Expansion and Contraction : Why does heat and cold make things expand and contract ? Also why do some metals expand more than others?

Asked by: Marcus Mckean

Answer

Recall that all materials are made up of atoms. At any temperature above absolute zero (-273 degrees celsius) the atoms will be moving. In a solid they will be vibrating in fixed positions, in a liquid thy will be jostling past each other and in a gas they will be whizzing past each other at very high speeds. When a material is heated, the kinetic energy of that material increases and its atoms and molecules move about more. This means that each atom will take up more space due to its movement so the material will expand. When it is cold the kinetic energy decreases, so the atoms take up less space and the material contracts.

Some metals expand more than others due to differences in the forces between the atoms / molecules. In metals such as iron the forces between the atoms are stronger so it is more difficult for the atoms to move around . In brass the forces are a little weaker so the atoms are free to move about more. These differences in contraction are used in a bimetallic strip, which is composed of a strip of brass laid along side strip of Iron. When the strip is heated the brass expands more than the iron so the strip beds. It is used in devics such as fire alarms and circuit breakers to either make or break contacts in an electric circuit.

The differences in expansion and contraction are even more visible in different states, again due to the amount of force holding the atoms together. A gas will expand the most as its atoms are free from each other so are free to increase in speed the most.
Answered by: Sara Al-Assam, Student, Tiffin Girls' School, Kingston