Applications of thermal expansion
With few exceptions, substances expand when heated, and very large forces may be set up if there is an obstruction to the free movement of the expansion or contracting bodies.
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| Rolling-Stock |
If concrete road surfaces were laid down in one continuous piece cracks would appear owing to expansion and contraction brought about by the difference between summer and winter teperatures. To avoid this, the surface is laid in small sections, each one being seperated from the next by a small gap which is filled in with a compound of pitch. On a hot summer day, expansion often squeezes this material out of the joints.
To ensure a tight fit the tyre is made slightly smaller in diameter than the wheel. Before being fitted the tyre is heated uniformly by special gas burners arranged in a ring. The resulting expansion enables the tyre to be slipped easily over the wheel, and on cooling it contracts and makes a tight fit.
The forece of contraction when hot metal cools is also utilized in riveting together the steel plates and girders used in shipbuilding and other constructional work.
The rivets are first made hot. This softens them so that they are easily burred into a head by pneumatic hammers, and their contraction on cooling serves to pull the plates tightly together.
Cool, is not it? 🙂
Expansion of various substances
When rods of the same length but different substances are heated through the same range of temperature, experiment shows that their expansions are not equal. Brass, for example, expands about one and a half times as much as steel; aluminium expands about twiceas much as steel.
An alloy of steel and nickel known as invar has an exceptionally small expansion when its temperature rises, and is used in watches and thermostats.
Glass has a smaller expansion than iron. Fused silica and Pyrex glass have very low expansion.
The bimetallic strip
The difference in the expansion of brass and iron may be shown by riveting together a strip of brass and an equal strip of iron ( see the fig.) . On heating this bimetallic strip it bends so that the brass is on the outside of the curve. The brass thus becomes longer than the iron, showing that it expands more than iron for the same temerature change.
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| bimetallic strip |
Useful applications of the bimetalic strip
One of the most important applications is the electric thermostat. A thermostat is a device for maintaining a steady temperature. Fig. shows the principle of a thermostat used for controlling the temperature of a room warmedby an electric heater.
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| the principle of a thermostat |
The heater circuit is completed through the two silver contacts of the thermostat, one of which is attached to a metal strip S and the other to a bimetallic strip M. If the room becomes too warm the bimetallic strip bends, seperates the contacts and cuts off the current. On cooling, contact is remade and the heater switched on again.
The bimetallic thermometer
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| bimetallic thermometer |
Fig. shows the principle of the bimetallic thermometer. One end of a thin bimetallic spiral is fixed, the other end being attached to the spindle of a pointer which moves over a scale of degrees. The metals used are brass and invar, and the spiral tends to curl in a clockwise direction as the temperature rises. You, my dear reader can see
Types of thermometer