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Heat


Temperature range and thermal sensations

In our daily lives, we are used to feeling hot or cold as we touch some objects. They are thermal sensations.

We are used to associating these thermal sensations with the concept of temperature. We say, for example, that the temperature of an ice stone is lower than that of a roast beef that has just been removed from the oven.

Imagine yourself in a place of ceramic flooring. You take off your shoes and socks and, barefoot, place one foot on a rug and the other directly on the ceramic floor. One foot will feel cold and the other will not. Do this experiment!

It turns out that both the carpet and the ceramic floor are at the same temperature! This proves that our touch is not always fully reliable for comparing the temperature of two objects.

A scientific method is required to measure the temperature of an object and express it by means of a numerical scale. This is how the concept of temperature scale came about.

Thermal expansion and contraction

A common way to construct a temperature scale is to use some material that has temperature-dependent properties.

Consider, for example, a metal bar being heated in a flame, as shown in the figure below. As it is heated, the bar has increased in length. The increase in volume of a material caused by heating is called thermal expansion.

If the heating is stopped, the bar will gradually cool and, as it occurs, will return to its original volume. Reducing the length of a material when its temperature decreases is called thermal contraction. If the bar is placed in the refrigerator, it will continue to cool and heat shrink.

The occurrence of thermal expansion and contraction allows us to state that the length of a metal bar depends on the temperature. It is therefore possible to use this bar to build a thermometer. Each length would be associated with a temperature. But the length of a solid metal bar during heating or cooling usually varies very little.

The most common thermometers are based on thermal expansion and contraction concepts.but instead of a solid bar a liquid column is used, which is usually colored alcohol or mercury.

When subjected to different temperatures, the liquid column changes considerably in volume. The longer the spine, the greater the swelling and therefore the higher the temperature.