Answers to Shower Questions - aF

[americanforest]

1. What is the mechanism for transfer of heat from the water to my body, ie. When the hot water hits me, does it lose energy during contact, thus warming my skin and cooling down the water. This would mean that the water in the bottom of the tub is cooler than that flying from the shower-head. If this is right how does it work?

2. Why is it harder to breathe in a really hot shower? My theory is that the hot water vapor pushes the oxygen up, so that it is especially hard to breathe when taking a bath, not a shower.

3. Why do I feel cold immediately after getting out of the shower?

Thanks,
aF

 

[Panda]

Thats a lot of questions. I'll try and give some insight into them...

1. The heat energy from the water will warm your skin but only fractionally. The warm water stimulates the nerves under your skin that detect the heat and pass that information to your brain, but they need very little energy transfer to trigger. Most of the heat energy will be lost into the air, but still only a small amount, if you collect water at the shower head, and then collect water at the shower floor, the difference in temperature will be quite small. Back to you feeling warm, the body detecting the heat moves blood to the outer surface of the body to make sure the body stays at a constant temperature, this is why you look red after a hot shower, or why a burn is red and feels hot, it's the blood not the applied heat.

2. Breathing air in a hot wet environment feels harder mainly due to the water vapour content. The air is denser and your lungs need to expel as much of the water vapour as it can as well. You get the same effect in a sauna or jungle.

3. You feel cold due to water evaporation. Evaporation is how your body naturally stays cool, i.e. sweating, your body releases moisture this moisture is warmed by your body and then taken away by the air, so your body has lost the heat required to warm the water. Any air movement across your skin will evaporate moisture, even the air movement caused by you walking. In the shower the air is saturated with water vapour so evaporation is small, if you move out of the shower the air is not saturated so evaporation happens faster and you feel colder. Go outside in a gale and you will get really cold (and arrested if you forgot to get dressed) even on a sunny day. Remember from answer 1 that the blood rushed to the surface of your skin, well now some of the heat lost through evaporation is taken from the blood so this cooler blood is passed back to the body core, your body detects this change in temperature and triggers an over cold reaction, your body needs to move so that heat generated in the muscles can warm the body, you will get involuntary muscle spasms or shiver.

 

[americanforest]

Thats a lot of questions. I'll try and give some insight into them...

1. The heat energy from the water will warm your skin but only fractionally. The warm water stimulates the nerves under your skin that detect the heat and pass that information to your brain, but they need very little energy transfer to trigger. Most of the heat energy will be lost into the air, but still only a small amount, if you collect water at the shower head, and then collect water at the shower floor, the difference in temperature will be quite small. Back to you feeling warm, the body detecting the heat moves blood to the outer surface of the body to make sure the body stays at a constant temperature, this is why you look red after a hot shower, or why a burn is red and feels hot, it's the blood not the applied heat.

2. Breathing air in a hot wet environment feels harder mainly due to the water vapour content. The air is denser and your lungs need to expel as much of the water vapour as it can as well. You get the same effect in a sauna or jungle.

3. You feel cold due to water evaporation. Evaporation is how your body naturally stays cool, i.e. sweating, your body releases moisture this moisture is warmed by your body and then taken away by the air, so your body has lost the heat required to warm the water. Any air movement across your skin will evaporate moisture, even the air movement caused by you walking. In the shower the air is saturated with water vapour so evaporation is small, if you move out of the shower the air is not saturated so evaporation happens faster and you feel colder. Go outside in a gale and you will get really cold (and arrested if you forgot to get dressed) even on a sunny day. Remember from answer 1 that the blood rushed to the surface of your skin, well now some of the heat lost through evaporation is taken from the blood so this cooler blood is passed back to the body core, your body detects this change in temperature and triggers an over cold reaction, your body needs to move so that heat generated in the muscles can warm the body, you will get involuntary muscle spasms or shiver.

First off, thanks for the response.

1. You said most of the heat is lost to the air. Why is this? Is this because it evaporates into water vapour as it's falling, breaking some molecular bonds and losing energy in the process? Also, this isn't Physics anymore, but I can't help asking: Isn't blood blue inside of veins. Why would you turn red when blood came to your skin, wouldn't you turn blue?

2. I thought that this might have something to do with the pressure of hot water vapour as opposed to oxygen. Would the water vapour be more dense, thus pushing oxygen up?

3. I wouldn't say the evaporation was small in the shower. It's just that as much is evaporating off of you as is condensing onto you. So there is little net evaporation.

 

[brewnog]

1. You said most of the heat is lost to the air. Why is this? Is this because it evaporates into water vapour as it's falling, breaking some molecular bonds and losing energy in the process?

There will be some evaporation, which is a mechanism of heat transfer, but by that logic alone the 'cold water' would no longer be in liquid form. The water is hotter than the air around it, the droplets are quite small (large surface area), and are moving quickly (high air flow over the surface) so it's not hard for the water droplets to lose heat to the air.

Also, this isn't Physics anymore, but I can't help asking: Isn't blood blue inside of veins. Why would you turn red when blood came to your skin, wouldn't you turn blue?

Blue blood?! Ever cut yourself?! Nope, it's red. Darker red than oxygenated blood (ie arteries) but it's definitely red. Scattering and absorption of light in the skin and flesh around the vein just makes it look more blue than the surrounding tissue.

2. I thought that this might have something to do with the pressure of hot water vapour as opposed to oxygen. Would the water vapour be more dense, thus pushing oxygen up?

No, Panda's answer was perfect. This has nothing to do with the oxygen content. Oxygen is very well mixed in air at these temperatures.