid into a liquid and is called the latent heat of fusion.

We would get this heat back if we froze water at 0o

 

Section CD of the curve represents a similar situation with boiling. This is the latent heat of evaporation (or sometimes condensation).

This represent the heat we must put in to change a liquid into a gas.

 

post-74263-0-43691000-1412858970_thumb.jpg

 

Can this latent heat be considered potential energy?

 

That is a good question since we need to put it in to melt or boil and can recover it by freezing or condenstation.

We can do this repeatedly so it is a reliable store for energy.

So yes in that sense it is potential energy.

 

However getting energy by freezing water is less useful at normal temperatures than storing energy in steam (which is hotter than ambient) and using it to drive machinery, by allowing it to cool all the way abck to liquid.

This is actually done in modern central heating boilers known as condensing boilers to make them more efficient by recovering the latent heat as well as the other forms of energy due to the combustion of the fuels.

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solid to liquid and liquid to gas each involve a change in some kind of attraction/bonding between the atoms or molecules, and forming or breaking those bonds involves a release or absorption of energy that's unrelated to a change in the temperature of the material. That's the latent heat — the energy you have to put in or take out to go from one phase to another, because of the level of "stickiness" the molecules have for each other.

hi, THANK YOU VERY MUCH for answering my questions. Your answer is simple and direct , and make me have a clear concept on latent heat and what it is .you explain it really clear .and thanks again

I wouldn't call it potential energy (as that is defined as Ep = m*g*h), but internal energy. This energy is present in liquid water of 0°C and has to be removed to form ice of 0°C. Same for all other phase transitions.

excuse me . i have a follow-up question , what is internal energy and what s the differences between potential energy, kinetic energy and internal energy . thank you

OK, I'm glad you have done a cooling curve. This makes things easier.

 

I am going to talk about a heating curve because it makes the explanation flow better.

Heating curves are just the reverse of cooling ones. You can go up and down them as many times as you want.

So if you heated some water (ice) up in a beaker and measured temperature v time and then cooled it down again you would get the similar curves.

The heating one would be quicker simply because you are supplying heat with a burner so can change the temperature more quickly.

 

OK so if we set the burner so it heats the beaker constantly.

That is we are supplying a constant amount of heat per second to first the ice and then the water.

We ensure this constancy by adjusting the burner then leaving it.

Understanding this is quite important and it is easier to do with heating than cooling.

 

So looking at my curve, We see that the temperature remains constant from A to B, then rises steadily from B to C then remains constant from C to D and if we are able to measure in the steam it rises again more steeply from D to E.

 

Looking first at the section B to C (I'll come back to AB)

 

We have ensured that the amount of heat input per second is constant.

(We could easily measure this with an electric heater instead of the burner)

 

Now I have drawn BC as a straight line and in a real experiment it would be very nearly so.

 

This means that the temperature rise is proportional to time and therefore to the total heat input.

 

The temperature rise = a constant times the total heat input.

 

If we turn this round we can also say that the total heat input = a constant times the temperature rise. (Where the second constant is the reciprocal of the first)

 

If we divide through by the mass of water we are heating we get an important constant called the specific heat.

The specific heat and tells us the amount of heat needed to raise the 1 kilogramme of water temperature 1 degree.

 

Now along the line AB we note that the temperature is not rising.

 

Yet we are inputting heat, which we can calculate in the same way as we did for the specific heat.

 

We should also notice that the temperature does not start rising until all the ice has melted.

 

This tells us that the heat we are putting in is doing something different from the specific heat.

 

Indeed it is the heat we have to put in to turn a solid into a liquid and is called the latent heat of fusion.

We would get this heat back if we froze water at 0o

 

Section CD of the curve represents a similar situation with boiling. This is the latent heat of evaporation (or sometimes condensation).

This represent the heat we must put in to change a liquid into a gas.

 

attachicon.gifheating1.jpg

 

Can this latent heat be considered potential energy?

 

That is a good question since we need to put it in to melt or boil and can recover it by freezing or condenstation.

We can do this repeatedly so it is a reliable store for energy.

So yes in that sense it is potential energy.

 

However getting energy by freezing water is less useful at normal temperatures than storing energy in steam (which is hotter than ambient) and using it to drive machinery, by allowing it to cool all the way abck to liquid.

This is actually done in modern central heating boilers known as condensing boilers to make them more efficient by recovering the latent heat as well as the other forms of energy due to the combustion of the fuels.

hello , once i saw your explanation i feel warm that u have spent so many times not only explaining my question,but also tell me more information. thank you and i understand your explanation well . thank you for helping me and spending your time on my questions . thanks

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excuse me . i have a follow-up question , what is internal energy and what s the differences between potential energy, kinetic energy and internal energy . thank you

 

 

Internal energy is a concept that you will not need until university, but it is really a very simple idea.

Internal energy is not another particular form of energy like kinetic energy or potential energy under gravity, which you may have met.

It is a sort of energy piggy bank to take account of all possible sorts of energy that a body or bunch of molecules or whatever may have.

This includes potential energy, which is probably why fuzzwood said what he said. But it also includes kinetic energy, electric energy, chemical energy and heat energy.

So if we heat something up the specific heat energy we input adds to its internal energy. This also applies to latent heat.

 

The branch of science that studies how one form of energy can be exchanged for another is called thermodynamics.

So you can imagine the internal energy bank as like your money account where you put dollars in and take euros out.

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