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Name: Nati
Status: student
Grade: 9-12
Location: IL
Country: USA
Date: Summer 2009



Question:
My chemistry teacher taught us that ceramics are heat insulators (that is why we can touch glass tubing when we fire polish it) and that metals are heat conductors, so we need to hold metal samples with tongs. I know it works, but I do not understand how the metal tongs prevent the heat from harming your hand. How does the metal act as an insulator?


Replies:
Hi Nati,

The reason little heat is conducted from the hot metal object to the tongs, is mostly because the tongs do not make good thermal contact to the hot metal. The tongs touch the hot metal at only a few small points. Any heat being conducted to the tongs, must pass though only these few very small contact points, so the result is rather like a river that has a very narrow point; very little water can get past the resistance of the narrow area. Even a small air gap between the rest of the tongs and the hot metal object, will dramatically increase the resistance to heat flow.

Regards,
Bob Wilson


Hi,

The reasons are that metal tongs used are long, thin, not highly conductive, and loosely grip the hot object. It is a combination of these that make it possible to hold a hot object with them.

The length puts a relatively large distance between the hot object and your hand holding it.

The thin cross section of the tong restricts the flow of heat.

The contact between the hot object and the tong's jaws holding it is not a good one: the contact AREA between the tong and the hot object is rather small, and this restricts the amount of heat per unit time that can go flow the hot object to the tong.

Lastly, metal tongs are not made of very conductive metals (such as copper). They typically conduct 10 times less than copper or worse.

So, the combination of these makes it possible to hold a hot object with a metal tong.

As you can imagine, if you have a very HOT object, a SHORT and THICK tong made of a GOOD conductor that makes GOOD contact at its jaws with the hot object, your hand may burn your hand.

Ali Khousnary, PhD
Argonne National Laboratory


Hi Nati

I think you misunderstand your chemistry teacher in regard to being protected from heat by holding hot objects with metal tongs.

Metal is a heat conductor, and if you hold a hot item with metal tongs, the heat will eventually pass up the metal tongs to your hand. So this is a safety warning. It may take some time for the heat to work its way up, but it eventually will work its way up.

In practice and by the Second Law of Thermodynamics, Heat, travels from hot things to cold things and as a measure of disorder (entropy), the disorder will always increase for an isolated system. That is, the heat will spread throughout the tongs and the hot thing you are holding with the tongs, until the tongs and the thing you are holding arrives at an even temperature throughout.

SO BE CAREFUL. AS YOU HOLD A HOT OBJECT WITH METAL TONGS, THE TONGS WILL EVENTUALLY GET HOT AS THE HEAT SPREADS FROM THE HOT OBJECT TO THE SYSTEM OF HOT OBJECT AND TONGS.

Now, about ceramics as an insulator. At this URL

http://en.wikipedia.org/wiki/Ceramic

we find this definition of a ceramic:

A ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling.[1] Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous (e.g., a glass). Because most common ceramics are crystalline, the definition of ceramic is often restricted to inorganic crystalline materials, as opposed to the non-crystalline glasses.

The earliest ceramics were pottery objects made from clay, either by itself or mixed with other materials. Ceramics now includes domestic, industrial and building products and art objects. In the 20th century new ceramic materials were developed for use in advanced ceramic engineering, for example, in semiconductors.

Types of ceramic products:

Ceramic products are usually divided into four sectors, and these are shown below with some examples:

* Structural, including bricks, pipes, floor and roof tiles

* Refractories, such as kiln linings, gas fire radiants, steel and glass making crucibles

* Whitewares, including tableware, wall tiles, pottery products, and sanitary ware

* Technical, is also known as Engineering, Advanced, Special, and in Japan, Fine Ceramics. Such items include tiles used in the Space Shuttle program, gas burner nozzles, ballistic protection, nuclear fuel uranium oxide pellets, bio-medical implants, jet engine turbine blades, and missile nose cones. Frequently the raw materials do not include clays.

So true ceramics do serve as heat insulators. But at some point in time, the SECOND LAW OF THERMODYNAMICS will prevail and the heat source and its ceramic insulators will reach an even (HOT) temperature. It will just take a longer time to reach this point because the ceramics are insulators. So you should BE CAREFUL again, just because a hot object is held by a ceramic, you can't assume that it is SAFE to pick up.

Sincere regards,
Mike Stewart


Hi Nati,

As you infer, the tongs are not insulators. This works for at least two reasons:

1) The tongs put your hand further away from the hot metal

2) The tongs have very poor thermal contact with the hot metal so only a fraction of the heat gets into them

Often the tongs are smaller diameter than the metal you are working with and so do a better job of radiating heat to the environment instead of transferring it up their length to your hand

Greg Bradburn


It is a matter of contact area. The metal tongs only come into contact with the hot part of the sample over a few ( or fraction ) square millimeters. So the amount of heat transferred from the hot zone is limited by the minimal physical contact.

Vince Calder



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