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[Phys-L] Re: water and internal energy



On Tuesday, Feb 22, 2005, at 10:25 America/New_York, Anthony Lapinski
wrote:

I tell my students that steam burns at 100°C are more harmful than
water
burns at 100°C because the steam has more internal energy (latent heat
--
mL). Similarly, water at 0°C has more internal energy that ice at 0°C.
Phase changes (+Q) increase the internal PE of the atoms, but not the
KE
(since the temperature does not change).

So here's my dilemma. How do you discuss this conceptually to students,
especially when talking about water? Ice is clearly LESS dense than
water
because of its expansion (increased volume) upon freezing. But if
water at
0°C has more internal energy, the WATER molecules be farther apart and
thus become less dense. But they're not! What causes this? I'm looking
for
an "easy" explanation for my students to understand that the expansion
has
LESS internal energy.

How harmful burning is depends on how much of the released energy
"goes into" the tissue during the (usually short) time interval. This
does
not necessarily correlates with how much thermal energy is released.
Skin might react differently to the vapor than to the liquid.

Ludwik Kowalski
Let the perfect not be the enemy of the good.