Thread: THE FIRST LAW OF THERMODYNAMICS.......

The internal energy has two components the kinetic energe and the potential energy

while studying the 1st law of thermodynamics my teacher said that when there is no increase in the temperature the change in the internal energy = zero

I am thinking how is that?
when we heat water for example and plot a graph of temperature against time we will see that even heating there is no changing in the temperature at 0 and 100 because all the energy is going to the PE to change the state
So even there is no change in the temperature the internal energy is increasing because of increasing the PE......

Am I right?????????
Or there is something i did not understand????

As you heat water, the water molecules increase in vibration until at 100C their energy is so great the hydrogen bonds break and it changes to gas. And if you measure the temperature of water while you heat it, the water does increase in temperature, it just does not change to gas until the H bonds break.

No my dear
What i meant is although the water is heated at 100 , there is no increase in the temperature
The curve is horizontal at 100 and after changing to the gas phase the temperature will increase.....
so the energy that i provide at 100 C is not used in increasing the temperature or in other wards is not used in increasing the vibration or the kinetic energy.
It is used in icreasing the potential energy..........and so the internal energy.....

Oh, you mean at 100C.

That energy there is known as heat of vaporization. It's a barrier that is overcome at that point to break the hydrogen bonds. It's not getting hotter because it's overcoming this barrier. The hottest water can be at sea level, under 1atm, is 100C. Any hotter it changes to gas.

Does that make sense?

So is there any increase in the internal energy at this point , where hydrogyn bonds r broken with no more being hotter ,........

Well, that's a bit of a loaded question.

There is an increase in energy, but if you're measuring it's temperature it won't rise above 100C until it is steam.

I will give u an example

Just wait...............

Are you sure there is no change in internal energy?
According to the equation (internal energy= change in temp. + work done), when water is boiled, work will done on the water to break down the hydrogen bond of the water molecules.

you mean if you increase pressure or put the water in a magnetic containment field (theory) it won't vaporise due to huge pressure, and due to change in it's boiling point.

Originally Posted by hokhay

Are you sure there is no change in internal energy?
According to the equation (internal energy= change in temp. + work done), when water is boiled, work will done on the water to break down the hydrogen bond of the water molecules.

No I think there should be change in the internal energy but my teacher said when the temperature is not changed there is no increase in the internal energy
I have an example
I will present it soon......