The Concept Of Heat Transfer Part III

The effects of thermal expansion and contraction occur in our daily lives activities. This as a result causes daily changes in our lives. We see some of the changes that thermal expansion and contraction cause as destructive nature to our property.

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For examples gaps are left in between the metal bridges to prevent the bending of metal as a result of over heating from the over increase in temperature. That's is why bridges are constructed by the experts of Engineers who have indepth knowledge about all these thermal expansion and contraction.

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The cracking noise is being heard from the Aluminum roof during hot weathers this due to expansion. For our knowledge about our previous lessons reveals that when heat is applied to solids for instance metals, they expand but contract when cooled. The nail that has been nailed to the Aluminum roof with the wood would try to relief itself during hot weathers and this result to the expansion causing it to make noise.

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The door and the frame keep making noise when shutting or opening during hot weathers but remain silence when weather cools. During hot weathers, the expansion is created in to door, but becomes normal at it fixed position when it contracts.

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The cracking of building and other concrete structures are due to the expansion of surfaces. Sometimes this cause great devastation in our societies. For instance the collapse of building can cause havoc to human nature and some properties.

A Thermostat

A Thermostat is a temperature or heat controlling for maintaining temperature as when the device should be cooled and hot. Some of the devices that use thermostat are electric pressing iron, electric kettle, electric cooker and refrigerator.

The thermostats are being controlled by "Bimetallic Strips" that's two metals such as steel and brass which have different temperatures (heating points) due to distinct hardness among them. Both metals expand at different rate of heating. An increase in temperature would increase the volume of the solid metals having different expansion rate at constant temperature. This would lead variation of the expansion of the two metals causing the device to cut due to the displacement of the two different metals.

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Construction of Bimetallic strips

Two different metals like steel and brass can be use to construct Bimetallic Strips. The Steel and Brass metal strips of equal lengths at room temperature are joined together into one bimetalic strip. When the 2 bonded metals are heated the brass expands widely than the steel, and the strip curves with the brass on the outside of the curve. When the strip is cooled from room temperature instead it would curve the other way, with the steel on the outside.

This causes temperature regulation in the devices that have been embedded with Bimetallic Strips. In most of devices such as thermostats, they are used as switches. This explains why electric pressing irons and refrigerators have automatic power cuts when they reach their optimum heat gains. For instance when using electric iron, it would be heated at certain point and the power would cut as soon as the heat increases above certain point. In the same way when the heat cools during contractions of the Bimetallic Strips, it would switch again. What happens if the Bimetallic Strips is damaged? The person using the iron may cause burns to his clothes.

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