engineers... got a question for you...

To all the engineers out there.. Hopefully someone will be able to help me out. I'm currently finishing up my bachelors in Mechanical Engineering and we were recently challenged with the question.. "Your car has been parked all afternoon in the sun and the interior temperature is a steady 110 degrees F. The drivers seat is covered with a cotton towel and the passengers seat is not.. it is bare vinyl. When you feel the cotton towel and the bare vinyl, one feels hotter than the other.." Can anyone explain why this might be? My guess is.. it has something to do with the fact that the cotton has a relatively rough and airy surface which would dramatically increase the contact resistance between the cotton towel and your hand resulting in a slower heat transfer thus making the cotton towel feel cooler. Does this seem like a good answer? Your thoughts?
Thanks,
Jason

Good answer. Fluffy cotton surface is a poor conductor of heat and has small heat capacity as well, so surface in contact with your hand quickly drops to temperature of your hand and hotter whatever behind can't get enough heat through the insulating cotton/air contact surface to raise your skin temperature.

Welcome to the fraternity/sorority..

Several processes make up how hot something feels to your skin... One is the ability for thermal transfer--Cotton fiber is very poor at thermal transfer--as well as much of a towel is "air" trapped between the fibers--again, trapped air (with no chance of thermal convection currents) is also a very good insulator.

Another is "thermal mass". A cotton towel has very little mass and thermal mass. Vinyl is heavier (at least at the surface) and has more thermal mass from which to source heat flow.

Neither material has "great" thermal transfer properties--turns out that free electrons (i.e., the ability to conduct electricity) is, usually, a very good indicator of a material's ability to conduct heat.

By the way, are you going to stay in school for a MS or higher degree? And, do you a have good dose of software engineering eduction under your belt? Both (from a few years ago) can increase your ability to get a good paying job. The ability to customize your CAD/Design tools and their outputs is a great differentiator in today's job markets (you are competing against people in China, India, and other places now).

-Bill

Another factor here is that actual surface temperatures of the two materials may be different due to differences in their ability to give up and/or absorb heat. If both surfaces are exposed to sunlight, then one will probably absorb better than the other (depends on color). The towel has more surface area for losing heat to the air by convection, and also there could be differences in the rate at which the surfaces can lose heat via convection due to orientation or boundary layer effects.

Overall though, I think the explanations given cover the major effects - these I have listed would be minor unless one surface was black and the other was white and they were both in direct sunlight.

I'm no engineer but I slept at that hotel last night..same deal with tinfoil in a hot oven; you can grab the (single sheet of) foil soon as you open the oven, but not the stuff in it.

I've no idea why you feel only qualified "experts", such as engineers should be involved in the reasoning.

The reason you'd rather carry a hot cup of coffee in a paper cup or a styrofoam cup is the same. It's thermal conductivity. Different materials absorb or give up heat at a different rate. The eventuality is that dissimilar materials will reach the same temperature as it's surroundings, and the same mass will absorb or transfer the same amount of heat, but it will be at different rates based on thermal conductivity. The physical texture can become involved if it permits an intermix of insulating materials, such as air, into the system. In that case, you're dealing with multiple factors and complexities.

Basic, and a surprising degree of sophistication, can be found in a Wikipedia entry on thermal conductivity.

A material with low thermal conductivity and low thermal capacity is going to feel much closer to body temperature than one with either high thermal conductivity, high thermal capacity, or both. The material used for the space shuttle tiles (low thermal conductivity, low thermal capacity) is an extreme example. It can be held in the hand when red hot. At the other end of the spectrum metals (high thermal conductivity but fairly low thermal capacity) feel colder or hotter than materials like wood. Even when they are only a few tens of degrees above or below room temperature they can be uncomfortable to touch. Water (very high thermal capacity but low thermal conductivity) also exhibits these properties. Some foods like popcorn can be eaten very hot without any problems because they don't hold much heat and are poor thermal conductors. On the other hand, drinking a very hot beverage can cause burns.

The color of the towel also matters. From what I have seen, most vinyl is black and most cotton is not. Lets presume the cotton towel is white and the vinyl seat is black. Since black absorbs more electromagnetic radiation then white, black seats will heat up faster when exposed to light then white cotton. So if all things being equal, the black seats will feel hotter due to more energy absorbed then the white cotton towel.

But if they are the same color, then due to the fact vinyl transfers heat better then cotton, the vinyl seat will feel hotter since more heat is being transfered to your hand.

My answer is not as specific as the everyone else's, but I hope I'm right.

Just to clarify the comment about space shuttle tiles - you can hold one in your hand while the center is red hot, but not the surface. If the surface you touch is red hot you will get burned! I have actually done this and it is a fun demonstration (not the burning of myself - the former). You can also hold a blow torch to one side while your hand is on the other.

Wow.. Thanks everyone for the the input! It's nice to be a member of a site with so many people who share the same interests/education as necessary to provide such an answer. Not something i've come across too often.

I've no idea why you feel only qualified "experts", such as engineers should be involved in the reasoning.

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Sorry about that.. like I said, sometimes it's tough finding a random person to talk to/help me out with this stuff. I wasn't expecting to find this many people who share the same interests. All i knew is that there were at least a few engineers here on CPF. Thanks for the input.

By the way, are you going to stay in school for a MS or higher degree? And, do you a have good dose of software engineering eduction under your belt? Both (from a few years ago) can increase your ability to get a good paying job. The ability to customize your CAD/Design tools and their outputs is a great differentiator in today's job markets (you are competing against people in China, India, and other places now).

Click to expand...

Planning on working after college for now. A buddy and I are seriously considering going back for Masters soon. I am very experienced with SolidWorks, AutoCAD, and Microsoft Office (mostly Excel) and I have a basic knowledge of ProEngineer, Maple, and MATLAB. Hopefully this will help. I am hearing more and more that the MSME's might make more but they usually end up with office jobs and management positions which doesnt sound too appealing to me. Thoughts?

While an advanced degree can open doors into management, you don't have to take those positions. I got a masters because I wanted more options, and now I have a job I really enjoy that pays more than one I might have gotten with only a bachelors. This job would not have been available to me without my masters because it was in my masters work that I got the specific experience my employer was after. I don't want to go into management at this time either, so I am going to stay in R&D until it is no longer interesting. I have colleagues choosing the management route, but typically only after they have ascended the technical ladder until bumping their heads on the old glass ceiling.

It has been a few years... But at least in the SF Bay Area, it was getting harder and harder to get a mechanical engineering job with just a BS...

I understand wanting to get out and work (I did between HS and College). And, from what I have seen, it is very difficult for someone that has been working to get back into college again (bills, married, etc.).

In general, I am a big believer in engineers getting some practical experience--too many engineers I have seen have very little common sense when it comes to try and solve problems and foresee issues.

Yes, getting experience in one or more CAD / Solid modeling software packages is pretty much a requirement these days--I was also suggesting some programming knowledge too. Learning C, scripting, and/or other languages associated with a Solid Works / AutoCAD / ProEngineer type package will really be a help when it comes to generating documentation packages and undertanding how to interface with Machining Centers, Simulation Packages, and even the (seemingly simple requirements) of documenting to archive and creating packages for vendors.

In the old days (30+ years ago), an engineer who did not know these things would just have a drafting department of junior engineers / draftsperson / specialists for support. Today--the best engineers (I have seen) are able to "do it all" and understand the requirements of their "customers". I have had, otherwise very smart designers ignore the requirements of the outsourced vendors (and even tell me that the vendors are wrong)--and I have had outsourced ME/CAD vendors lose very lucrative contracts when they turned simple designs over to a senior drafts person who knew how to drive a mouse but knew nothing about modeling, scaling, and not to use sample fastener drawings that came with the CAD package (drew a complete inch based design using mm units--19" rack mount computer was 19mm wide--sample fasteners did not match dimensions of real fasteners--not enough metal to even swag them in--sheetmetal shop went nuts when they got the model files to unfold the sheetmetal for fabrication. Drawings looked fined, but the models were just a bit larger than a postage stamp--required 100% redrawing / redesign by our internal designer to fix the problems).

Very few smaller companies can absorb the overhead of a support department--and if downsizing occurs, those that require support personnel are not going to last long.

Lastly, when you get the good job--save your money so that you have 6-12 months of savings to live on (keep your expenses low). Allows you to enjoy the job more and have more choices in life (try new job, leave job where they treat you like poop).

My two cents worth. Good Luck and have Fun!

-Bill

ask the guy who gave you the question "why not?"

if he reply with a correct answer, that is your answer.

I think it's more of an interesting question "why does a puddle freeze when it's 34°F outside?"

EngrPaul said:

I think it's more of an interesting question "why does a puddle freeze when it's 34°F outside?"

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...'cause the wind's blowing?...

The key to answering this question involves math. This is why it's difficult to answer.

Keep in mind that heat transfer from the water to the environment can involve several heat transfer methods:

Convection (but the air is warmer than freezing)
Conduction
Radiation
Phase Change (Evaporative cooling)

Another hint is to take an infrared thermometer out on a clear night and aim it at the sky. What does it read?

EngrPaul said:

...snip...
Another hint is to take an infrared thermometer out on a clear night and aim it at the sky. What does it read?

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Average background radiation.... 3K

Perhaps one seat is in the shadoiw of the roof and the other isn't...

Sorry. Is this the life-drawing class?

w

Freezing doesn't exactly happen at any specific temperature. Statistically it's much more likely to happen around 32 degrees farenheit, but even that isn't always true.

When we're talking about puddles, the water generally isn't pure H2O, and the puddle isn't necessarily at sea level (atmospheric pressure). The puddle may be 34 degrees farenheit, but that is just an average temperature. Some of the water will be colder, and some will be warmer - it just averages out to 34 degrees.

Some of the water may have a higher concentration of impurities, and some may have a lower concentration. When the conditions are right, the water can nucleate and begin solidification. It could take a while, but it is possible.

And depending on the atmospheric pressure, the freezing temperature of water won't be 32 degrees f. If you have the right combination of pressure and temperature, you can have water existing in 3 different phases - ice, water, steam. There are a lot of reasons why this puddle could have frozen...


But at the level of the car seat discussion...that kind of question doesn't leave a lot of room for guess work. You don't need to look into it that far...the interior is a constant temperature and you can assume everything inside the car is the same temp. Heat transfer is what will usually make something feel hotter, especially since our senses can't tell 'temperature' - they only get a feeling of heat transfer...

The puddle of water is normal water at STP. Let's assume it's pure enough and has the right conditions to freeze at 32F.

Hint: The water isn't 34F, the ambient air around it is.