# Expansivity

Welcome to class!

In today’s class, we will be talking about expansivity. Enjoy the class!

### Expansivity

Most substances (whether solid, liquid or gas) expand when heated and contracts when their temperature fall. When a body is heated, it gains some energy as a result of the fast movement of its kinetic energy. This result in an increase in the speed and vibration of the molecules of the system. When the system is cooled after being heated, there is a decrease in the speed of the kinetic energy, the molecules and the vibration of the system. Thus, this is said to be contracting. Thermal expansion of solids can be demonstrated using ball and ring apparatus as well as a steel rod apparatus.

At room temperature, that ball is just able to pass through the ring. If however, the ball is heated it will no longer pass through the ring because its size has increased (expansion).

#### Application of expansion

• ##### Railway tracks:

The Rail-line expands during hot weather and contracts when the temperature falls. For this reason, the rails are not laid as one continuous line, rather a small gap is allowed between successive rails to allow for expansion. If this is not done, the rail will buckle and be damaged.

• ##### Bimetal strip:

A bimetal strip is made of two strips of different metals laid side by side and joined together to become one strip. The two metals have a different rate of expansion when heated.

If a bimetal shown above is heated, since brass expands more than iron, the strip will bend as shown below.

On cooling, the strip returns to its original shape.

Bimetal finds application in a thermostat. A thermostat is a device used in regulating temperature. It is used in pressing Iron, water heater, etc to ensure that the desired temperature is maintained. Bimetal is also used in automatic fire alarm and in the balance wheel of a watch to prevent the watch from losing time when the temperature rises.

• ##### Removing tight stopper of a bottle:

The phenomenon of thermal expansion can be employed in removing tight stopper of a bottle. If the bottle is slightly heated, its neck expands, allowing the stopper to be removed easily.

#### Advantages of expansion of solid

1. For making bimetallic strip used in a thermostat.
2. Removal of tight glass stopper by careful and gentle heating
3. Red-hot rivets in ship buildings
4. Expansion of metals is used in bimetallic thermometer
5. Fitting of wheels in rims
6. Fire-alarms e.g. electric bell

#### Disadvantages of expansion of solid

1. Expansion of metal or concrete bridges.
2. Cracking of glass when hot water is poured into it
3. Expansion of balance wheel or wristwatch, leading to loss of time.
4. Expansion of railway lines.
5. Bursting of water pipes

In our next class, we will be talking more about Expansivity.  We hope you enjoyed the class.

Should you have any further question, feel free to ask in the comment section below and trust us to respond as soon as possible.

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