# URGENT, help with internal energy for physics test

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#1
According to all videos I can find, internal energy is the sum of Kinetic energy and potential energies of a substance. However this doesn't make sense according to the idea a gas must always have a higher internal energy than a liquid and here is why.

Let's say we have 1kg of water at 100 degrees celsius in a closed system at standard atmospheric pressure. I think i would be correct in saying it has an electrostatic potential energy of 2.26 x 10^6 Joules and a kinetic energy of 4.2x10^5.
Using Internal energy = Kinetic energy + potential energies
We find an internal energy of 2680000 Joules for the water at 100 degrees.
Now let's say we add all of that energy required to turn into steam and now we have: Internal Energy= Kinetic energy + potential energy of substance.
Since apparently the kinetic energy hasn't changed we have
U = 4.2x10^5 + 0 as gas apparently has an electrostatic potential energy of zero. But this can't be true so i am immensely confused. Surely gas can't have a lower internal energy than its liquid counterpart?
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4 weeks ago
#2
(Original post by Jake-0402)
Let's say we have 1kg of water at 100 degrees celsius in a closed system at standard atmospheric pressure. I think i would be correct in saying it has an electrostatic potential energy of 2.26 x 10^6 Joules and a kinetic energy of 4.2x10^5.
where are you getting those figures from?
Last edited by Sinnoh; 4 weeks ago
1
4 weeks ago
#3
(Original post by Jake-0402)
According to all videos I can find, internal energy is the sum of Kinetic energy and potential energies of a substance. However this doesn't make sense according to the idea a gas must always have a higher internal energy than a liquid and here is why.

Let's say we have 1kg of water at 100 degrees celsius in a closed system at standard atmospheric pressure. I think i would be correct in saying it has an electrostatic potential energy of 2.26 x 10^6 Joules and a kinetic energy of 4.2x10^5.
Using Internal energy = Kinetic energy + potential energies
We find an internal energy of 2680000 Joules for the water at 100 degrees.
Now let's say we add all of that energy required to turn into steam and now we have: Internal Energy= Kinetic energy + potential energy of substance.
Since apparently the kinetic energy hasn't changed we have
U = 4.2x10^5 + 0 as gas apparently has an electrostatic potential energy of zero. But this can't be true so i am immensely confused. Surely gas can't have a lower internal energy than its liquid counterpart?
The potential component is conventionally negative and the latent heat of vaporisation is fairly significant (i.e. it's almost certain that the internal energy of the liquid should be considerably lower than that of gas, at least by your definition of it.) The result of this is that the sum of all the energy contributions will be more negative for a liquid (and even more for a solid) than a gas at the same temperature. The internal energy (again, same T) is highest for a gas, lowest for a solid.
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4 weeks ago
#4
(Original post by Sinnoh)
where are you getting those figures from?
They used the latent heat of vaporisation (something like 2,260 kJ/kilo or whatever) and the specific heat cap (something like 4,200 J/kgK)

Got me at first too... bit of a weird way of getting at it but their boat floats I guess
Last edited by Callicious; 4 weeks ago
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