Thermal expansion in real life

Use it, or leave room for it

Expansion can be useful, or it can break things. Engineers either put it to work or leave room so it does no harm.

Long things in the open, such as rails, bridges and pipes, change length the most, so they are the ones that need gaps and joints.

Key terms

Expansion gap

A small space left so metal can expand without bending.

Sun kink

A rail that buckles on a hot day because it had no room to expand.

Expansion joint

A joint that lets a bridge expand and contract safely.

Pressure relief valve

A valve that lets out gas before the pressure bursts the container.

Short questions with model answers

1. Q1. A metal lid is stuck on a glass jar. Why does running warm water over the lid help?

   • Heat the lid.
   • Compare.

   The lid grows a little more than the glass, so it becomes easier to open.

2. Q2. Why do bridges need expansion joints but a jar with a metal lid does not? (Constructed Response 10.1)

   • ΔL = α L₀ ΔT
   • Sizes.

   A long bridge changes length by a lot and would crack; a small lid changes by a tiny amount the glass can take.

Common mistakes

• ✗ Gaps between rails are a building mistake.

  ✓ They are left on purpose so the steel can expand in summer.

• ✗ Thermal expansion only causes damage.

  ✓ Thermometers, balloons and engines put it to use.

• ✗ Cooling a stuck metal lid loosens it.

  ✓ Cooling makes it contract and grip tighter; warm water loosens it.

MCQs

1. 1. Small gaps are left between railway tracks to:

   1. (a) Save steel
   2. (b) Let rain drain
   3. (c) Allow expansion in summer
   4. (d) Make the train quieter
   Show answer

   (c) p. 6: gaps let the tracks expand in summer, preventing them from bending.

2. 2. Which uses thermal expansion on purpose?

   1. (a) A thermometer
   2. (b) A bridge joint
   3. (c) A pipe loop
   4. (d) A relief valve
   Show answer

   (a) p. 6: expansion in a narrow tube measures temperature. The others protect against expansion.

3. 3. Aerosol cans carry warnings to keep them away from heat because:

   1. (a) The paint fades
   2. (b) The gas inside expands and the pressure can burst the can
   3. (c) The can shrinks
   4. (d) The gas freezes
   Show answer

   (b) p. 6: heat builds up pressure inside and can cause explosions.

4. 4. Flexible joints and loops are used in piping systems to:

   1. (a) Make water flow faster
   2. (b) Heat the water
   3. (c) Reduce the cost
   4. (d) Absorb expansion and contraction
   Show answer

   (d) p. 6: they absorb movement and prevent bursts or leaks.

5. 5. To loosen a stuck metal lid on a glass jar, you should:

   1. (a) Put it in the freezer
   2. (b) Run warm water over the lid
   3. (c) Shake it hard
   4. (d) Pour in cold water
   Show answer

   (b) p. 6: warm water makes the lid expand slightly, making it easier to open.

Quick revision

• Thermometers, jar lids, balloons and engines put expansion to use.
• Rail gaps, bridge joints, relief valves and pipe loops leave room for it.
• Long structures change length most, so they need the most room.