Transfer of Heat

• P – Point → In the above paragraph, I point out to the theory of: Heat being a form of energy rather than a fluid.

• E – Evidence → The evidence that supports the theory is: Heat is measured in joules, makes particles move faster, and can cause materials to expand.

• E – Explanation → Now I explain my point: Heat energy alters particle movement to raise temperatures and change states of matter. This energy transfers through three distinct methods: conduction, convection, and radiation.

• L – Link → I link my knowledge of transfer of heat to: Practical everyday applications like cooking food and engineering expansion joints in bridges.

1. Shanaya noticed that the ice melted quickly in summer. Due to heat, ice absorbs heat from the surroundings and changes into water. This change of a solid into a liquid is called melting.

2. According to Shanaya's grandmother, heat can change the state of matter in two ways. It can make solids turn into liquids, like ice changing into water (melting), and it can make liquids like water turn into gas, which is called evaporation.

3. When Shanaya picked up the metal spoon, it felt very hot and she quickly pulled her hand back. This happened because metal allows heat to travel through it easily. The heat moved from the hot soup to the spoon and then to her hand by conduction.

4. Conduction is the process by which heat travels easily through a solid, such as metal. In the story, heat moves from the hot soup to the metal spoon and then to Shanaya's hand by conduction.

5. After learning about the effects of heat, Shanaya felt amazed and was eager to learn more. This tells us that she is a curious child who is interested in understanding the science behind everyday things.

1. Sweeping
   Reason — Melting, boiling and freezing are changes of state caused by adding or removing heat, whereas sweeping is a cleaning activity and is not an effect of heat.
2. Drawing
   Reason — Expansion, evaporation and conduction are all effects or processes related to heat, whereas drawing has nothing to do with heat.
3. Stone
   Reason — Ice, steam and water are the three states of the same substance (water), whereas stone is a different substance.
4. Ruler
   Reason — Thermometer, temperature and heat are all related to thermal phenomena, whereas a ruler is used to measure length.
5. Fan
   Reason — The Sun, a candle and a heater are sources of heat, whereas a fan does not produce heat; it only gives a cooling effect.

1. Heat is a form of energy which results in the sensation of hotness or coldness.
2. One calorie is the amount of heat required to raise the temperature of 1 g of water by 1°C.
3. The higher the temperature, the greater the rate of evaporation.
4. To allow for thermal expansion and contraction.
5. Generally, the density of a substance decreases with an increase in temperature. Water is an exception from 0°C to 4°C, where its density increases and is maximum at 4°C.

1. The change of a substance from its liquid state to its gaseous state by absorbing heat from the surroundings at any temperature below its boiling point is called evaporation.
2. When a solid is heated, there is an increase in the average kinetic energy of the molecules of the solid.
3. There is more increase in the volume of a liquid when there is a greater rise in temperature of a liquid.
4. The molecules in gases possess negligible intermolecular forces of attraction as compared to solids and liquids.
5. Bimetallic strips are used in electrical devices, thermometers and heat engines to control temperature.

1. True
2. False
   Corrected Statement — The increase in the area of a solid when heat is supplied to it is called superficial expansion.
3. False
   Corrected Statement — Boiling takes place at a particular fixed temperature called the boiling point. (It is evaporation that takes place at all temperatures.)
4. False
   Corrected Statement — Sublimation is the process of change of a solid directly into its gaseous state, without changing to its liquid state.
5. True

1. Ice melts when it absorbs heat from the surroundings.
2. Heat flows from a hotter object to a colder object.
3. A thermometer is used to measure the temperature of an object.
4. Condensation is the process wherein a gas (vapour) changes into liquid.
5. Metals are good conductors of heat.

During solidification, the molecules of the liquid lose heat energy to the surroundings. As a result, they come closer and arrange themselves into a fixed pattern. This change from liquid to solid takes place at a fixed temperature called the freezing point.

When perfume or spirit is sprayed on the hand, it evaporates by absorbing heat from the surroundings, including from the hand. As heat is taken away from the hand, the temperature of the hand decreases, which gives a cool sensation.

At mountains, the atmospheric pressure is lower than at the plains. Since the boiling point of a liquid decreases with a decrease in atmospheric pressure, water boils at a lower temperature on mountains. As the food cooks at this lower temperature, it takes longer to cook vegetables or boil rice.

Steel expands in summer and contracts in winter. A gap is left between successive rail tracks so that the tracks have space to expand in the summer. If no gap is left, the tracks would bend outwards due to expansion, which may lead to the derailment of a train.

Liquids expand on heating. In summer, the liquid in a bottle gets hot and expands. If the bottle is filled to the top, there is no space for the liquid to expand and it would overflow. Therefore, some space is left so that the liquid can expand without spilling.

On heating, a substance can change its state, for example a solid can melt into a liquid and a liquid can change into a gas. Heat also increases the size of a substance, as most substances expand (their length, area and volume increase) on heating. On cooling, the reverse changes take place.

Vaporisation (boiling) is the change of a substance from its liquid state into its vapour state (gas) at a fixed temperature by absorbing heat. For example, the change of water into steam is vaporisation.

The factors affecting the cubical expansion of liquids are:

1. Original volume of the liquid.
2. Rise in temperature of the liquid.
3. Nature of the liquid.

Sublimation is the change of a substance from its solid state directly into its gaseous state, without changing into the liquid state. For example, camphor and naphthalene undergo sublimation.

A thermometer uses a thermometric liquid which has the ability to expand on heating. When the bulb is placed in contact with a body, the liquid expands and rises in the capillary tube. The temperature of the body is read from the scale graduation corresponding to the level of the liquid.

Matter is made up of molecules which are constantly vibrating. When a substance is heated, its molecules gain energy and start vibrating faster. Because of this, the intermolecular distance between the molecules increases, which causes the substance to expand.

The extent of expansion is different for the three states of matter:

1. Solids expand the least, because their molecules are tightly packed with strong intermolecular forces.
2. Liquids expand more than solids, as their molecules are less tightly packed.
3. Gases expand the most, because their molecules possess negligible intermolecular forces of attraction.

The factors that affect evaporation are:

1. Area of the exposed surface — The greater the exposed surface area of the liquid, the faster is the evaporation.
2. Temperature of the liquid — The higher the temperature of the liquid, the greater the rate of evaporation.
3. Temperature of the surroundings — Evaporation increases when the temperature of the surroundings is higher.
4. Nature of the liquid — Different liquids have different rates of evaporation.
5. Presence of wind — The presence of wind increases the rate of evaporation.
6. Presence of moisture in the air — Evaporation slows down when there is more moisture in the air.

There are three types of expansion in solids.

1. Linear expansion — It is the increase in the length of a solid when it is heated. For example, a metal rod becomes slightly longer on heating.
2. Superficial expansion — It is the increase in the area of a solid when heat is supplied to it. For example, a metal plate increases in area on heating.
3. Cubical expansion — It is the increase in the volume of a solid on heating. For example, a metal cube increases in volume on heating.

For a given solid, the coefficients of linear, superficial and cubical expansion are in the ratio α : β : γ = 1 : 2 : 3.

Some applications of expansion of solids in daily life are:

1. A small gap is left between concrete slabs while making cement roads or floors to allow for their expansion.
2. Electricity and telephone wires are laid a little loose to allow for their expansion in summer and contraction in winter.
3. A gap is left between successive railway tracks to allow for the expansion of the steel tracks.
4. One end of a metal bridge is kept on rollers so that it is free to expand and contract.
5. Pipes carrying hot liquids are arranged in the form of loops to allow for their expansion and contraction.
6. Bimetallic strips are used in electrical devices, thermometers and heat engines to control temperature.
7. An iron rim is heated to expand it before fixing it on a wooden wheel; on cooling, it contracts and fits tightly.

1. Sam observed several everyday effects of heat: the air near the window felt hot because the sun heated the ground and air outside, the steel jug became warm due to conduction, and the water in the earthen pot became cool due to evaporation.

2. The steel jug became warm because metal is a good conductor of heat, so it quickly absorbed and passed on the heat. The earthen pot kept the water cool because its walls are porous; water seeped through the walls and evaporated, taking away heat and cooling the water inside.

3. The water slowly seeped through the porous walls of the earthen pot and evaporated from the surface. During evaporation, the water absorbed heat from the remaining water in the pot. This loss of heat lowered the temperature of the water, keeping it cool.

4. The steel jug became hot because heat travelled through the metal by conduction, since metal is a good conductor of heat. This shows that heat can travel through a material by conduction and cause a change in its temperature.

5. Sam learnt that heat has many effects in daily life. He understood that heat can travel through metals by conduction, that it can warm the air and objects, and that evaporation has a cooling effect. In this way, he connected the chapter on heat with his real-life experiences.

1. Thermal expansion is the property of matter in which the temperature of a substance is increased, resulting in the expansion of the substance.

2. In solids, on heating, the molecules gain kinetic energy and start vibrating faster about their mean positions. This increases the intermolecular distance between the molecules, causing the solid to expand in length, area and volume.

3. Building materials composed of aggregates, such as concrete and stone, should be used, as they do not shrink or expand significantly when heated or cooled.

Only the metal spoon became hot because metal is a good conductor of heat. It quickly conducted the heat from the hot water to its other end. The plastic spoon did not become hot because plastic is a bad conductor of heat and does not allow heat to pass through it easily. This tells us that metals are good conductors of heat, while plastic is a bad conductor (insulator) of heat.

When heat is applied, all three states of matter expand, but to different extents.

1. Solids expand the least, as their molecules are tightly packed. A solid can show linear, superficial and cubical expansion.
2. Liquids expand more than solids. A liquid shows only cubical (volume) expansion.
3. Gases expand the most, as their molecules have negligible intermolecular forces. A gas also shows only cubical expansion.

In addition, on continued heating, a solid can change into a liquid and a liquid can change into a gas.

When the sealed bottle was left in the sun, both the water and the glass bottle expanded on heating. Since water expands much more than glass and the bottle was completely filled and sealed, the expanding water exerted great pressure on the bottle. This unequal expansion of water and glass caused the bottle to crack.

I would store the drinking water in an earthen pot. The walls of an earthen pot are porous, so water slowly seeps through them to the outer surface and evaporates. During evaporation, the water absorbs heat from the water inside the pot. This loss of heat lowers the temperature of the water, keeping it cool even without a fridge.

Aim: To show that heat causes expansion in solids.

Materials required: A Bunsen burner, an iron stand, a metal ball attached to an iron chain, and a ring whose diameter is only slightly more than the ball, so that the ball can just pass through it.

Procedure:

1. First, allow the metal ball to pass through the ring. It passes easily.
2. Now heat the metal ball over the burner for some time.
3. Again try to pass the ball through the ring and observe.
4. Cool the ball and try once more.

Observation: After heating, the metal ball does not pass through the ring because its diameter increases on heating. After cooling, it passes through the ring again.

Conclusion: This shows that solids expand on heating and contract on cooling.

I would design the lunch box with double walls and air or a vacuum between them, as they are poor conductors of heat and reduce heat loss. The inner walls would be lined with a shiny layer, and the box would have a tight-fitting insulating lid. The outer body would be made of plastic, a bad conductor of heat. Thus, the lunch box would keep food warm for a long time by reducing heat transfer to the surroundings.

1. Heat is a form of energy which results in the sensation of hotness or coldness.

2. On heating, a substance can change its state. For example, a solid can change into a liquid and a liquid can change into a gas.

3. The measurement of how hot or cold an object is, is called temperature.

4. The three physical states — solid, liquid and gas — can change into one another due to heat.

5. When heat is supplied to water, it is absorbed by the water. On reaching its boiling point, the water changes into steam (vapour). Thus, heat is used to change the state of water from liquid to gas.

1. Factors affecting the rate of evaporation:

• Area of the exposed surface.
• Temperature of the liquid.
• Temperature of the surroundings.
• Nature of the liquid.
• Presence of wind.
• Presence of moisture (humidity) in the air.

2. Evaporation occurs faster in sunlight because sunlight increases the temperature of the liquid and of the surroundings. A higher temperature increases the rate of evaporation, so the wet towel in sunlight dries faster.

3. The increase in the size of materials when heated is called thermal expansion.

4. Engineers leave gaps between railway tracks to allow for the expansion of the steel tracks in summer. If no gaps are left, the tracks would bend due to expansion, which may cause derailment.

5. Gases show the maximum thermal expansion.

1. The metal ball activity shows that solids expand on heating, as the heated ball no longer passes through the ring.

2. Thermal expansion is the property of matter in which the temperature of a substance is increased, resulting in the expansion of the substance.

3. A thermometer is used to measure the temperature of a body.

4. A rise in the thermometer reading indicates an increase in the temperature of the body.

5. The metal ball did not pass through the ring after heating because its diameter increased on heating due to thermal expansion.

1. When the air inside a hot air balloon is heated, it expands and becomes less dense than the surrounding cool air. As a result, the buoyant force (upthrust) exerted by the surrounding air on the balloon becomes greater than its weight. Therefore, the balloon rises into the sky.

2. Gases expand more than solids and liquids because the molecules of a gas possess negligible intermolecular forces of attraction. On heating, they move vigorously in the available space, so the intermolecular distance increases much more than in solids and liquids.

3. In bridge construction, one end of a metal bridge is kept on rollers and special joints (gaps) are left so that the bridge is free to expand in summer and contract in winter without getting damaged.

4. If thermal expansion is not considered in engineering, structures such as railway tracks and bridges may bend, crack or get damaged due to the expansion of the material in hot weather.

5. The knowledge of heat and its effects helps in real life in designing safe structures like bridges and railway tracks, in making thermometers, in cooking, and in avoiding damage caused by thermal expansion.

1. Heat is a form of energy which flows from hotter body to colder body.
2. The expansion of substances due to heating is called thermal expansion.
3. The increase in area of a solid on heating is called superficial expansion.
4. One calorie is equal to 4.2 joules.
5. The change of state of a substance without a change in temperature is called change of state (interconversion of states of matter).

1. True
2. False
   Corrected Statement — Evaporation occurs only at the surface of a liquid, but boiling takes place throughout the bulk of the liquid.
3. False
   Corrected Statement — Railway tracks are laid with gaps to allow for expansion during summer.
4. False
   Corrected Statement — Heat and temperature are different physical quantities; heat is a form of energy, while temperature is the degree of hotness or coldness of a body.
5. False
   Corrected Statement — Expansion of gases on heating is more than that of solids.

Heat is a form of energy which results in the sensation of hotness or coldness. Temperature is the degree of hotness or coldness of a body.

The SI unit of heat is the joule (J). Two other units of heat are the calorie (cal) and the kilocalorie (kcal). Their relationships are:

1. 1 cal = 4.184 J ≈ 4.2 J
2. 1 kcal = 1000 cal
3. 1 kcal = 4184 J ≈ 4200 J
4. 1 J = 0.24 cal

The three effects produced by heat on a substance are:

1. Change in temperature.
2. Change in state of matter.
3. Change in dimensions (thermal expansion).

Gaps are left between railway tracks to allow for the expansion of the steel tracks in summer and their contraction in winter. If no gaps are left, the tracks would bend outwards due to expansion, which may lead to the derailment of a train.

Glass is a poor conductor of heat. When very hot water is suddenly poured into a glass tumbler, the inner surface heats up and expands quickly, while the outer surface remains cool and does not expand at the same rate. This unequal expansion of the inner and outer surfaces causes the glass tumbler to crack.

Thermal expansion is the property of matter in which the temperature of a substance is increased, resulting in the expansion of the substance. It is different for the three states of matter.

1. Thermal expansion in solids — Solids expand the least on heating. A solid shows linear, superficial and cubical expansion. For example, a gap is left between railway tracks to allow for their expansion.
2. Thermal expansion in liquids — Liquids expand more than solids and show only cubical (volume) expansion. For example, the thermometric liquid in a thermometer expands and rises in the capillary tube on heating.
3. Thermal expansion in gases — Gases expand the most and show only cubical expansion, because their molecules have negligible intermolecular forces. For example, the air inside a hot air balloon expands on heating and makes the balloon rise.

Three applications of thermal expansion in daily life are:

1. Railway tracks — A gap is left between successive railway tracks so that the steel tracks can expand in summer without bending.
2. Metal bridges — One end of a metal bridge is kept on rollers so that it is free to expand and contract with changes in temperature.
3. Bimetallic strips — Bimetallic strips, which bend on heating because the two metals expand differently, are used in electrical devices, thermometers and heat engines to control temperature.

When heat is supplied to a body, the rise in its temperature depends on:

1. The amount of heat supplied to the body.
2. The mass of the body.
3. The material of which the body is made.

In a change of temperature, the heat supplied increases the temperature of a substance, but the substance remains in the same physical state. For example, when water at 20°C is heated to 80°C, its temperature rises but it stays a liquid.

In a change of state, the heat supplied changes the physical state of the substance, while its temperature remains constant. For example, ice melts into water at 0°C, and water changes into steam at 100°C; during these changes, the temperature does not change.