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Tongs as Insulators
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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