Saturday, February 4, 2012

Useful applications of thermal expansion

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.

     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.

Although expansion can be troublesome, it often proves very useful.

    The wheels of rolling-stock, particularly the large driving wheels of locomotives, are fitted with steel tyres, which have to be renewed from time to time owing to wear.

    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.

Ok, these are good informations. But what is the point here?

Please read this carefully:

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.
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.
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 temperature at which the contacts open is controlled by a screw which presses against an insulating sleeve on the strip S. If S is moved to the left the bimetallic strip will have to bend further before the contacts open, and so a higher temperature will maintained in the room.

Thermostats working on the bimetal principle are also used to control the temperature of laundry irons, hot-water storage tanks, aquaria for tropical fish and for many other purposes.

The bimetallic thermometer
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

For simplicity, the coil in the diagram is shown as a flat spiral, but in practice it is much longer than is shown and is wound in a cylindrical form.