Thermal expansion

Thermal expansion is a process where objects expand as a result of being heated.

How thermal expansion works

1. As a substance is heated its molecules gain kinetic energy. 
2. This causes them to vibrate more and move slightly further apart. 
3. This is known as thermal expansion. 

How thermal expansion varies for solids, liquids, and gases

How much a substance expands depends on the intermolecular forces between its molecules. 

1. Solids - Thermal expansion occurs least in solids because atoms are kept in fixed positions and have strong intermolecular forces.
2. Liquids - Thermal expansion occurs more in liquids because the particles are free to move.
3. Gases - Thermal expansion occurs most in gases due to the weak intermolecular forces.  

Applications of thermal expansion

You need to be able to explain thermal expansion in applications such as riveting, expansion joints, and power lines. 

Riveting

Rivets are pieces of metal which are used to tightly fix metal sheets together. An example of rivets is in the assembly of aircraft fuselages. 

1. A metal rivet is heated to increase its length.
2. The rivet is threaded through holes in metal plates and the ends are hammered flat.
3. As the rivet cools, it contracts in length and pulls the metal plates tightly together. 

Expansion joints

Expansion joints are used in civil engineering when the size of the construction materials may vary depending on their temperature.

1. Structures like bridges and some buildings are built with expansion gaps. 
2. When the materials get hot, they expand and they fit tightly together.
3. When the materials cool, they shrink and leave a small gap known as an expansion gap.

These expansion gaps are essential to avoid permanent damage to the structures.  

Power lines

Power lines are essential for power distribution around the country. Overhead power lines are suspended from pylons.

1. Power lines are deliberately built to sag slightly.
2. On hot days, the power lines extend and sag more.
3. On cold days, the power lines contract. The lines need to be sufficiently long to account for this contraction to avoid snapping or damaging the pylons.