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Answers to Readers' Questions on Physics:

Thermal Insulation

The following 141 comments and questions have been sent in. They are listed according to date.

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We have to build a thermos

Question

March 23, 2010

We have to build a thremos, and I was wondering how I could make the lid of the thermos. What materials must I use, and how do I make it?

S - Canada

19423

Answer

See the lesson on Thermos at:
http://www.school-for-champions.com/science/thermos.htm

Also, look at the Reader Feedback on the page to see what other students have done.

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Radiation and conduction from a black body material

Question

February 25, 2010

Case A: If a perfect vacuum exists between two, non-touching, "black bodies" that are at uniform but different temperatures, heat transfer between the bodies can only occur via radiation. Integrating Planck's law over all frequencies (zero to infinity) and including the Lambertian nature of black-body radiation from a planar surface (i.e., the radiation into a solid angle is proportional to the COSINE of the angle the solid angle direction makes with the normal to the black-body surface), provided two differential unit areas (one on each body) have line-of-sight visibility, one can generate a formula for the net power radiated between a differential unit of area on one black body to a differential unit of area on the other black body. By applying this formula over the entire surface areas of both bodies, one can in principle generate a formula for the heat flow between the two bodies. Suppose by hook or by crook we correctly generate such a formula.

Case B: Now fill the "vacuum of space" that existed between the two black bodies with a fixed material such as concrete. The fixed material will support heat transfer between the bodies via conduction but not convection. The formulas for conduction heat flow and radiation heat flow between two bodies are different. However, assume again by hook or by crook we can generate a formula for the conductive heat transfer between the bodies.

My question is this: To compute the "Case B" total heat transfer, do we algebraically sum (i) the "vacuum" radiative heat transfer formula of Case A...and... (ii) the conductive heat transfer formula of Case B? Put another way, when the vacuum is filled with a non-convective substance, are the radiative heat transfer properties affected by the substance? The follow-up question is: "If the radiative heat transfer properties are different for "vacuum" versus "fixed substance" conditions, how should the radiative transfer formula be modified?

Thank you for your time,

Reed - USA

19292

Answer

What this comes down to is the question of whether there is any radiation heat transfer between two materials in contact to add to the conduction heat transfer.

Conduction is caused by molecules in one material striking molecules in the other, transferring kinetic energy. However, the black body molecules are also emitting radiation, which is also providing thermal energy to the concrete material.

Neither the conduction heat transfer from a black body material to concrete nor the radiation heat transfer is very efficient. I'm not sure which is more efficient. I assume the total heat transfer would be the total of the two.

The black body emits radiation independent of the other material. However, the loss of energy due to conduction would lower the temperature of the balk body and reduce the amount and form of its radiation.

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Teaching 7 and 8 year olds about insulation

Question

January 17, 2010

Hi,
Your information on insulation was very helpful.

I am teaching 7 and 8 year olds about insulation and wanted to role play the effect of insulation and materials that are poor or good insulators, do you have any suggestions on how to show it?I was using the children to be a warm or cold product and then some to be a material and show effect.
Thanks for any ideas you might suggest.

jennie - UK

19069

Answer

The role of clothing in protecting you from heat or cold is one good exercise for the children. Putting on a pair pf gloves and then holding an ice cube or getting near to a heater can show the insulation effect. Then try to same thing with a thin cotton cloth wrapped around the hand.

Likewise, holding a paper cup with warm or cold water in it can be compared to holding a better insulating cup.

I hope this helps. Best wishes in your demonstration.

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Confused about thernal radiation

Question

September 25, 2009

Hi,
I found you page on thermal insulation very helpful, so thank you but I am confused about how you say that "thermal radiation (heat) cannot pass through a vacuum", but if we receive heat from sun, through space, which I have been told is a vacuum.
Thanks for your help,

Anna - UK

18382

Answer

The paragraph where I mention that heat cannot pass through a vacuum was on thermal conduction, which requires matter. This form of heat comes from moving molecules.

The paragraph on "Insulation from radiation" concerns thermal radiation, and that can pass through a vacuum. However, the reflective coatings in a thermos are used to reflect thermal radiation.

Note that heat can be transferred by convection, conduction and radiation.

Perhaps I should clarify the paragraph on heat, so there is no misunderstanding.

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I don't understand insulation

Question

May 20, 2009

i don't anderstand the ideas about insulation system

hamza - Ethiopia

17778

Answer

Thermal insulation is preventing the flow of heat through a material. Metal conducts heat from one end to another quickly. But good insulators don't/

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What would insulate a 12 oz aluminum can?

Question

March 13, 2009

okay so i have a project coming up and i was wondering what would insulate a 12 oz aluminum can? would duck type wrapped in pencils work or paper or foil??
what would work the best
we want the highest heat posible so ideas please
thank you

courtney - USA

17439

Answer

You could wrap the can with something that does not conduct heat very well. There are Styrofoam cup holders that are very good. You could wrap it in layers of newspaper, as long as the layers are thick enough. Even wrapping a wool scarf around the can could be good to insulate it.

Look at our Reader Feedback to see what other students are doing. Also see:
http://www.school-for-champions.com/science/thermos.htm
http://www.school-for-champions.com/science/heat_transfer_coffee.htm

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How can a stainless steel coffee mug hold heat?

Question

February 25, 2009

Hi I was wondering, since metals that conduct electricity have poor insulation why do they use stainless steel in thermos coffee mugs? Doesn't stainless steel conduct electricity?

- USA

17333

Answer

Stainless steel is a strange metal. Although it is an alloy of steel and nickel, it is not usually magnetic. It does conduct electricity, but also, the rate that it conducts heat is only 20% of that of steel. To add to its ability to hold heat, a stainless steel coffee mug has a shiny surface which does not radiate away heat as would a dark dull surface.

Although a stainless steel coffee mug is not as efficient as a ceramic mug in holding heat (except for the fact that you can put a cap on it), the stainless steel mug is more convenient to use.

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Testing for best insulator

Question

October 30, 2008

Thank you very much, information was extremely helpful and I am very appreciative. By the way, i'm doing a sci. fair project, and I'm testing what insulator would be the best insulation to put in buildings? Cellulose, Fiberglass, or rockwool?

- USA

16712

Answer

The effectiveness of insulation is determined by its R-value, which is related to the type of material and its thickness.

As a Science Fair project, you want to compare the same thickness of materials to determine which one works the best in a building. The chart on the Thermal Insulation page shows that 1 inch thick fiberglass board as an R-value of 4.35, while 1 inch of cellulose is 3.7. That means the fiberglass is a better insulator. I don't have numbers for rockwool, but I think they are a little lower.

Best wishes in your project.

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Temperature homework problem

Question

February 20, 2008

If a space is at a temperature of 55 deg F with Humidity of 90%, the adjacent space separated by R-5 wall of 2" thick is 35 deg F with a humidity of 75%, there is no air movement, what would the temperature be at 12" from the wall on the 55 deg side?

Sam - USA

15405

Answer

That sounds like a good problem. Unfortunately, we do not solve problems.

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Needs to keep ice cube from melting

Question

February 6, 2008

Hey,

I have a project due in about a week ( February 12th, 2008 ), and it's about making a container of some sort to keep an ice cube from melting. I have to try to make the ice cube last (not melt) for 6 hours, at least. What materials should I use? (Tinfoil, styrofoam, bubble wrap..ect) What container should I use? (Glass container, popcan, ect.) How would I put it together? What would be good insulators? Please answer as soon as possible!!!

Thanks, and sorry for asking to many questions.

Bob - Canada

15291

Answer

Take a look at the Reader Feedback in both Thermal Insulation and Thermos Container lessons to see what other students have done. There are plenty of examples. I think they will answer your questions.

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