Thermodynamics Question

goucho · May 11, 2006, 12:52 AM

Ok, i know this may be a little off topic, but i figured that this section would be the smartest place to ask this question. Right now i'm taking a thermodynamics class in college (my last week before i graduate with a physics and mechanical engineering Bachlors Degree) and i'm learning a whole lot about engine cycles (otto, diesal, brayton etc....). Anyways, the teacher presented us with the following question.

"When i shower, i take off my glasses and leave them on the sink. When i get out, their not fogged up. Although sometimes when i put them back on they fog up. Why would this happen?"

I know it has something to do with psychrometrics, but i'm not really sure why. I figured someone here might have an idea.... Sorry if this is to over the top for the people here.

l2r99gst · May 11, 2006, 08:17 AM

Why do you think it has something to do with psychometrics? Do you think he is trying to give you a trick question....like one on an IQ test?

If so, there can be a lot of different answers, like where is the sink, etc?

But, just think about it. What is the fog on the glasses? It's condensation, right? How does that happen and why? That happens because as air temperature rises, the water vapor saturation limit increases, so the air can hold more water. But, if the air touches something cooler, then the air at that interface with the object cannot hold more water anymore and the air has to reject that water vapor, which condenses it on the cooler object.

I can think of a few different answer to your question, but I'm not in your class, I have no clue exactly what your teacher is looking for, etc. But, you either need the glasses getting cooler or the air getting warmer, both already in an environment where the air satuaration limit is already reached.

Maybe you took a cold shower and your warm breath or body heat is causing a localized are of higher saturation that condenses on the cold glasses. There are numerous answers to the question.

Eric

goucho · May 11, 2006, 12:36 PM

See that's kinda along the lines that I was thinking, with the water vapor. Since the temperature of the water is most likely above 100 degrees and it's at atmospheric pressure, it's going to be saturated. Thus, condensation. Which is why the mirror would fog up. But then why would his glasses not fog up, until he put them on his face. I was thinking maybe because he has them on his sink, where it will be less likely to feel the temperature difference. That is until he puts them on his face where the sudden contact with his raised body heat causes it to fog them up. I mean, there are probably a hundred explanations for this, but I was just kinda curious what some other people might be thinking. It's nice to have other knowledgable people to talk to! Thanks for replying!

Also, i was thinking about psychrometrics because it has to do with dew points, vapor pressures and specific humiditys.....

zmeli · May 11, 2006, 12:47 PM

CONDENSATION is a warming process

EVAPORATION is a cooling process

The relative Humidity in the bathroom after the shower very high so the dew point is low. Maybe the evaporation of water from your body is collecting on the glasses?

SuperHatch · May 11, 2006, 01:34 PM

Both faces of the glass in the eyeglasses are exposed to the steamed air.

Only one face of the mirror is exposed. The temperature differential on the mirror (one side is attached to a cold wall/medicine cabinet) accelerates the condensation.

As far as why it condenses when you put them on? I'm not sure to be honest with you. My best guess would be that the humidity level in the bathroom is high due to the shower, yet that air temperature is still relatively low (~80F). The body temp would beupwards of ~100F, the temperature differential across the glasses with the high humidity would cause the condensation....

byron3636 · May 11, 2006, 02:33 PM

The glasses are warmer(less than body temp, but more than room temp) than the mirror(room temp) when you put them down therefore the water vapor will condense on the mirror faster....

as you put them back on your head, your body temp like superhatch said is much much higher than that of the steamy air. The temperature gradient from your the water on your body to that in the air is much bigger causing them to fog right away.

I don't think breath has anything to do with it at all.

Another thing to consider is the properties of the water. water vapor tends to condense faster on things that are "dirty/dusty". the water molecules cling to these other substanes more than pure clean glass... the glass are most likely much cleaner than the mirror.

thechoc · May 11, 2006, 02:53 PM

I think he's sofa king with you. One of my T.A.'s asked that same question but gave us a little more to work with when he presented it (aka volume of the room, air temp, water temp, etc). The answer ended up being the sink is lower than his head.

I wanted to stab him after that.

trinydex · May 11, 2006, 03:24 PM

Quote:

Originally Posted by l2r99gst

Why do you think it has something to do with psychometrics? Do you think he is trying to give you a trick question....like one on an IQ test?

If so, there can be a lot of different answers, like where is the sink, etc?

But, just think about it. What is the fog on the glasses? It's condensation, right? How does that happen and why? That happens because as air temperature rises, the water vapor saturation limit increases, so the air can hold more water. But, if the air touches something cooler, then the air at that interface with the object cannot hold more water anymore and the air has to reject that water vapor, which condenses it on the cooler object.

I can think of a few different answer to your question, but I'm not in your class, I have no clue exactly what your teacher is looking for, etc. But, you either need the glasses getting cooler or the air getting warmer, both already in an environment where the air satuaration limit is already reached.

Maybe you took a cold shower and your warm breath or body heat is causing a localized are of higher saturation that condenses on the cold glasses. There are numerous answers to the question.

Eric

if you took a warm shower and your body is warmer when you put on the relatively colder classes they will fog. but i'm guessing you said cold shower cuz then the glasses wouldn't get warmed up while they were waiting for you...

trinydex · May 11, 2006, 03:27 PM

Quote:

Originally Posted by thechoc

I think he's sofa king with you. One of my T.A.'s asked that same question but gave us a little more to work with when he presented it (aka volume of the room, air temp, water temp, etc). The answer ended up being the sink is lower than his head.

I wanted to stab him after that.

i would stab him for that

andenbre · May 13, 2006, 10:40 PM

I wrote the following before reading all the posts

how about this, the air at the sink level is cooler than the warmer water saturated air 3 feet higher. while you were in the shower the glasses stayed in this cooler air when you get out you dont even have to put them on your face to fog them up just hold them at arms length and eye level and water will condense on them. next time you take a hot shower after you get out of the tub stand there for a few seconds and take note of the temp at face level then sit on the floor, this will put your face at approx counter level and feel how much cooler the air feels on your face.... and the foor on your azz

Andrew LB · May 17, 2006, 06:52 PM

Quote:

Originally Posted by thechoc

I think he's sofa king with you. One of my T.A.'s asked that same question but gave us a little more to work with when he presented it (aka volume of the room, air temp, water temp, etc). The answer ended up being the sink is lower than his head.

I wanted to stab him after that.

I was going to give this exact answer to the question. I have a mirror in my bathroom that is the full length of the wall. When i take a shower, the steam can only go up and over the top of the glass shower doors and stays high for obvious reasons. When i get out, i look at the mirror which has condensation on the top 2/3 of it. The bottom is clear.

Adamk24 · May 18, 2006, 01:52 AM

Steam rises. When the glasses are down low in the sink they are in the cold air. When you put them on your head, your in the steam. seems pretty simple to me....

thechoc · May 19, 2006, 03:35 PM

Quote:

Originally Posted by Adamk24

Steam rises. When the glasses are down low in the sink they are in the cold air. When you put them on your head, your in the steam. seems pretty simple to me....

Nothing is that simple when you're an engineer, everything has to be broken down to the point of stupidity. I think it's an OCD thing that we pick up while in college.

FastEvoFL · May 21, 2006, 10:28 PM

Quote:

Originally Posted by thechoc

Nothing is that simple when you're an engineer, everything has to be broken down to the point of stupidity. I think it's an OCD thing that we pick up while in college.

Tell me about it. I always ask too many questions and it really screws up everything!

I wonder if this would work when using a psychometric chart assuming clsoe to 100% humidty? Couldnt you find dewpoints... wet bulb and dry build temps and figure out some actual answer to this all?

C6C6CH3vo · Jun 20, 2006, 10:56 PM

Quote:

Originally Posted by goucho

"When i shower, i take off my glasses and leave them on the sink. When i get out, their not fogged up. Although sometimes when i put them back on they fog up. Why would this happen?"[.

]

Sounds like your Prof is weeding out the kids that still don't get it even at the end of thier senior year. Even with a 4.0 GPA some graduates still can't apply squat for working knowledge - even in the simplest form. JK

Ever wish the mirror in the shower would just stay unfogged for 1 minute so you can shave, brush, or whatever? You turn the water cold and still it fogs from your breath - what do you do? (Ans) Heat the mirror up with hot water first and it will be good for most of the shower depending upon it's size of thermal conductivity.