Heat Transfer | ICSE Class 8 Physics Notes

This note covers heat and temperature, methods of heat transfer, boiling and evaporation, factors affecting evaporation, cooling by evaporation, thermal expansion, linear and superficial expansion, expansion of liquids and gases, and everyday applications.

What are heat and temperature, and what can heating change?

Temperature is a measure of the hotness or coldness of an object. An object at a higher temperature is hotter than one at a lower temperature. A thermometer is an instrument used to measure temperature.

Heat is energy transferred between objects, or between an object and its surroundings, because of a temperature difference. The surroundings are the materials and objects around the object being considered. Heat and temperature therefore describe different things.

Definition: Heat is energy transferred because of a temperature difference; temperature indicates how hot or cold an object is.

Which units are used?

SI stands for International System of Units. The SI unit of heat is the joule, written J. The SI unit of temperature is the kelvin, written K. The degree Celsius, written °C, is another commonly used temperature unit.

A cup of hot tea left on a table cools as heat passes to its cooler surroundings. A glass of ice-cold water on a hot summer day gains heat from its warmer surroundings. Heat transfer continues until the object and surroundings reach the same temperature.

What are the possible effects of heat?

Heating may raise an object's temperature, increase its size through expansion, or change its state. A state is a form of matter, such as solid, liquid or gas. A change of state changes one of these forms into another.

These effects must be distinguished. Water becoming hotter involves a temperature rise. A metal lid becoming larger involves expansion. Ice turning into liquid water involves a change of state. During a change of state, heat can be supplied without raising the temperature.

When temperature increases, the particles making up matter gain energy and move faster. This helps explain why heating can affect matter. However, a description of any particular observation must identify whether the observed change concerns temperature, size or state.

How does heat travel by conduction, convection and radiation?

Conduction transfers heat from a hotter part of an object to a colder part. A heated particle passes energy to its neighbour, which passes energy onwards. The particles do not themselves travel from the hot end to the cold end.

In solids, heat transfer takes place mainly through conduction. Materials that let heat pass through easily are good conductors of heat. Metals are examples. Materials that do not let heat pass through easily are poor conductors, also called insulators.

How does moving material carry heat?

Convection transfers heat through the actual movement of particles in liquids and gases. Water heated at the bottom of a beaker expands and rises. Comparatively cooler water at the sides moves down to take its place.

The descending water is then heated and rises in turn. This continuing movement heats the water throughout the beaker. Hot air also expands and rises. Convection therefore involves movement of the material that carries the heat.

Which method needs no material medium?

A medium is the material through which a transfer takes place. Conduction and convection require a material medium. Radiation is heat transfer that does not require a material medium. Heat from the Sun reaches the Earth by radiation.

All objects exchange heat with their surroundings through radiation. A hot utensil taken away from a flame radiates heat to its surroundings. The warmth felt around a fireplace is another example of heat reaching us by radiation.

The material and the method both matter. Air is a poor conductor. Air trapped in woollen fabric reduces heat flow from the body to the surroundings. This explains why woollen clothes help us remain warm without themselves being a source of heat.

What happens when a liquid boils?

Boiling is the change of a liquid into vapour throughout its volume at its boiling temperature. Vapour is the gaseous state of the substance. Vaporisation is the broader name for the change from liquid to vapour.

The boiling point is the temperature at which liquid and vapour coexist during boiling. For a given liquid at a fixed pressure, boiling occurs at a fixed temperature. Pressure means force acting at right angles to a surface per unit area; the pressure of the surrounding air affects boiling.

The SI unit of pressure is the pascal, written Pa, equivalent to a newton per square metre, $\mathrm{N\,m^{-2}}$.

What can be observed while water is heated?

1. At first, some small bubbles contain air that was dissolved in the water and is now escaping.
2. Later, bubbles of water vapour form near the heated bottom. In cooler water above, these bubbles can change back into liquid and disappear.
3. When the water reaches its boiling temperature, vapour bubbles reach the surface and boiling occurs throughout the liquid.
4. Continued heating changes liquid water into vapour while the boiling temperature remains constant, provided the pressure remains constant.

Condensation is the change from vapour to liquid. It explains why early vapour bubbles can disappear in cooler water. Water vapour itself is invisible. The foggy appearance near escaping steam comes from tiny liquid droplets formed by condensation.

Why does pressure need to be stated?

Pure water boils at 100 °C under standard atmospheric pressure, the standard reference pressure used for the surrounding atmosphere. Increasing pressure raises its boiling point. Decreasing pressure lowers it. The boiling point at standard atmospheric pressure is called the normal boiling point.

At high altitudes, atmospheric pressure is lower and water boils at a lower temperature than at sea level. Cooking is consequently more difficult. A pressure cooker raises the pressure and the boiling point of water, allowing faster cooking.

Note: Water does not boil at 100 °C under every pressure. State the pressure condition when giving its boiling temperature, and distinguish the first air bubbles from bubbles of water vapour.

How is evaporation different from boiling?

Evaporation is the conversion of a liquid into vapour at its surface. The surface is the boundary exposed to the air. Particles escape from this boundary while the rest of the liquid remains below it.

Evaporation occurs at all temperatures at which the substance is liquid. It happens even at room temperature. Water left on a washed utensil dries, a mopped floor becomes dry, and wet clothes lose water without the water having to boil.

Where do the changes occur?

In evaporation, particles at the surface escape to form gas; other particles inside the liquid do not have enough energy to escape in the same way. Boiling involves the whole volume of liquid at the boiling temperature.

Both processes change liquid into gas, but they differ in where and when that change occurs. The appearance of vapour does not by itself prove that boiling has occurred. Drying water at room temperature is an example of evaporation.

Does heating always increase temperature?

Melting, also called fusion, changes a solid into liquid. Freezing changes a liquid into solid. When ice melts at its melting temperature, the supplied heat changes its state. After the ice has melted, further heating raises the water's temperature.

The temperature becomes steady again when the water boils under constant pressure. Heat is then used in changing liquid water into vapour. A steady temperature can therefore accompany continuing heat transfer.

The vertical axis shows temperature in degrees Celsius and the horizontal axis shows time in minutes. The line is initially horizontal at 0 °C, rises to 100 °C, and becomes horizontal again. A dotted rising continuation follows. The figure is not to scale.

See Fig. 10.9 in your NCERT textbook

Which conditions make evaporation faster or slower?

The rate of evaporation means how quickly a liquid changes into vapour. Exposed surface area, temperature, air movement and humidity affect this rate. Humidity means the amount of water vapour present in the air.

A larger exposed surface makes evaporation faster. Water spread on a plate has more surface exposed to air than the same amount in a bottle cap. The comparison concerns exposed area, rather than simply the amount of water present.

How can the effect of surface area be investigated?

1. Take some water in a small bottle cap.
2. Put the same amount of water on a plate, where it has a larger exposed area.
3. Keep the cap and plate near each other so that they experience similar surroundings.
4. Record the time each takes to lose its water completely and compare the observations.

Water on the plate evaporates faster. A useful comparison changes the feature being investigated while keeping the other relevant conditions the same. Otherwise, a difference in drying time could be due to more than the intended change.

What roles do sunlight, wind and humidity play?

Water evaporates faster in sunlight than in shade. Wet clothes also dry faster on a hot sunny day. Greater air movement makes evaporation faster, which explains why clothes dry faster on a windy day.

When the air already contains more water vapour, evaporation is slower. Rainy days are more humid, and clothes dry slowly. Humidity and air movement must therefore be considered along with temperature when explaining drying.

The illustration shows one bottle cap on a shaded veranda and another outside in sunlight. An enlarged view shows a cap containing water. The Sun, house and tree establish the shaded and sunny surroundings.

See Fig. 8.6 in your NCERT textbook

For the sunlight comparison, use identical caps and equal amounts of water. Observe them every 15 minutes and record the time taken for complete evaporation. These are observation intervals, not a claim that all the water disappears in 15 minutes.

How does evaporation produce cooling in everyday life?

Evaporation causes cooling. Water changing into vapour needs heat. When evaporation takes place from a wet surface, the process can cool that surface. This connects a change of state with everyday experiences such as sweating and cool water in an earthen pot.

Why does an earthen pot keep water cool?

In an earthen pot, water seeps through the pot's surface and evaporates. This produces a cooling effect on the water. The explanation involves the escape and evaporation of water, rather than a supply of coldness from the pot.

Sprinkling water on a floor or roof during summer is another example of cooling by evaporation. The water changes into vapour, and the wet surface becomes cooler. The change of state is essential to the explanation.

Why can a fan help both cooling and drying?

Sweat evaporating from the body helps cool it. Air movement from a fan helps the sweat evaporate. Faster evaporation explains the cooling sensation, even though a fan does not make the sweat boil.

The same connection explains why moving air helps wet clothes dry. Drying concerns water leaving the clothes as vapour, while cooling concerns the heat needed for that change. These effects can occur together.

A pot-in-pot cooler is a cooling arrangement with a smaller earthen pot inside a larger one, with wet sand between them. Water must be added regularly to keep the sand moist. The arrangement uses evaporative cooling to help keep its contents cool.

The surrounding conditions still matter. High humidity slows evaporation, whereas greater air movement makes it faster. An explanation of evaporation should therefore connect the liquid's change of state with the conditions in which the change occurs.

What is thermal expansion, and what are its three forms?

Thermal expansion is an increase in an object's dimensions because its temperature increases. Dimensions describe its size, such as length, area or volume. Contraction is a decrease in size. Most substances expand on heating and contract on cooling.

The word most matters. Expansion on heating is a general behaviour with exceptions, rather than a rule without exceptions. Water over a particular temperature range provides an important example of unusual behaviour.

Which dimension is being considered?

Linear expansion is expansion in length. Superficial expansion, also called area expansion, is expansion in area. Area measures the extent of a surface. Volume expansion is expansion in volume, which measures the space occupied by a body.

These names specify the size change being studied. Calling a rod's length change linear expansion does not mean that its other dimensions cannot change. A sheet can increase in area because its length and breadth both increase.

Three panels show a rod increasing in length, a square surface increasing in area, and a block increasing in volume. The panels are labelled linear expansion, area expansion and volume expansion.

See Fig. 10.5 in your NCERT textbook

How is expansion different from a change of state?

A heated metal lid can expand while remaining solid. A liquid in a thermometer can rise as its volume increases while remaining liquid. Expansion describes a change in size; melting and vaporisation describe changes of state.

In many heating observations, expansion occurs before a change of state. Identify the actual observation before naming the effect. A larger solid object shows expansion, while the production of liquid from a solid shows melting.

Heating also increases the energy of the particles of matter. This links thermal behaviour with particle motion. However, the observable measurements remain distinct: temperature describes hotness, while length, area and volume describe size.

How does expansion of solids explain familiar applications?

A solid's expansion can be small yet useful. Copper expands more than glass for the same temperature rise. A rise in temperature can therefore alter the fit between solid objects, even though they remain in the solid state.

Why can warming a metal lid make it easier to open?

A tightly screwed metallic lid can be loosened by placing the lid in hot water for some time. The lid expands on heating, making it easier to unscrew. This is an application of thermal expansion.

The explanation does not require the lid to melt. The useful change is in its size. This separates expansion from a change of state and connects a visible practical result with heating a solid.

Why is an iron ring heated before fitting a wheel?

A blacksmith heats an iron ring before fitting it around the rim of a wooden wheel. Heating increases the ring's size so that it can fit over the rim. The application uses the expansion of solid iron.

The rim is the outer edge of the wheel. In explaining the fitting process, identify the object being heated, the resulting change in its dimensions, and the reason that change helps the ring fit.

What if expansion is prevented?

A heated rod held rigidly at both ends cannot expand freely. Its supports exert forces on it, creating compression, or squeezing within the rod. With steel rails prevented from expanding, the forces can be large enough to bend the rails.

This illustrates why thermal expansion matters even when a size change seems small. The same tendency to expand that helps loosen a lid can create difficulties when movement is prevented. The result depends on how the heated solid is supported.

For a complete explanation, connect the temperature rise to expansion and then to the practical effect. Merely stating that a solid becomes hot does not explain why a lid loosens, a ring fits or a restrained rail can bend.

How do liquids expand, and why is water an exception?

For a liquid, thermal expansion is considered as a change in volume. Liquids take the shape of their containers, so an increase in volume can appear as a rise in liquid level. This behaviour is useful in liquid-in-glass thermometers.

Such a thermometer contains a liquid whose volume changes with temperature. Mercury or alcohol may be used. When a mercury thermometer is placed in slightly warm water, the mercury level rises; after removal, the level falls again as it cools.

Do all liquids expand by the same amount?

Different liquids do not expand equally for the same temperature rise. Alcohol, specifically ethanol, expands more than mercury for the same rise. The nature of the liquid therefore matters, as well as the temperature change.

Density is mass per unit volume. Mass means the quantity of matter in a body. For a given mass, occupying a larger volume means a lower density. Occupying a smaller volume means a higher density.

The SI unit of mass is the kilogram, written kg. The SI unit of density is the kilogram per cubic metre, written $\mathrm{kg\,m^{-3}}$.

What happens to water between 0 °C and 4 °C?

Water shows anomalous expansion, meaning expansion behaviour that differs from the usual pattern. It contracts when heated from 0 °C to 4 °C. A given amount then occupies less volume as its temperature rises through this interval.

Water has its maximum density at 4 °C. When water is cooled from room temperature towards 4 °C, its volume decreases. When cooled below 4 °C towards freezing, its volume increases and its density decreases.

Two graphs place temperature on the horizontal axis. The volume graph falls to a minimum near 4 °C and then rises. The density graph rises to a maximum near 4 °C and then falls.

See Fig. 10.7 in your NCERT textbook

This behaviour helps explain why lakes and ponds freeze at the top first. Surface water cooling towards 4 °C becomes denser and sinks. Once the surface water cools below 4 °C, it becomes less dense, stays above the water beneath, and eventually freezes.

The exception has a definite temperature range. Do not replace “most substances expand on heating” with “all substances expand on heating”, or describe water as contracting whenever it is heated.

How does gas expansion compare with expansion of solids and liquids?

At ordinary temperatures, gases expand more than solids and liquids. The broad comparison is that thermal expansion is least in solids, greater in liquids and greatest in gases. Specific materials and conditions still matter when making a comparison.

What happens to air in a balloon?

A balloon partially inflated in a cool room may expand to full size when placed in warm water. The air inside expands as it is heated. The balloon's larger size makes this volume change visible.

A fully inflated balloon placed in cold water starts shrinking because the air inside contracts. This gives a paired observation: heating can increase the volume of the enclosed gas, while cooling can reduce it.

A partially inflated balloon placed in the Sun also becomes larger as the air warms and expands. These examples show expansion of a gas, rather than a solid or liquid changing into gas.

How should the three states be compared?

Expansivity describes how much a substance expands for a temperature rise. Comparing expansivity means comparing the response to heating. It does not mean comparing the starting sizes of unrelated objects.

Keep the distinction between expansion and vaporisation clear. A gas can expand while remaining a gas. Liquid water becoming water vapour is a change of state. Both can be associated with heating, but they are different observations.

The connection with convection is also useful. Heated air expands and rises; cooler air moves into its place. Expansion can therefore help explain movement during heat transfer as well as a change in the size of an enclosed gas.

Glossary

• Heat — Energy transferred between objects or their surroundings because their temperatures are different.
• Temperature — A measure of the hotness or coldness of an object.
• Conduction — Heat transfer between neighbouring particles without those particles travelling through the material.
• Convection — Heat transfer through the actual movement of particles in liquids and gases.
• Radiation — Heat transfer that can occur without any material medium between objects.
• Boiling — Conversion of liquid into vapour throughout its volume at its boiling temperature.
• Evaporation — Conversion of liquid into vapour from its surface, including at room temperature.
• Condensation — Conversion of vapour into liquid, as when water droplets form on a cold surface.
• Humidity — The amount of water vapour present in the surrounding air.
• Thermal expansion — Increase in the dimensions of a body because its temperature increases.
• Linear expansion — Increase in the length of a body as its temperature rises.
• Superficial expansion — Increase in the area of a body as its temperature rises.
• Volume expansion — Increase in the space occupied by a body as its temperature rises.
• Density — Mass per unit volume, relating the quantity of matter to the space it occupies.
• Anomalous expansion of water — Water's unusual behaviour of contracting on heating from 0 °C to 4 °C.

Common errors and misconceptions

• Misconception: Heat and temperature are two names for the same thing. Correct: Heat is energy transferred because of a temperature difference; temperature measures hotness or coldness. Their SI units are different.
• Misconception: Water must boil before it can become vapour. Correct: Evaporation occurs at the surface even at room temperature. Drying clothes provides an everyday example of this change.
• Misconception: Boiling happens at the surface just like evaporation. Correct: Boiling involves the whole volume of the liquid. Vapour bubbles form within it and reach the surface.
• Misconception: Supplying heat must keep raising temperature. Correct: During boiling at constant pressure, supplied heat changes liquid into vapour while the temperature remains constant.
• Misconception: Water boils at 100 °C at every location and pressure. Correct: This is its normal boiling point at standard atmospheric pressure. Lower pressure lowers the boiling point.
• Misconception: Every substance expands whenever it is heated. Correct: Most substances expand on heating. Water contracts when heated from 0 °C to 4 °C and is densest at 4 °C.
• Misconception: A balloon growing larger in warm water shows liquid changing into gas. Correct: The air already inside the balloon expands. It remains a gas during this size change.
• Misconception: All three methods of heat transfer require material between the objects. Correct: Conduction and convection require a medium, but radiation does not. Solar heat reaches the Earth by radiation.

Exam-style questions with model answers

Q1. Distinguish between heat and temperature by defining each. [2 marks]

1. Heat is energy transferred between objects, or between an object and its surroundings, because of a temperature difference.
2. Temperature is a measure of the hotness or coldness of an object.

Q2. Compare boiling and evaporation in terms of where they occur, their temperature requirements, and particle behaviour. Assume a fixed pressure when describing boiling. [3 marks]

1. Evaporation takes place at the liquid's exposed surface, whereas boiling takes place throughout the volume of the liquid.
2. Evaporation can occur at all temperatures at which the substance is liquid, including room temperature. Boiling occurs at a fixed boiling point for the stated pressure.
3. During evaporation, surface particles escape into the gaseous state. During boiling, vapour bubbles form within the liquid and reach its surface.

Q3. Equal amounts of water are placed in a bottle cap and on a plate, kept beside each other under the same conditions. The plate exposes a larger water surface. Explain which dries first, identify the changed factor and a kept-same factor, and state what should be measured. [4 marks]

1. The water on the plate dries first because its larger exposed surface makes evaporation faster.
2. The factor deliberately changed is the area of water exposed to the surrounding air.
3. The amount of water is kept the same, allowing the effect of exposed area to be compared under the same surrounding conditions.
4. Measure and record the time taken for the water in each container to evaporate completely.

Q4. Define thermal expansion and distinguish linear, superficial and volume expansion by naming the dimension involved in each. [4 marks]

1. Thermal expansion is an increase in a body's dimensions because its temperature rises. Most substances expand when heated.
2. Linear expansion is an increase in length, such as the length change considered for a heated rod.
3. Superficial expansion, also called area expansion, is an increase in surface area, as when a sheet's length and breadth increase.
4. Volume expansion is an increase in the space occupied by a body, as represented by an expanding block.

Q5. Water is heated from 0 °C to 4 °C. Describe its volume and density changes, state the temperature of maximum density, explain why this is an exception to usual expansion, and describe what happens when it cools from 4 °C towards freezing. [5 marks]

1. When heated from 0 °C to 4 °C, water contracts. A given amount of it therefore occupies a smaller volume as its temperature rises.
2. Its density increases during this heating interval because the same mass is contained in a decreasing volume.
3. Water has its maximum density at 4 °C, where a given mass occupies its minimum volume.
4. This is an exception because most substances expand on heating, whereas water contracts over this particular temperature range.
5. When water cools from 4 °C towards freezing, its volume increases and its density decreases. This reversal is part of its anomalous expansion behaviour.

Q6. Pure water is heated at constant standard atmospheric pressure. Its normal boiling point is 100 °C. Explain the first small air bubbles, later vapour bubbles that disappear, sustained boiling, the temperature during continued boiling, and the foggy appearance near escaping steam. [5 marks]

1. The first small bubbles can be air that was dissolved in the water and comes out as the water is heated.
2. Later, vapour bubbles form near the heated bottom. When they rise into cooler water, they condense into liquid and disappear.
3. At the stated boiling point of 100 °C, vapour bubbles reach the surface and boiling occurs throughout the liquid.
4. During continued boiling at the stated constant pressure, temperature remains constant because the supplied heat changes liquid water into vapour.
5. Water vapour itself is invisible. The foggy appearance near escaping steam is produced by tiny water droplets formed when the vapour condenses.

Q7. A tightly screwed metallic lid is placed in hot water for some time. Explain why it becomes easier to unscrew, naming the heated part, its size change, and the practical effect. [3 marks]

1. The metallic lid is the part being warmed by placing it in hot water. Its temperature rises as it receives heat.
2. The metal undergoes thermal expansion, so the dimensions of the lid increase. This is a size change while the lid remains solid.
3. The expanded lid becomes loosened, making it easier to unscrew. The useful effect therefore follows from the increased size of the heated lid.

Q8. Explain how each of these affects evaporation or its cooling effect: increasing exposed area, placing water in sunlight rather than shade, increasing air movement, increasing humidity, and allowing water to seep through an earthen pot. Keep other relevant conditions the same in each comparison. [5 marks]

1. Increasing exposed surface area makes evaporation faster, as shown by equal amounts of water spread on a plate and held in a bottle cap.
2. Water placed in sunlight evaporates faster than water placed in shade, as shown using equal amounts in identical bottle caps.
3. Increasing air movement makes evaporation faster. This helps wet clothes dry and helps sweat evaporate from the body.
4. Increasing humidity slows evaporation because the surrounding air already contains more water vapour. Clothes therefore dry slowly on humid rainy days.
5. Water seeping through an earthen pot evaporates from its surface. This evaporation produces a cooling effect on the water in the pot.

Key takeaways

• Heat is energy transferred because of a temperature difference; temperature indicates how hot or cold an object is.
• Conduction and convection need a material medium, whereas radiation transfers heat without requiring a material medium.
• Boiling occurs throughout a liquid at its boiling point for the given pressure; evaporation occurs at its surface.
• Larger exposed area, sunlight and greater air movement make evaporation faster; greater humidity makes it slower.
• Evaporation produces cooling, helping explain cool water in earthen pots and the cooling effect of evaporating sweat.
• Most substances expand on heating. Linear, superficial and volume expansion describe changes in length, area and volume respectively.
• Water contracts when heated from 0 °C to 4 °C and has its maximum density at 4 °C.
• At ordinary temperatures, gases expand more than solids and liquids; heating air in a balloon makes its expansion visible.

Test yourself

What distinguishes heat from temperature?

Heat is energy transferred because of a temperature difference; temperature measures an object's hotness or coldness.

Why can a mopped floor dry without its water boiling?

Water evaporates from the floor's exposed surface even at room temperature. It does not need to reach its boiling point.

What pressure condition belongs with water's normal boiling point of 100 °C?

Pure water has this normal boiling point under standard atmospheric pressure. Its boiling point changes when the pressure changes.

What is another name for superficial expansion?

Superficial expansion is area expansion: an increase in a body's area as its temperature rises.

Does water expand or contract when heated from 0 °C to 4 °C?

Water contracts over this interval. Its volume decreases and its density increases, reaching maximum density at 4 °C.

Why does a balloon partially inflated in a cool room become larger in warm water?

The air inside gains heat and expands, increasing its volume. The air remains a gas during this change.

Why do wet clothes dry slowly on a humid day?

Humid air already contains more water vapour, so evaporation is slower and the clothes take longer to dry.

Which method carries heat from the Sun to the Earth?

Radiation carries solar heat to the Earth and does not require a material medium for its transfer.