mechanical · 24 minutes · Official OETT Aligned
Heating & Cooling
Why materials grow or shrink
What to know
- expansion: getting bigger
- contraction: getting smaller
- space for materials to expand
- two-metal strips
Machines and bridges need room for parts to grow as they warm up.
The core rule
Most materials get bigger when heated and smaller when cooled.
Expansion means growing in size; contraction means shrinking. A heated metal rod usually gets slightly longer. Gaps between bridge sections leave room for this growth. Different metals grow by different amounts, so two metals joined in a strip can bend when heated.
Worked examples
Example 1: easy
A heated metal rod becomes slightly longer.
- The rod gets hotter and has room to expand.
- Its ends move slightly farther apart, so it becomes longer.
A higher temperature increases the length of most metal rods.
Example 2: medium
A bridge gap narrows on a hot day as sections expand.
- On a hot day, both bridge sections grow toward the gap.
- The gap between them becomes smaller.
The gap leaves room for temperature increases to make the bridge sections expand.
Example 3: hard
A bimetal strip bends toward the lower-expansion material when heated because the other layer lengthens more.
- The two joined metals grow by different amounts.
- The layer that grows more goes around the outside of the bend. The strip bends toward the layer that grows less.
The same temperature increase makes the two metals grow by different amounts, so the joined strip bends.
Common mistakes
Heating a metal plate makes a hole in it smaller.
The hole gets larger along with the rest of the plate, as long as the plate can expand freely.
Put it into practice
Use guided explanations, check your answers, and try a new variation.