Applying the Principles Expansion and Contraction of Matter


1. The effects of expansion and contraction of matter can be very troublesome, and precautions have to be taken against these effects.


2. We can also apply the principle of expansion and contraction of matter in making instruments that are useful in daily life.


3. The following are some examples to show the use of expansion and contraction of matter:

(A) Mercury in a thermometer

Mercury is a liquid metal that can expand and contract continuously when there is a change in temperature. This makes it suitable for temperature measurement and it is used to make thermometers.


(B) The bimetallic strip in a fire alarm

An automatic fire alarm uses a bimetallic strip to switch on the electric bell when there is a fire. The heat from the fire causes the bimetallic strip to expand and bend towards the contact point. When the bending strip touches the contact point, it completes the circuit and the fire alarm rings.


(C) The bimetallic strip as a thermostat

The bimetallic strip is used as a thermostat in an electric iron for keeping a steady temperature. As temperature rises, the bimetallic strip expand and bends away from the contact point and cuts off the current. When the bimetallic strip cools down, it contracts and contact is made again, causing the current to flow once more to heat up the iron.


(D) Bimetallic thermometer

A bimetallic strip wound in spiral form as shown in Figue 7.28 can be used to measure temperature. Brass (an alloy of copper and tin) and invar (an alloy of nickel and steel) are used to make the bimetallic strip. Brass is on the outside and invar is on the insisde. As the temperature increases, the brass expands more than invar and causes the spiral to wound tighter. The pointer will then move to the right.

This type of thermometer is not very accurate.



4. When laying railway tracks, gaps have to be left between succesive lengths of rail to allow for expansion on hot days. If not, this can lead to buckling of the track.


5. When concrete roads are laid down, gaps (normally filled with tar) are left between sections in order to allow for expansion, as shown in the picture below.


6. Structures like steel bridges and overhead bridges are built with gaps to allow for expansion. Sometimes the end is supported by rollers to allow bridges to expand easily when heated.



7. Electric transmission cables and cables of a cable car line sag during the hot day and tighten during the cold night. Therefore, allowances have to be made for the expansion and contraction of the cables.



The use of the principle of expansion and contraction of matter in solving simple problems

1. The force of contraction when hot metal cools is used in riveting two metal plates together. Steel plates such as those used in ship building or in large boilers are riveted together using red hot rivets. Holes are made in the overlapping plates. A red hot rivet is pushed through and its head is held tightly against one plate, while the other end of the rivet is hammered tight against the other plate. On cooling, the rivet contracts and holds the plates even more tightly together.


2. A very tight cap on a bottle can be removed easily by wrapping the cap with a hot towel. The cap expands much faster than the bottle and so the cap can be removed easily when it expands.


3. The hub of the wheel is usually slightly smaller than the size of the axle. To fix the axle into the hub of the wheel, the axle has to be placed in liquid nitrogen (at - 190°C) to cool it so that it contracts until it can be fitted into the hub.


4. The metallic wheels of a train are fitted with steel tyres. To ensure a tight fit, the tyre is made slightly smaller in diameter than the wheel. Before being fitted the tyre is heated uniformly and the resulting expansion enables the tyre to be slipped over the wheel. Upon cooling, the steel tyre contracts and make a tight fit.