Friday, 6 July 2018

Principles of Expansion: Advantages,Bimetallic strip, Uses in Electric iron, Fire alarm and thermostat

All about Expansion

Expansion of objects 

The expansion of an object is the process of increasing in size, amount or number. It is relative, when we tall about expansivity


Advantage of expansion

Advantages of expansion are utilised in the fitting of wheels on metal rims. The principle of fire alarms, thermostat, bimetallic thermometer, pendulum.

Disadvantages of expansion

These are in the sagging of overhead cables, the bucking of railway lines, in the gaining or losing of time, as a result of expansion of balance wheel of watches or clocks.

Read: Linear, area and cubic Expansivity 

Expansion of materials.          
Bimetallic strip

The bimetallic strip consists of a strip with different metals such as iron and brass attached rigidly side by side.

Bimetallic strip


This occurs because the metals have different expansivity. They expand at different degrees when the strip is heated. The unequal expansivity is as a result of the brass having a greater linear expansivity.

Uses of bimetallic strip

1) Thermostat

The bimetallic strip is used in the electric thermostat, Gas oven thermostat and the electric fire alarm. The bimetallic strip of an electric thermostat is used to control the temperature of an electric laundry iron as shown below;

Electric iron works on expansion

Bimetallic strip in Electric iron

The thermostat is set to desired temperature. When the current is switched on, it flows through the circuit, and produces heat in the heater coils. As the coil heats up, the bimetallic strip bends away from the contact point, which is fixed, until it reaches that set temperature, then the contact is completely broken and switches off the current. As the laundry iron cools, the strip straightens up again and remarks contact at the fixed point, thus switching on the electric current once more. This make-and-break regulates the temperature of an electric iron.

2) Electric fire alarm

The circuit of an electric fire alarm is shown below;

Electric fire alarm

When a fire breaks out in a building, it causes the strip to bend towards the contact, thus completing the circuit and causing the bell to ring out a fire alarm.

Expansion of glass

A thick glass cup cracks when hot water is pour into it. The inside of the cup expands more rapidly than the outside and causes a strain in the glass. A type of glass known as Pyrex, is therefore used for making laboratory beakers and flak to avoid the above effect. Pyrex has a low thermal expansivity.

We can remove a tight glass stopper of a glass bottle without cracking either the bottle or the stopper, by either standing the bottle in hot water or b warming the neck of the bottle with a flame. As the bottle expands, the stopper becomes loose.
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