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B.Sc (Hons, USJ) (Polymer Science and Technology, Chemistry, Physics)

This article mainly discusses the linear expansion of solids. But to properly enter that topic, we have to get a proper idea about what is the thermal expansion of matter and why it happens.

When the matter is supplied heat, if the volume of the matter increases with the temperature, it is called a thermal expansion.

The thermal expansion can be a,

- solid expansion,
- liquid expansion, or
- gaseous expansion.

Very matter exists made out of atoms. When heat energy is provided to any kind of matter including solid, liquid, and gaseous matter, the kinetic energy of atoms that are in the material will be increased. As a result of this increment of kinetic energy, the vibration frequency of atoms will be increased. Also moving distance of atoms will be increased. So the distances between atoms will be increased.

Thermal expansion will affect differently to solids, liquids, and gases. Therefore because of thermal expansion in matter,

- increment in length, surface area, and volume in solids
- Increment in the volume of liquids
- Increment in pressure in gas (in constant volume), increment in volume of gas (at constant pressure).

When heat is supplied to a solid, a solid matter will increase its length, surface area, and volume. So there are three types of solid expansion.

- Linear expansion of solids
- Superficial expansion (Area expansion) of solids
- Volume expansion of solids

When the heat is supplied to an object like a rod, it shows an increase in length. This is known as the linear expansion of solids. Mathematically, when the temperature is increased by ** Δθ** degrees, of a rod-like object with the length of '

Where,

- l - Initial length of the rod
- Δ l - Increase in length
- Δθ - Increase in temperature
- α - Linear expansion coefficient

The linear expansion coefficient is the proportional coefficient that expresses the ratio between the increased length and the original length when the temperature is increased by one degree of Kelvin or Celsius. Linear temperature expansion coefficient is a material property.

Consider a rod with the length of ** l_{1}** at temperature

Material | Linear expansion coefficients (×10^{-6} ^{0}C^{-1}) |

ABS thermoplastic | 72 - 108 |

Aluminum | 21 - 24 |

Brass | 18 - 19 |

Cellulose nitrate | 80 - 120 |

Portland cement | 11 |

Chromium | 6 - 7 |

Concrete | 13 - 14 |

Copper | 16 - 16.7 |

Copper, Beryllium 25 | 17.8 |

Diamond | 1.1 - 1.3 |

Glass, hard | 5.9 |

Glass, Pyrex | 4.0 |

Gold | 14.2 |

Ice, 0^{o}C water | 51 |

Iron, cast | 10.4 - 11 |

Lead | 29 |

Glass fiber reinforced Nylon | 23 |

Polyamide | 110 |

Polycarbonate | 65 - 70 |

source: engineeringtoolbox.com

Two metallic strips with the same dimensions but different linear expansion coefficients are joined together. When it is heated, two metals are expanded according to their linear expansion coefficients. The metal with a higher thermal expansion coefficient will show a higher increase in length than the other metal. This will result in a bend in the bimetallic strip.

Bimetallic strips are used in some electric circuits to protect the device from high heat. When the temperature increases, the bimetallic strip will bend and the circuit will be disconnected. Thus, the temperature is decreasing and the bimetallic strip will become the original state. Therefore, the circuit is connected again.

**Question 01**

When the temperature of a rod is increased by 100 °C, it will show 0.002 of rational increase in length. What is the linear expansion coefficient of the material?

**Answer**

Sciencedirect.com - *Thermal expansion*

Engineeringtoolbox.com - *Linear expansion coefficients*

Amesweb.info - *Linear thermal expansion coefficient of metals*

The cover image was designed using an image by Virtual gamma, licensed under CC BY-SA 3.0, via Wikimedia Commons

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