Engineers Leave Gaps in Rails on Purpose
Learn to separate heat from temperature and convert between Celsius and Kelvin, compare how solids, liquids and gases expand, see expansion put to work in a bimetallic strip and a cart wheel, and how its damage is prevented.
Why are gaps deliberately left between railway rails?
Look closely at an older stretch of railway track and you will find a small gap where one rail meets the next. It looks like poor workmanship. It is the opposite.
Steel expands when it is heated. A rail lying in the sun on a hot afternoon is longer than the same rail at dawn. If the rails were laid end to end with no room to grow, each one would push against its neighbours, and with nowhere to go the track would buckle sideways off the sleepers.
The gap gives the expansion somewhere to happen. On a hot day the gaps close up a little; on a cold night they open again.
That single idea runs through this whole page. Heating a body makes its particles vibrate more strongly, so they push each other a little further apart and the body grows — and engineers either use that growth, as in a bimetallic strip or a cart wheel's iron rim, or allow for it, as in a rail gap or a bridge's rollers.
Before any of that, though, two words have to be kept apart: heat and temperature are not the same quantity, and mixing them up makes the rest of the chapter impossible.
This page covers the first part of the ICSE Class 9 Physics chapter on heat and energy — heat and temperature, the expansion of the three states, and the uses and hazards of thermal expansion.
Steel expands when it is heated. A rail lying in the sun on a hot afternoon is longer than the same rail at dawn. If the rails were laid end to end with no room to grow, each one would push against its neighbours, and with nowhere to go the track would buckle sideways off the sleepers.
The gap gives the expansion somewhere to happen. On a hot day the gaps close up a little; on a cold night they open again.
That single idea runs through this whole page. Heating a body makes its particles vibrate more strongly, so they push each other a little further apart and the body grows — and engineers either use that growth, as in a bimetallic strip or a cart wheel's iron rim, or allow for it, as in a rail gap or a bridge's rollers.
Before any of that, though, two words have to be kept apart: heat and temperature are not the same quantity, and mixing them up makes the rest of the chapter impossible.
This page covers the first part of the ICSE Class 9 Physics chapter on heat and energy — heat and temperature, the expansion of the three states, and the uses and hazards of thermal expansion.
Formula
What is the difference between heat and temperature?
Heat is the total thermal energy a body contains; temperature is the degree of hotness that decides which way heat flows.
- Heat — a form of energy. SI unit the joule (J). Depends on the mass, the material and the temperature of the body
- Temperature — a measure of the average energy of the particles. SI unit the kelvin (K). Does not depend on how much substance there is
Heat always flows from the body at higher temperature to the body at lower temperature, and stops when the two are equal.
The conversion between the two scales.
Worked conversions.
- K — the melting point of ice
- K — the boiling point of water
- K — a warm Indian afternoon
- K
- K — normal body temperature
Absolute zero is K, which is , and no body can be cooled below it. That is why the Kelvin scale has no negative values.
A temperature DIFFERENCE is the same number on both scales. The divisions are the same size, so a rise from to is a rise of , and in kelvin it is K to K — still a rise of K. **So add for a temperature and never for a temperature change.
Worked example — where the scale really matters.** A gas is heated from to . Has its temperature doubled? On the Celsius scale is more than twelve times . In kelvin:
The absolute temperature has exactly doubled. Any calculation involving a ratio of temperatures must use kelvin, because the Celsius scale's zero is an arbitrary choice and a ratio is meaningless without a true zero.
Two bodies at the same temperature can hold very different amounts of heat. A bucket of water and a cup of water both at are equally hot, and the bucket contains far more heat because it has far more mass. So "hotter" and "more heat" are different claims — and a cup of boiling water will not warm a bathtub, even though it is at a much higher temperature.
- Heat — a form of energy. SI unit the joule (J). Depends on the mass, the material and the temperature of the body
- Temperature — a measure of the average energy of the particles. SI unit the kelvin (K). Does not depend on how much substance there is
Heat always flows from the body at higher temperature to the body at lower temperature, and stops when the two are equal.
The conversion between the two scales.
Worked conversions.
- K — the melting point of ice
- K — the boiling point of water
- K — a warm Indian afternoon
- K
- K — normal body temperature
Absolute zero is K, which is , and no body can be cooled below it. That is why the Kelvin scale has no negative values.
A temperature DIFFERENCE is the same number on both scales. The divisions are the same size, so a rise from to is a rise of , and in kelvin it is K to K — still a rise of K. **So add for a temperature and never for a temperature change.
Worked example — where the scale really matters.** A gas is heated from to . Has its temperature doubled? On the Celsius scale is more than twelve times . In kelvin:
The absolute temperature has exactly doubled. Any calculation involving a ratio of temperatures must use kelvin, because the Celsius scale's zero is an arbitrary choice and a ratio is meaningless without a true zero.
Two bodies at the same temperature can hold very different amounts of heat. A bucket of water and a cup of water both at are equally hot, and the bucket contains far more heat because it has far more mass. So "hotter" and "more heat" are different claims — and a cup of boiling water will not warm a bathtub, even though it is at a much higher temperature.
Which expands more on heating: a solid, a liquid or a gas?
Gases expand the most, liquids much less, and solids least of all — for the same rise in temperature.
Why the order is that way. Heating makes the particles vibrate more vigorously, and whether they can move apart depends on how strongly they are held:
- In a solid the intermolecular forces are strongest, so the particles can only vibrate a little more about fixed positions. The expansion is very small
- In a liquid the forces are weaker and the particles can slide past one another, so the expansion is larger
- In a gas the forces are negligible, so the particles move apart freely and the expansion is very large
Solids expand in three ways. A heated rod grows in length (linear expansion), a heated sheet grows in area (superficial expansion), and a heated block grows in volume (cubical expansion). All three happen together; which one is talked about depends on the shape.
Everyday observations of each.
- Solid — a tight metal lid loosens when hot water is run over it, because the metal expands more than the glass beneath it
- Liquid — mercury or alcohol rising in a thermometer is a liquid expanding into a narrow tube
- Gas — a partly inflated balloon left in the sun swells noticeably
Worked comparison — the thermometer. A thermometer works only because the liquid expands more than the glass around it. If the two expanded equally the level would never move. The glass bulb expands too, and the reading is really the difference between the liquid's expansion and the container's.
A hole in a solid expands as well. Heat a metal plate with a hole in it and the hole gets bigger, not smaller — every part of the plate grows in proportion, including the empty part. That surprises most people, who expect the surrounding metal to close in. It is why a tight metal ring can be loosened from a rod by heating the ring, and why the lid trick above works.
Water is the great exception among liquids. Between and water contracts on heating instead of expanding — the anomalous behaviour that the next part of this chapter is devoted to, and the reason fish survive a frozen winter.
Why the order is that way. Heating makes the particles vibrate more vigorously, and whether they can move apart depends on how strongly they are held:
- In a solid the intermolecular forces are strongest, so the particles can only vibrate a little more about fixed positions. The expansion is very small
- In a liquid the forces are weaker and the particles can slide past one another, so the expansion is larger
- In a gas the forces are negligible, so the particles move apart freely and the expansion is very large
Solids expand in three ways. A heated rod grows in length (linear expansion), a heated sheet grows in area (superficial expansion), and a heated block grows in volume (cubical expansion). All three happen together; which one is talked about depends on the shape.
Everyday observations of each.
- Solid — a tight metal lid loosens when hot water is run over it, because the metal expands more than the glass beneath it
- Liquid — mercury or alcohol rising in a thermometer is a liquid expanding into a narrow tube
- Gas — a partly inflated balloon left in the sun swells noticeably
Worked comparison — the thermometer. A thermometer works only because the liquid expands more than the glass around it. If the two expanded equally the level would never move. The glass bulb expands too, and the reading is really the difference between the liquid's expansion and the container's.
A hole in a solid expands as well. Heat a metal plate with a hole in it and the hole gets bigger, not smaller — every part of the plate grows in proportion, including the empty part. That surprises most people, who expect the surrounding metal to close in. It is why a tight metal ring can be loosened from a rod by heating the ring, and why the lid trick above works.
Water is the great exception among liquids. Between and water contracts on heating instead of expanding — the anomalous behaviour that the next part of this chapter is devoted to, and the reason fish survive a frozen winter.
How is thermal expansion put to work?
Three standard applications, and each one works by letting a metal grow or shrink at a controlled moment.
A bimetallic strip. Two strips of different metals — commonly brass and iron — are riveted firmly together along their length. Brass expands more than iron for the same temperature rise, so on heating the brass side becomes longer than the iron side, and since they cannot separate, the strip has no option but to bend.
Which way does it bend? The metal that expanded more ends up on the outside of the curve, because the outer edge of a bend is the longer one. So on heating the strip bends with brass on the convex side and iron on the inside. On cooling below its original temperature it bends the other way, because brass now contracts more.
Where it is used.
- A thermostat in an electric iron, a refrigerator or a room heater — the bending strip makes or breaks the circuit at a set temperature, switching the heating on and off automatically
- A fire alarm — the strip bends on heating and completes the circuit of a bell
- A flashing indicator in some circuits — the strip heats, bends, breaks the circuit, cools, straightens and reconnects, repeating the cycle
Riveting two metal plates together. Rivets are heated until they are red hot, pushed through the holes in the plates, and hammered flat at both ends. As each rivet cools it contracts, pulling the two plates powerfully together. A cold rivet hammered into place could never grip as tightly — the grip comes from the contraction, not from the hammering.
Fitting an iron rim on a cart wheel. The rim is deliberately made slightly smaller in diameter than the wooden wheel. It is then heated strongly, so that it expands and slips over the wheel. Cold water is poured on it at once, and as it contracts it clamps the wheel tightly, holding the wooden spokes and segments firmly in place.
The same trick fits a gear or a bearing onto a shaft, and it is the reason a workshop heats a ring rather than forcing it.
All three uses depend on expansion being reliable and predictable, not on it being large. The movement of a bimetallic strip is a fraction of a millimetre, and it is enough to open a circuit because the contact only has to part. So a small effect is useful when it is made to control something, which is the general lesson of this section and the reason the same small effect becomes a hazard in the next one.
A bimetallic strip. Two strips of different metals — commonly brass and iron — are riveted firmly together along their length. Brass expands more than iron for the same temperature rise, so on heating the brass side becomes longer than the iron side, and since they cannot separate, the strip has no option but to bend.
Which way does it bend? The metal that expanded more ends up on the outside of the curve, because the outer edge of a bend is the longer one. So on heating the strip bends with brass on the convex side and iron on the inside. On cooling below its original temperature it bends the other way, because brass now contracts more.
Where it is used.
- A thermostat in an electric iron, a refrigerator or a room heater — the bending strip makes or breaks the circuit at a set temperature, switching the heating on and off automatically
- A fire alarm — the strip bends on heating and completes the circuit of a bell
- A flashing indicator in some circuits — the strip heats, bends, breaks the circuit, cools, straightens and reconnects, repeating the cycle
Riveting two metal plates together. Rivets are heated until they are red hot, pushed through the holes in the plates, and hammered flat at both ends. As each rivet cools it contracts, pulling the two plates powerfully together. A cold rivet hammered into place could never grip as tightly — the grip comes from the contraction, not from the hammering.
Fitting an iron rim on a cart wheel. The rim is deliberately made slightly smaller in diameter than the wooden wheel. It is then heated strongly, so that it expands and slips over the wheel. Cold water is poured on it at once, and as it contracts it clamps the wheel tightly, holding the wooden spokes and segments firmly in place.
The same trick fits a gear or a bearing onto a shaft, and it is the reason a workshop heats a ring rather than forcing it.
All three uses depend on expansion being reliable and predictable, not on it being large. The movement of a bimetallic strip is a fraction of a millimetre, and it is enough to open a circuit because the contact only has to part. So a small effect is useful when it is made to control something, which is the general lesson of this section and the reason the same small effect becomes a hazard in the next one.
How is the damage from thermal expansion prevented?
By deliberately leaving room for the movement, or by keeping the structure slack enough to absorb it. Every case below is the same idea in a different setting.
Railway tracks. Small gaps are left between successive rails, so that each can lengthen on a hot day without pushing its neighbours. Without them the track would buckle sideways. The fish-plates joining the rails have oval holes so the bolts can slide, and modern long-welded track is instead held under carefully managed tension.
Bridges. One end of a steel girder rests on rollers rather than being fixed, so the girder can slide as it lengthens and shortens. The roadway carries expansion joints — toothed metal strips with gaps between them that open and close. Fixing both ends rigidly would bend or crack the structure as the seasons changed.
Pipelines. Long steam or water pipes are fitted with loops or bends, which flex to take up the change in length. A perfectly straight fixed pipe would tear itself from its supports.
Overhead electric and telephone wires. They are strung with a deliberate sag in summer, when they are at their longest. In winter they contract and the sag reduces — and a wire pulled tight in summer would snap on the first cold night.
Concrete roads and pavements. They are laid in slabs with gaps between them, filled with pitch or tar, which stays soft enough to be squeezed as the slabs expand.
Glass and hot water. Pouring boiling water into a thick glass tumbler can crack it. The inner surface heats and expands at once while the outer surface is still cold, and the unequal expansion sets up a stress that the glass cannot take. A thin glass heats right through almost immediately, so the two surfaces stay nearly equal, and heat-resistant glassware expands very little for a given temperature rise.
The same reason explains the telegraph-wire hum and the creaking of a roof. A metal roof sheet expands in the morning sun and slips slightly against its fastening, and the small sudden movements are heard as creaks.
Worked example — why the gap has to be there. Suppose a rail is warmed from a night temperature of to an afternoon temperature of . That is a rise of
Steel expands only slightly for such a rise, but a rail is long and there are many rails in a kilometre, so the total movement along a stretch of track is what matters. A tiny expansion repeated along a long structure is a large expansion, and that is exactly why the problem appears in rails, bridges and pipelines rather than in a spoon.
Prevention never means stopping the expansion. Nothing can prevent a heated metal from growing; the engineering choice is only where the growth is allowed to happen. A design that tries to hold it back converts the expansion into a force, and that force is what buckles a rail or cracks a bridge.
Railway tracks. Small gaps are left between successive rails, so that each can lengthen on a hot day without pushing its neighbours. Without them the track would buckle sideways. The fish-plates joining the rails have oval holes so the bolts can slide, and modern long-welded track is instead held under carefully managed tension.
Bridges. One end of a steel girder rests on rollers rather than being fixed, so the girder can slide as it lengthens and shortens. The roadway carries expansion joints — toothed metal strips with gaps between them that open and close. Fixing both ends rigidly would bend or crack the structure as the seasons changed.
Pipelines. Long steam or water pipes are fitted with loops or bends, which flex to take up the change in length. A perfectly straight fixed pipe would tear itself from its supports.
Overhead electric and telephone wires. They are strung with a deliberate sag in summer, when they are at their longest. In winter they contract and the sag reduces — and a wire pulled tight in summer would snap on the first cold night.
Concrete roads and pavements. They are laid in slabs with gaps between them, filled with pitch or tar, which stays soft enough to be squeezed as the slabs expand.
Glass and hot water. Pouring boiling water into a thick glass tumbler can crack it. The inner surface heats and expands at once while the outer surface is still cold, and the unequal expansion sets up a stress that the glass cannot take. A thin glass heats right through almost immediately, so the two surfaces stay nearly equal, and heat-resistant glassware expands very little for a given temperature rise.
The same reason explains the telegraph-wire hum and the creaking of a roof. A metal roof sheet expands in the morning sun and slips slightly against its fastening, and the small sudden movements are heard as creaks.
Worked example — why the gap has to be there. Suppose a rail is warmed from a night temperature of to an afternoon temperature of . That is a rise of
Steel expands only slightly for such a rise, but a rail is long and there are many rails in a kilometre, so the total movement along a stretch of track is what matters. A tiny expansion repeated along a long structure is a large expansion, and that is exactly why the problem appears in rails, bridges and pipelines rather than in a spoon.
Prevention never means stopping the expansion. Nothing can prevent a heated metal from growing; the engineering choice is only where the growth is allowed to happen. A design that tries to hold it back converts the expansion into a force, and that force is what buckles a rail or cracks a bridge.
Exam tip
Exam tip: add 273 for a temperature, never for a change
** for a temperature. For a change**, the number is the same on both scales — a rise of is a rise of K.
Use kelvin whenever a ratio is involved. to is K to K, a doubling.
Heat is in joules, temperature in kelvin. State the unit, and never say a body "has more temperature".
Two bodies at the same temperature can hold different heat — the bucket and the cup. Say mass when explaining it.
Learn the order: gas expands most, then liquid, then solid, and give intermolecular forces as the reason.
A hole in a heated plate gets BIGGER. This is asked directly.
For a bimetallic strip, name the metals and say which is on the outside — brass expands more, so brass is on the convex side on heating.
For riveting and the cart wheel, the grip comes from CONTRACTION on cooling, not from the heating.
Say what would happen without the remedy: rails would buckle, wires would snap, a fixed bridge would crack.
Overhead wires sag in summer, and that sag is deliberate.
And for the cracked tumbler, say unequal expansion of the inner and outer surfaces — thin glass is safer because it heats through at once.
Use kelvin whenever a ratio is involved. to is K to K, a doubling.
Heat is in joules, temperature in kelvin. State the unit, and never say a body "has more temperature".
Two bodies at the same temperature can hold different heat — the bucket and the cup. Say mass when explaining it.
Learn the order: gas expands most, then liquid, then solid, and give intermolecular forces as the reason.
A hole in a heated plate gets BIGGER. This is asked directly.
For a bimetallic strip, name the metals and say which is on the outside — brass expands more, so brass is on the convex side on heating.
For riveting and the cart wheel, the grip comes from CONTRACTION on cooling, not from the heating.
Say what would happen without the remedy: rails would buckle, wires would snap, a fixed bridge would crack.
Overhead wires sag in summer, and that sag is deliberate.
And for the cracked tumbler, say unequal expansion of the inner and outer surfaces — thin glass is safer because it heats through at once.
Did you know
Why a hole gets bigger when you heat the metal around it
Take a flat metal plate with a circular hole punched in the middle and heat it. Does the hole shrink as the metal grows into it, or widen?
It widens.
The reason becomes obvious with one change of viewpoint. Imagine the disc of metal that was punched out, still sitting in place. On heating, that disc would expand — and the plate around it expands by exactly the same proportion, since it is the same material at the same temperature. So the boundary between them moves outward either way. Taking the disc away changes nothing about how the surrounding metal behaves.
Put differently: every distance marked on a heated body grows in the same proportion, whether there is metal along that distance or empty space. The diameter of the hole is such a distance.
That is why running hot water over a stubborn metal lid loosens it. The lid's diameter grows, the glass jar beneath expands far less, and the grip eases. It is also why a workshop heats a ring to slide it onto a shaft, rather than heating the shaft.
And it explains a trick worth knowing. A metal ring stuck tightly on a finger or a rod cannot be freed by heating the whole assembly, because both parts grow together. Heating only the outer ring is what works, and that is a statement about which part you warm, not about the physics of holes.
The cart wheel of the previous section is the same idea deliberately reversed. The rim is heated so that its inner diameter grows enough to pass over the wheel, and then cooled so that the diameter shrinks back and clamps tight. One hole, expanded on purpose and contracted on purpose, doing a job that no amount of hammering could.
It widens.
The reason becomes obvious with one change of viewpoint. Imagine the disc of metal that was punched out, still sitting in place. On heating, that disc would expand — and the plate around it expands by exactly the same proportion, since it is the same material at the same temperature. So the boundary between them moves outward either way. Taking the disc away changes nothing about how the surrounding metal behaves.
Put differently: every distance marked on a heated body grows in the same proportion, whether there is metal along that distance or empty space. The diameter of the hole is such a distance.
That is why running hot water over a stubborn metal lid loosens it. The lid's diameter grows, the glass jar beneath expands far less, and the grip eases. It is also why a workshop heats a ring to slide it onto a shaft, rather than heating the shaft.
And it explains a trick worth knowing. A metal ring stuck tightly on a finger or a rod cannot be freed by heating the whole assembly, because both parts grow together. Heating only the outer ring is what works, and that is a statement about which part you warm, not about the physics of holes.
The cart wheel of the previous section is the same idea deliberately reversed. The rim is heated so that its inner diameter grows enough to pass over the wheel, and then cooled so that the diameter shrinks back and clamps tight. One hole, expanded on purpose and contracted on purpose, doing a job that no amount of hammering could.
Exam relevance
How does thermal expansion feed into JEE Main and NEET?
Because the Kelvin scale is compulsory in every gas calculation, and expansion becomes a formula-driven topic with three coefficients.
This is the foundation for Class 11 Physics Thermal Properties of Matter and Class 11 Chemistry States of Matter, both examined in JEE Main and NEET. The qualitative statements here become quantitative there:
with the linear, superficial and cubical coefficients related by and . The three kinds of expansion named on this page are exactly those three coefficients, and questions asking for one coefficient given another are a standard type.
The expanding hole reappears as a formal result. Class 11 shows that a cavity in a solid expands with the same as the solid itself, and the reasoning is the disc argument from the previous section. Assertion-reason questions on it appear in both papers precisely because the intuitive answer is wrong.
Thermal stress is the new topic that grows out of the rail-gap discussion. When expansion is prevented, the stress developed is
with the Young's modulus — which is the formal version of this page's closing point that a design holding back expansion converts it into a force. Numericals on the force in a clamped rod are recurring JEE Main material.
Kelvin is where the scale work pays off. The gas laws take their simple forms only in kelvin: Charles's law is constant and the ideal gas equation is , and using Celsius in either gives nonsense. The point made here — that ratios require an absolute zero — is the whole reason the Kelvin scale exists, and it is examined in Class 11 Chemistry as well.
Where heat and temperature separate formally. Class 11 introduces specific heat capacity, so that — and the bucket-and-cup comparison on this page is that equation with and fixed and varying. Calorimetry numericals in both exams begin with that distinction.
What the questions look like. For board work, expect distinguish heat from temperature with units, convert between the scales, compare the expansion of the three states with a reason, explain a bimetallic strip, riveting or a cart wheel rim, and describe the remedy for rails, bridges, pipelines or wires. These are reasoning questions and the marks are in the wording. For JEE Main and NEET, expect linear-expansion numericals, coefficient relations, thermal stress, cavity expansion, and gas-law problems needing kelvin.
How board and competitive emphasis differ. A board paper rewards a clearly explained application — which metal is on the outside, and why the rivet grips. A competitive paper assumes all of it and tests the arithmetic of , and and the stress formula.
The single trap that costs the most marks. Adding to a temperature difference. A rise from to is a rise of K, not K, and putting the wrong value into gives an expansion several times too large. The defence is to write the two temperatures in kelvin first and subtract them there — — so the cancels itself and can never be added twice.
This is the foundation for Class 11 Physics Thermal Properties of Matter and Class 11 Chemistry States of Matter, both examined in JEE Main and NEET. The qualitative statements here become quantitative there:
with the linear, superficial and cubical coefficients related by and . The three kinds of expansion named on this page are exactly those three coefficients, and questions asking for one coefficient given another are a standard type.
The expanding hole reappears as a formal result. Class 11 shows that a cavity in a solid expands with the same as the solid itself, and the reasoning is the disc argument from the previous section. Assertion-reason questions on it appear in both papers precisely because the intuitive answer is wrong.
Thermal stress is the new topic that grows out of the rail-gap discussion. When expansion is prevented, the stress developed is
with the Young's modulus — which is the formal version of this page's closing point that a design holding back expansion converts it into a force. Numericals on the force in a clamped rod are recurring JEE Main material.
Kelvin is where the scale work pays off. The gas laws take their simple forms only in kelvin: Charles's law is constant and the ideal gas equation is , and using Celsius in either gives nonsense. The point made here — that ratios require an absolute zero — is the whole reason the Kelvin scale exists, and it is examined in Class 11 Chemistry as well.
Where heat and temperature separate formally. Class 11 introduces specific heat capacity, so that — and the bucket-and-cup comparison on this page is that equation with and fixed and varying. Calorimetry numericals in both exams begin with that distinction.
What the questions look like. For board work, expect distinguish heat from temperature with units, convert between the scales, compare the expansion of the three states with a reason, explain a bimetallic strip, riveting or a cart wheel rim, and describe the remedy for rails, bridges, pipelines or wires. These are reasoning questions and the marks are in the wording. For JEE Main and NEET, expect linear-expansion numericals, coefficient relations, thermal stress, cavity expansion, and gas-law problems needing kelvin.
How board and competitive emphasis differ. A board paper rewards a clearly explained application — which metal is on the outside, and why the rivet grips. A competitive paper assumes all of it and tests the arithmetic of , and and the stress formula.
The single trap that costs the most marks. Adding to a temperature difference. A rise from to is a rise of K, not K, and putting the wrong value into gives an expansion several times too large. The defence is to write the two temperatures in kelvin first and subtract them there — — so the cancels itself and can never be added twice.
Key takeaways
Heat, temperature and thermal expansion: quick revision
- Heat is total thermal energy, in joules; temperature is the degree of hotness, in kelvin, and decides the direction of heat flow.
- ****: K, K, K, K, and K .
- **Absolute zero is K , so the Kelvin scale has no negative values.
- A temperature CHANGE is the same number on both scales** — a rise of is a rise of K.
- Ratios need kelvin: to is K to K, an exact doubling.
- Same temperature, different heat: a bucket and a cup of water at are equally hot and hold very different amounts of heat.
- Expansion order: gas > liquid > solid, because intermolecular forces are weakest in a gas and strongest in a solid.
- Solids expand in length, area and volume at once.
- Everyday cases: a hot metal lid loosens; a thermometer's liquid rises; a balloon swells in the sun.
- A thermometer works because the liquid expands more than the glass — the reading is the difference.
- A hole in a heated plate gets BIGGER, since every marked distance grows in proportion.
- Water is anomalous between and , contracting on heating — the next part of this chapter.
- Bimetallic strip: brass and iron riveted together; brass expands more, so on heating the strip bends with brass on the convex side. Used in thermostats, fire alarms and flashers.
- Riveting: red-hot rivets are hammered in and contract on cooling, pulling the plates tight.
- Cart wheel rim: made slightly small, heated to slip on, then cooled so it clamps the wheel.
- Railway tracks: gaps between rails, or they would buckle; fish-plates have oval holes.
- Bridges: one end on rollers, plus toothed expansion joints.
- Pipelines: loops or bends absorb the change in length.
- Overhead wires: strung with a deliberate sag in summer, or they would snap in winter.
- Concrete roads: slabs with gaps filled with pitch.
- A thick tumbler cracks with boiling water because the inner and outer surfaces expand unequally; thin glass heats through at once.
- A rail warming from to rises by ** K, and a small expansion repeated along a long structure becomes a large one.
- Prevention never stops the expansion** — it only chooses where the growth happens, since holding it back turns it into a force.
Find a metal jar lid that will not open, run hot water over the lid alone, and see which of the two materials you just made bigger.
- ****: K, K, K, K, and K .
- **Absolute zero is K , so the Kelvin scale has no negative values.
- A temperature CHANGE is the same number on both scales** — a rise of is a rise of K.
- Ratios need kelvin: to is K to K, an exact doubling.
- Same temperature, different heat: a bucket and a cup of water at are equally hot and hold very different amounts of heat.
- Expansion order: gas > liquid > solid, because intermolecular forces are weakest in a gas and strongest in a solid.
- Solids expand in length, area and volume at once.
- Everyday cases: a hot metal lid loosens; a thermometer's liquid rises; a balloon swells in the sun.
- A thermometer works because the liquid expands more than the glass — the reading is the difference.
- A hole in a heated plate gets BIGGER, since every marked distance grows in proportion.
- Water is anomalous between and , contracting on heating — the next part of this chapter.
- Bimetallic strip: brass and iron riveted together; brass expands more, so on heating the strip bends with brass on the convex side. Used in thermostats, fire alarms and flashers.
- Riveting: red-hot rivets are hammered in and contract on cooling, pulling the plates tight.
- Cart wheel rim: made slightly small, heated to slip on, then cooled so it clamps the wheel.
- Railway tracks: gaps between rails, or they would buckle; fish-plates have oval holes.
- Bridges: one end on rollers, plus toothed expansion joints.
- Pipelines: loops or bends absorb the change in length.
- Overhead wires: strung with a deliberate sag in summer, or they would snap in winter.
- Concrete roads: slabs with gaps filled with pitch.
- A thick tumbler cracks with boiling water because the inner and outer surfaces expand unequally; thin glass heats through at once.
- A rail warming from to rises by ** K, and a small expansion repeated along a long structure becomes a large one.
- Prevention never stops the expansion** — it only chooses where the growth happens, since holding it back turns it into a force.
Find a metal jar lid that will not open, run hot water over the lid alone, and see which of the two materials you just made bigger.