Basic Physics Question

[Trixxie]

Ok maybe I am just a little dense. I never took Physics in H/S just Physiology. (went to college to be an auto mechanic but that is another story)

Just started my D/M classes and am doing the diving encyclopedia workbook. One of the first questions has to do with Convection and Conduction. asking which one affects a diver more in terms of heat loss.

Well if conduction is the movement of heat through direct contact and convection is the movement of heat through fluids (gas or actual fluid) then if a diver is in the water and is losing heat why is this conduction and not convection? OK I realize the water touching the skin is conduction but why is it not convection since fluids are involved.........

 

[pir8]

Your body is whats losing the heat. even though it is composed of a high percentage a water, it is still not considered a fluid.

 

[miketsp]

Convection would be the case if the transfer of heat from your body to the surrounding medium then caused an instability in that medium which caused the heated part of the medium to move away. Example a radiator heating air which expands and then rises away from the radiator to be replaced by colder air.
In the case of the diver the prime mechanism is conduction. The heat transfer to the surrounding water does not cause it to move away from the contact area.
A good example is the water in your wet suit. Assume it stays in place, it still conducts heat away from your skin 25 times faster than an air layer at the same temperature would.

 

[radinator]

Think of a nail that you hold one end in a candle flame. Because of conduction, the heat is transferred to the other end, even though the medium (the metal) is not moving to the other end. That is one sort of heat transport.

Now think of a tub of cold water, and you run the faucet on hot. At first, the only warm part of the water is the part near the faucet. The other end of the tub is still cold. Over time, though, the water will move around, and the heat will be transported to the other end. This heat transport happened because the medium (the water) actually moved to take the heat to the other end.

Now think about the diver in the water. The water is in contact with her skin, and the heat is being transferred from skin to water through conduction. The skin cells aren't transporting the heat by moving into the water, it is just in contact with it.

Once the water near the skin has warmed up, it is then swept away by a current or by movement of the diver. That moving water takes the heat with it, and the water is replaced by cold water near the skin. That part of the process is convection, because the water is moving and taking the heat with it.

Note that there is also another conduction part of the proces here where the water near the skin, without moving, can conduct heat to the next layer of water farther from the skin. So even a diver stationary in perfectly still water with no convection can lose heat by heating up the surrounding water.

A wetsuit helps because it retards the convection part of the process. The water is instead held near the body and cannot easily flow away.

The 'second part of the conduction" I mentioned is the reason that as the water gets colder the wetsuits have to get thicker. If convection were the only way the water carried heat away from your body, a thin plastic sheet would work as a wetsuit. However, conduction can still take the heat away from one layer of water to the next (or through the plastic sheet), so the thicker wetsuit helps prevent this conduction.

But no matter how the heat is transported away from the vicinity of he diver's skin, the heat is initially transferred from skin to water through conduction, because the skin cells are not moving to carry the heat.

 

[Walter]

(gas or actual fluid)

Gas is an actual fluid. A fluid is something which flows - a gas or a liquid.

 

[AggieDad]

MauiGal
Thanks for your question. I too never took Physics in High School, nor Algebra or Geometery. When I see the formulas for the physic's of the Dive Master all I see is a jungle, I don't even see a forest for the trees. :shakehead
I went to school to be an aircraft mechanic and ended up flying them instead. Algebra, Physic's in aviation? I add, subtract, multiply and divide and look at charts. This half life stuff I understand but controlling compartments? I just wish they would put a simple formula out with some examples so I can work them out. Verbiage with a math problem is WRONG for those like me. The workbook is great, I think. I also tried to learn Technical Analysis for stock picking and when I thought it said sell it was saying buy and vice versa.
Hang in there, even us mathmatically challenged will figure it out.

 

[paolov]

asking which one affects a diver more in terms of heat loss. ....if a diver is in the water and is losing heat why is this conduction and not convection? .... but why is it not convection since fluids are involved.........

it is assumed that the diver has proper exposure suit protection , a WETSUIT.

with the premise that a diver is wearing a wetsuit , therefore convection is no longer the determining factor as the wetsuit protects the diver from convection as the water trapped between the divers skin and wetsuit would be prevented from moving


now the only thing that has to conduct heat loss is thru conduction. from body to the thin layer of water then tru the wetsuit...

.

 

[pescador775]

As far as the wetsuit, rubber is a dielectric as is the air trapped in bubbles. That is, the material inhibits the movement of electrons (electricity). Dielectric materials are typically poor conductors of heat as well. This is the principal mode of protecting the diver from heat loss, eg heat can not be easily transmitted through foam rubber. The implication is that conduction, not convection is the mechanism for heat loss. Convection requires an intermediate. For example, when human skin is exposed to cold air, the body heat is carried away by the warm air which is subject to buoyancy generated currents. When the air contacts something else, like a car window, some heat is transferred to the glass. That is convection. Since there are no currents within the gap between the diver's skin and suit it would appear that convection is a minor issue. When bare skin is exposed to water, heat is transferred to the water through conduction. Currents of water convect the heat away. However, the real problem is the high heat conduction, and high heat capacity, of water.

 

[Trixxie]

But no matter how the heat is transported away from the vicinity of he diver's skin, the heat is initially transferred from skin to water through conduction, because the skin cells are not moving to carry the heat.

I knew that gas and fluids were both fluids *but* does convection only take place through movement?

 

[radinator]

I knew that gas and fluids were both fluids *but* does convection only take place through movement?

Yes, convection requires movement of the heat transport medium relative to the heat source.

Note It doesn't require the diver to move. The fluid on its own will move because (usually) warmer fluids are less dense than colder fluids, so the warm water (or air) will flow up away from the diver in the (otherwise) absence of movement.

OR the diver can move, and the fluid not move. But there must be relative motion.