Expansion
Physics · प्रसार
📋Quick Overview
When substances are heated, they expand (get bigger). When cooled, they contract (get smaller). This is called Thermal Expansion. Solids, liquids, and gases — all expand on heating, but gases expand the MOST and solids expand the LEAST. This simple concept explains many daily life things — why railway tracks have gaps, why bridges have expansion joints, etc.
📖Types of Expansion in Solids
| Type | What Expands | Coefficient | Example |
|---|---|---|---|
| Linear Expansion | Length only (1 dimension) | α (alpha) | Metal rod becomes longer when heated |
| Superficial/Area Expansion | Area (2 dimensions) | β (beta) = 2α | Metal plate becomes larger in area |
| Cubical/Volume Expansion | Volume (3 dimensions) | γ (gamma) = 3α | Metal ball expands in all directions |
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Relationship: α : β : γ = 1 : 2 : 3. This means area expansion coefficient is twice the linear, and volume is thrice the linear.
📝Anomalous Expansion of Water
- •Most substances expand on heating. But WATER behaves differently between 0°C and 4°C!
- •Water CONTRACTS (shrinks) from 0°C to 4°C instead of expanding. After 4°C, it expands normally.
- •Water has MAXIMUM DENSITY at 4°C. This is called anomalous (abnormal) expansion.
- •WHY this matters: In winter, lake water at 4°C (heaviest) sinks to bottom. Ice (0°C, lighter) floats on top. Fish survive in the water below the ice!
- •This is why ice floats on water — ice is LESS dense than liquid water at 4°C
📝Applications of Thermal Expansion
- •Railway tracks have GAPS between rails — so tracks can expand in summer without bending
- •Bridges have expansion joints — to allow concrete/metal to expand and contract
- •Electric wires sag in summer (expand) and become tight in winter (contract)
- •Bimetallic strip: Two different metals bonded together bend when heated (used in thermostats, fire alarms)
- •Hot water loosens a tight metal lid on a glass jar (metal expands more than glass)
- •Riveting: Hot rivets are used to join metal plates — they shrink on cooling and hold plates tightly