internal energy

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Thread starter 4 years ago
#1
I have the following question: 'Which is more dangerous, your skin being exposed to 100degrees C water or 100degrees C steam? Explain your answer.'

My knowledge so far, is that internal energy is composed of both potential and kinetic energy. Kinetic energy manifests itself as heat/temperature, and at 0K there will be no kinetic energy and therefore no heat, but there will still be potential energy.

Therefore I would say that at 100 degrees, both water and steam are equally dangerous to skin, as their kinetic energies are equal and burning yourself (temperature) is only dependent on the kinetic energy of the system.

Unfortunately the book says otherwise, but I cant quite see how the larger potential energy of steam effects your skin burning?

Thanks for any help.
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4 years ago
#2
(Original post by Sir_Malc)
I have the following question: 'Which is more dangerous, your skin being exposed to 100degrees C water or 100degrees C steam? Explain your answer.'

My knowledge so far, is that internal energy is composed of both potential and kinetic energy. Kinetic energy manifests itself as heat/temperature, and at 0K there will be no kinetic energy and therefore no heat, but there will still be potential energy.

Therefore I would say that at 100 degrees, both water and steam are equally dangerous to skin, as their kinetic energies are equal and burning yourself (temperature) is only dependent on the kinetic energy of the system.

Unfortunately the book says otherwise, but I cant quite see how the larger potential energy of steam effects your skin burning?

Thanks for any help.
The liquid properties of water are the result of water molecules loosely bound to each other (molecular cohesion) because the electric force holding individual atoms together (hydrogen and oxygen) to form a water molecule, are polarised.

i.e. one side of the molecule is slightly positive and the opposite side slightly negative meaning that the two hydrogen atoms of one molecule will be attracted to the oxygen atom of another molecule.

Up to 100oC, water molecules do not have sufficient kinetic energy to break free from each other and so remain as a collective bulk liquid.

As the thermal energy of the water increases, the kinetic energy of individual molecules also increases and when a threshold is reached (specific heat capacity), individual water molecules overcome the attractive force holding the liquid together and can break free from the bulk. (rather like a space-rocket attaining sufficient velocity to escape orbit). Free molecules are no longer bound to the bulk and are described as steam.

Steam molecules by definition, therefore have a higher kinetic energy than liquid molecules bound together in a bulk, and therefore have more kinetic energy to damage human skin.
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4 years ago
#3
(Original post by Sir_Malc)
I have the following question: 'Which is more dangerous, your skin being exposed to 100degrees C water or 100degrees C steam? Explain your answer.'

My knowledge so far, is that internal energy is composed of both potential and kinetic energy. Kinetic energy manifests itself as heat/temperature, and at 0K there will be no kinetic energy and therefore no heat, but there will still be potential energy.

Therefore I would say that at 100 degrees, both water and steam are equally dangerous to skin, as their kinetic energies are equal and burning yourself (temperature) is only dependent on the kinetic energy of the system.

Unfortunately the book says otherwise, but I cant quite see how the larger potential energy of steam effects your skin burning?

Thanks for any help.
have you done anything on phase changes or latent heat ?

it's related to the reason 5 grams of ice at 0 deg C is better at cooling your drink than 5 grams of water at 0 deg C

There's a considerable amount of latent heat in steam... for equal masses of water and steam at the same temperature the steam will contain considerably more heat - and that will come out as sensible heat when it condenses on your skin.
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Thread starter 4 years ago
#4
(Original post by uberteknik)
The liquid properties of water are the result of water molecules loosely bound to each other (molecular cohesion) because the electric force holding individual atoms together (hydrogen and oxygen) to form a water molecule, are polarised...
(Original post by Joinedup)
have you done anything on phase changes or latent heat...
Thankyou both for your replies but I am still struggling a little to fully understand.

Am I right in thinking that the temperature is only dependent on the kinetic energy part of the internal energy? Therefore, if the temperature (kinetic energy) of both Steam and water is 100degreesC surely not one has more energy to burn you than the other?

My textbook says that the thermal energy in heating during a change of phase effects the potential energy, not the kinetic energy, and therefore temperature remains constant during this change of phase.

I could understand if the steam was at say, 101degreesC, then the steam would be worse as its got more kinetic energy.

Maybe I am incorrect in thinking only kinetic energy effects temperature?
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4 years ago
#5
(Original post by uberteknik)
The liquid properties of water are the result of water molecules loosely bound to each other (molecular cohesion) because the electric force holding individual atoms together (hydrogen and oxygen) to form a water molecule, are polarised.

i.e. one side of the molecule is slightly positive and the opposite side slightly negative meaning that the two hydrogen atoms of one molecule will be attracted to the oxygen atom of another molecule.

Up to 100oC, water molecules do not have sufficient kinetic energy to break free from each other and so remain as a collective bulk liquid.

As the thermal energy of the water increases, the kinetic energy of individual molecules also increases and when a threshold is reached (specific heat capacity), individual water molecules overcome the attractive force holding the liquid together and can break free from the bulk. (rather like a space-rocket attaining sufficient velocity to escape orbit). Free molecules are no longer bound to the bulk and are described as steam.

Steam molecules by definition, therefore have a higher kinetic energy than liquid molecules bound together in a bulk, and therefore have more kinetic energy to damage human skin.
at 100 degrees, the temperature of steam and water are the same obviously. Water becomes steam by a transfer of thermal energy into potential energy, so the kinetic energy does not increase, but the internal energy does. So even if they are different states, they still have the same kinetic energy.
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4 years ago
#6
(Original post by Sir_Malc)
Thankyou both for your replies but I am still struggling a little to fully understand.

Am I right in thinking that the temperature is only dependent on the kinetic energy part of the internal energy? Therefore, if the temperature (kinetic energy) of both Steam and water is 100degreesC surely not one has more energy to burn you than the other?

My textbook says that the thermal energy in heating during a change of phase effects the potential energy, not the kinetic energy, and therefore temperature remains constant during this change of phase.

I could understand if the steam was at say, 101degreesC, then the steam would be worse as its got more kinetic energy.

Maybe I am incorrect in thinking only kinetic energy effects temperature?
When you're boiling a kettle the water heats up to 100 deg C and if the kettle doesn't switch off the water will slowly boil away as steam... what doesn't happen is that all the water in the kettle hits 100 deg C and instantly all turns to steam at the same time.

The element in a UK kettle is putting heat into the water at a rate of probably about 2700 Watts and the temperature of the water is staying the same at 100 deg C

What this means is that when you've got the water up to 100 deg C you still need to keep putting energy in to break the water molecules apart and go from liquid water to steam, that energy is called the latent heat of vaporisation and when the steam condenses back to water that energy needs to come back out again.

if the steam is condensing on your skin the energy will go into heating your skin up.
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Thread starter 4 years ago
#7
(Original post by Joinedup)
When you're boiling a kettle the water heats up to 100 deg C and if the kettle doesn't switch off the water will slowly boil away as steam... what doesn't happen is that all the water in the kettle hits 100 deg C and instantly all turns to steam at the same time.

The element in a UK kettle is putting heat into the water at a rate of probably about 2700 Watts and the temperature of the water is staying the same at 100 deg C

What this means is that when you've got the water up to 100 deg C you still need to keep putting energy in to break the water molecules apart and go from liquid water to steam, that energy is called the latent heat of vaporisation and when the steam condenses back to water that energy needs to come back out again.

if the steam is condensing on your skin the energy will go into heating your skin up.
So it is the extra latent potential energy that can cause more burning in steam than water at 100degC. Not the kinetic energy?
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4 years ago
#8
(Original post by Sir_Malc)
So latent energy causes more burning from steam than water at 100 °C, not KE?
Steam's latent energy converts to thermal in the skin, raising its temp moreso than hot water does. It follows condensed water cools slower than hot water.

'5 g of ice at 0 °C is better at cooling your drink than 5 g of water at 0 °C'. Ice withdraws more energy to melt, hence a cooler drink (less KE, lower temp).

See: https://www.thestudentroom.co.uk/sho...42&postcount=2
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