Why Railway Tracks Are Laid With Gaps Between Them

Because steel expands when it gets hot. On a summer afternoon the rails grow slightly longer, and without a gap to grow into they would push against each other and buckle.

So the gap is not sloppy work — it is designed expansion room. This page covers everything in the ICSE Class 7 Physics chapter's first half: heat and temperature, the Celsius and Kelvin scales, thermometers, the effects of heat, and the expansion of solids, liquids and gases.

Both use the expansion of mercury in a fine glass capillary, but they are built for different jobs.

A laboratory thermometer has a wide range, typically $-10°C$ to $110°C$, so it can measure ice-cold and boiling liquids. It has no kink, so the mercury falls back as soon as it is removed — which is why it must be read while still in the liquid.

A clinical thermometer has a short range of $35°C$ to $42°C$, because human body temperature never travels far outside it. The narrow range lets the scale be stretched out, so it can be read to $0.1°C$.

Its defining feature is the kink — a sharp constriction just above the bulb. When the thermometer is taken out of the mouth, the kink stops the mercury thread from running back, so the reading is held and can be read at leisure.

That is also why a clinical thermometer must be shaken with sharp jerks before the next use: the mercury will not return past the kink on its own.

Normal body temperature is $37°C$, which is $98.6°F$ on the Fahrenheit markings many thermometers also carry.

Supplying heat to a substance can produce four effects.

- Rise in temperature — water in a kettle grows hotter as heat is supplied.
- Expansion — a metal lid loosens when the jar's neck is warmed under a tap.
- Change of state — ice melts to water at $0°C$, and water boils to steam at $100°C$.
- Change in physical or chemical properties — an iron rod becomes red hot and softens, milk turns sour, and a chapati browns on the tawa.

Running warm water over a stuck steel lid is the everyday use of the second effect: the metal expands more than the glass, and the lid turns freely.

The important boundary case is the third one. During a change of state the temperature does not rise, even though heat is still being supplied. Ice and water together stay at $0°C$ until all the ice has melted, because the heat goes into breaking the solid structure rather than into raising the temperature.

So "adding heat" and "raising temperature" are not the same statement — a point that decides several exam questions in this chapter.

Because the particles of a gas are far apart with almost no forces holding them together, so they move apart easily. Solids hold their particles most tightly of all.

The order of expansion for the same rise in temperature is therefore:

gases > liquids > solids

Each has its familiar consequences.

Solids. Gaps between railway tracks allow the rails to lengthen in summer. Overhead electric and telephone wires are strung slightly loose so they can contract in winter without snapping — which is why they sag more on a hot day. Bridges rest on roller supports for the same reason.

A bimetallic strip puts unequal expansion to work. Two different metals, such as brass and iron, are riveted together; brass expands more than iron for the same heating, so the strip bends towards the iron side. That bending makes or breaks a circuit, which is how an electric iron or a fire alarm switches itself.

Liquids. A bottle filled to the brim overflows when warmed, and thermometers work only because mercury expands measurably.

Gases. A partly inflated balloon left in the sun swells, and a sealed tin can bulge if heated.

Water has one famous exception, worth remembering: between $0°C$ and $4°C$ it contracts on heating rather than expanding, which is why ice forms at the top of a pond and the water below stays liquid.