Random science question (Bioshock related)

SylasGaunt · Post by **SylasGaunt** » 2007-09-10 01:21pm

I was messing around with Winter blast and my spiffy icy wrench of doom and got to wondering.. just how much energy would have to be pulled out of a human body to freeze it solid, does anyone know?

Junghalli · Post by **Junghalli** » 2007-09-10 01:40pm

You could probably get a good ballpark estimate by multiplying the specific heat of water by the weight of an average person, and then multiplying that by the temperature difference between the human body and the freezing point of water. But I'm really to lazy to do that right now.

Isolder74 · Post by **Isolder74** » 2007-09-10 01:50pm

Junghalli wrote:You could probably get a good ballpark estimate by multiplying the specific heat of water by the weight of an average person, and then multiplying that by the temperature difference between the human body and the freezing point of water. But I'm really to lazy to do that right now.

It would have to be the freezing point of salt water.

Eris · Post by **Eris** » 2007-09-10 03:17pm

Since freezing point is a colligative property, there isn't a single freezing point for water, just range of points depending on the molarity of the solute present in the sample, and good luck getting a number for the molarity of all the liquid in one's body. For the moment let's pretend it all acts like water for a ballpark figure.

Average body temp is is 37 degrees Celsius. The specific heat of liquid water is 4.2 kJ/(kg K) and the heat of fusion is 334 kJ/kg. Let's say the person is 70 kg. The energy should come out to about 34 MJ. Someone check my math, though. I'm currently about 20 hours without sleep and probably did something wrong there. All of this is from memory, which I wouldn't rely upon at the moment.

Eris · Post by **Eris** » 2007-09-10 03:18pm

Ghetto edit: That should begin "Since freezing point depression is a colligative property."

Steel · Post by **Steel** » 2007-09-10 03:25pm

Takes 4.2kJ to change the temperature of 1kg of water by 1C

So to take a person (80kg) from 37 C (normal temp) to -10 C (popsicle)

Would be a net gain of ~47*80*4.2kJ=15MJ to the environment.

15MJ is sufficient to raise the temperature of a 5kg (pretty damn hefty) iron wrench by ~6600 C (=15MJ/0.45(j/gC)*5kg)

So that wrench better not be taking in all that energy or you're going to have a slightly fried hand. (And a bit of a tan as thats hotter than the surface of the sun...)

Eris · Post by **Eris** » 2007-09-10 03:33pm

Steel wrote:Takes 4.2kJ to change the temperature of 1kg of water by 1C

So to take a person (80kg) from 37 C (normal temp) to -10 C (popsicle)

Would be a net gain of ~47*80*4.2kJ=15MJ to the environment.

You're forgetting enthalpy change during fusion, since this is about freezing a person solid rather than just reducing them to freezing point. Since water takes a good deal of energy to melt, you're looking a much greater value. In your example it'd come out to be another 26.7 MJ on top of what you have predicted, for about 42 MJ in all.

Steel · Post by **Steel** » 2007-09-10 03:34pm

Bah forgot heat of fusion, which is worth 26MJ in my case.... so net change of ~ 42MJ for my example

What are the odds that after an hour 2 people would start posting at the same time...

Connor MacLeod · Post by **Connor MacLeod** » 2007-09-10 09:41pm

Uh, something just occured to me.

If you "rapidly" cool someone down to the point he's frozen solid, isn't that going to have a nasty effect on the cells of the body? I'm pretty sure cells might burst if exposed to a rapid temperature change.

Ariphaos · Post by **Ariphaos** » 2007-09-10 11:15pm

Connor MacLeod wrote:Uh, something just occured to me.

If you "rapidly" cool someone down to the point he's frozen solid, isn't that going to have a nasty effect on the cells of the body? I'm pretty sure cells might burst if exposed to a rapid temperature change.

Wrong idea - crystal formation doesn't occur with rapid cooling, but that's not the problem. The problem, instead, is with freezing between cells.

Starglider · Post by **Starglider** » 2007-09-10 11:34pm

Connor MacLeod wrote:If you "rapidly" cool someone down to the point he's frozen solid, isn't that going to have a nasty effect on the cells of the body? I'm pretty sure cells might burst if exposed to a rapid temperature change.

Rapidly cooling someone down is much better than slowly cooling them down in this respect. Slow cooling causes large ice crystals to form, which pop cells and chew up membranes. Fast cooling causes much smaller and more numerous crystals to form, which do less damage. This is a real problem for medical cryogenics; since it's impossible to flash-freeze people in reality (you can drop them in liquid helium but it still takes a while for the body core to freeze due to low thermal conductivity), chemicals are used instead to limit ice crystal size.