Heat

1. Conduction is the process of transfer of heat from a body at a higher temperature to a body at a lower temperature when both are in direct contact with each other.
2. A thermos flask keeps liquids warm because the vacuum between its double walls minimises heat transfer by conduction and convection. Its silvered walls reduce heat loss by radiation, and its insulating stopper reduces heat loss through the mouth. Thus, the liquid remains warm for a long time.
3. In conduction, heat is transferred through a substance without the actual movement of its molecules, as the molecules only vibrate about their fixed positions, and it mainly takes place in solids. In convection, heat is transferred by the actual movement of molecules, and it takes place in liquids and gases.

1. Kettle
   Reason — Heat, temperature and cold are all related to the thermal state of a body, whereas a kettle is an object used to heat water.
2. Solid
   Reason — Radiation, conduction and convection are the three modes of heat transfer, whereas a solid is a state of matter.
3. Freezing Reason — Heating, thermal expansion and melting involve the supply or absorption of heat, whereas freezing takes place when heat is removed from a substance.
4. Metal
   Reason — Metal is a good conductor of heat. Wood and water are poor conductors, while conduction and convection cannot occur through a vacuum because it has no particles.

1. Heat
2. Boiling
3. Deposition
4. Convection

1. The process during which a substance directly changes from its gaseous state into solid state at a fixed temperature is called deposition.
2. When the temperature of a substance is increased, it results in expansion of the substance.
3. Water and steam pipes often have a U-bend in them to allow thermal expansion.
4. In radiation, no medium is required to transfer heat.

1. False
   Corrected Statement — This process of transfer of heat from a body at a higher temperature to a body at a lower temperature when both are in direct contact with each other is called conduction.
2. False
   Corrected Statement — Substances which allow heat to flow through them easily are called good conductors.
3. True
4. False
   Corrected Statement — When the temperature of a substance is decreased, it results in its contraction.

1. The process during which a substance changes from its gaseous state to liquid state at a particular temperature is known as condensation.
2. Vaporisation is a process in which a liquid changes into its vapour state by absorbing heat at a constant temperature.
3. When heat is supplied to matter, the energy of its molecules increases and they start vibrating vigorously. Due to this, the state of matter may change.
4. The freezing point of water is taken as 273 K and the boiling point of water is marked as 373 K.

A thermos flask keeps tea hot for a long time because it minimises heat loss through all three modes of heat transfer. The vacuum between its double walls reduces conduction and convection, the silvered walls reduce heat loss by radiation, and the insulating lid reduces heat transfer through the mouth. Hence, very little heat escapes and the tea stays hot.

When water boils, the heat supplied is used to change its state from liquid to vapour and not to raise its temperature. Therefore, the temperature of boiling water remains constant at its boiling point even when heating continues.

The air near the heater gets heated, expands and becomes less dense (lighter). Being lighter, this hot air rises up, while the cooler and denser air comes down to take its place. This sets up convection currents, due to which the hot air rises above the heater.

Metals expand on heating and contract on cooling. Small gaps are left between metal benches to provide space for the metal to expand in hot weather. If no gaps are left, the expansion may bend or damage the benches.

A thermometer consists of a graduated glass stem with a thin, long and uniform capillary bore. At one end, it has a bulb containing a thermometric liquid such as mercury or alcohol.

Deposition is a process during which a substance directly changes from its gaseous state into its solid state at a fixed temperature. For example, naphthalene vapour changes directly into solid naphthalene on cooling.

Eskimos make igloos from snow because snow contains a large amount of trapped air. Air is a bad conductor of heat, so it does not allow the body heat to escape. This keeps the inside of the igloo warm.

Sea breeze, land breeze and monsoon are examples of convection currents in nature.

1. The base of a cooking vessel may be painted black so that it absorbs more heat radiation and helps the food cook faster.
2. The inner surface of a solar cooker is painted black so that it absorbs the maximum amount of heat radiation from the Sun.

The qualities of a good thermometer are as follows.

1. The stem of the thermometer should be made of thick glass to protect the capillary tube against any physical damage.
2. The bulb of the thermometer should be thin-walled so that the liquid inside it quickly attains the temperature of the body.
3. The bore of the capillary should be narrow so that the liquid moves through a large length even if the increase in volume of the liquid is small.
4. The thermometric liquid used should expand uniformly, be opaque and clearly visible, be practically non-volatile, not stick to the walls of the capillary tube, and have a low freezing point and a high boiling point so that it can be used over a wide range of temperature.

The three temperature scales used to measure temperature are as follows.

1. Celsius scale — In this scale, the upper fixed point is 100°C, which is the boiling point of water, and the lower fixed point is 0°C, which is the freezing point of water. The interval between the two fixed points is divided into 100 equal parts.
2. Fahrenheit scale — In this scale, the upper fixed point is 212°F, which is the boiling point of water, and the lower fixed point is 32°F, which is the freezing point of water. The interval between the two fixed points is divided into 180 equal parts.
3. Kelvin scale — This is the SI scale of temperature. Its zero (0 K) is set at absolute zero, the lowest possible temperature in the universe. On this scale, the freezing point of water is 273 K and the boiling point of water is 373 K.

Heat is a form of energy, so when an object is heated, it can bring about several changes in the object.

1. Change in temperature — On heating, the temperature of a substance increases. This change can be measured with a thermometer.
2. Change in state of matter — On heating, a substance can change its state. For example, a solid can melt into a liquid and a liquid can change into vapour (gas).
3. Change in dimensions — On heating, most substances expand, that is, their length, area and volume increase. This is called thermal expansion. On cooling, they contract.

A thermos flask is a device that prevents the transmission of heat by minimising conduction, convection and radiation.

1. It consists of a double-walled glass vessel with a vacuum between the two walls. The vacuum greatly reduces heat transfer by conduction and convection. The walls are made of glass, which is a poor conductor of heat.
2. The mouth of the flask is closed with an insulator such as cork or plastic, which reduces heat transfer through the opening.
3. The sides of the double wall facing each other are silvered (made shiny). Shiny surfaces are poor absorbers and radiators of heat, so they reduce heat transfer by radiation.
4. The whole system is enclosed in a thick plastic or stainless steel case to protect it from damage.

In this way, a thermos flask keeps hot things hot and cold things cold for a long time.

1. Vineeta's mother used a thermos flask because it minimises heat transfer between the tea and the surroundings. The vacuum between its double walls reduces conduction and convection, so the hot tea remains warm for a long time.

2. The metal bench felt hot because metal is a good conductor of heat. The heat from the sun made the metal warm, and being a good conductor, the metal quickly passed this heat to Vineeta's hand, so it felt hotter than the wooden bench.

3. Convection is the process of transfer of heat by the actual movement of molecules in liquids and gases. Vineeta observed it when the warm air and water vapour above the hot tea rose upwards.

4. The temperature of the tea was measured using a thermometer.

5. Small gaps are left in the metal railings to allow for thermal expansion. Metals expand on heating, so the gaps give the metal the space to expand without bending or breaking.

1. Heat is a form of energy that is transferred from one body to another as a result of a difference in temperature.

2. A substance can absorb heat without an increase in temperature by changing from one physical state (phase) to another, such as from a solid to a liquid (melting), from a solid to a vapour (sublimation), or from a liquid to a vapour (boiling).

3. Sublimation is the process during which a substance directly changes from its solid state into its gaseous state at a fixed temperature, without passing through the liquid state. For example, camphor changes directly into its gaseous form.

4. The two commonly used units of heat are the calorie and the British thermal unit (BTU).

5. When two bodies at different temperatures are brought together, heat energy flows from the hotter body to the colder body. This flow continues until both bodies reach the same temperature.

The metal spoon felt hot because metal is a good conductor of heat. It quickly conducted the heat from the hot soup to its other end. The plastic spoon did not feel hot because plastic is a bad conductor (insulator) of heat and does not allow heat to pass through it easily.

Each part reduces heat loss in a different way.

1. Vacuum layer — The vacuum between the double walls greatly reduces heat transfer by conduction and convection because it contains no particles.
2. Shiny surface — The silvered surfaces are poor absorbers and radiators of heat, so they reduce heat loss by radiation.
3. Tight lid — The tight insulating lid closes the mouth of the flask and reduces heat transfer through it.

I would choose metal to make a cooking utensil. This is because metals are good conductors of heat, so they allow heat to pass through them quickly and cook the food faster. Wood and glass are bad conductors of heat and would not let heat reach the food easily.

The water in the black container will heat up faster. This is because black (dark) surfaces are good absorbers of heat, so the black container absorbs more heat. A silver (shiny) surface is a poor absorber and a good reflector of heat.

A thermos flask can be drawn as a double-walled glass vessel enclosed in an outer case. The parts that help prevent heat loss are as follows.

1. Double glass walls with a vacuum in between — greatly reduce heat transfer by conduction and convection.
2. Silvered (shiny) walls — reduce heat transfer by radiation.
3. Insulating cork or plastic lid — reduces heat transfer through the mouth.
4. Outer plastic or stainless steel case — protects the glass vessel from damage.

Aim: To compare the rates of heat transfer through metal and wood.

Materials required: A thermometer, hot water, a beaker, a metal spoon, a wooden spoon, two small pins and equal amounts of wax.

Procedure:

1. Pour hot water into a beaker and use the thermometer to record its temperature.
2. Fix one small pin to the upper end of each spoon using an equal amount of wax. Keep the pins at the same distance from the lower ends of the spoons.
3. Place equal lengths of the metal and wooden spoons in the hot water at the same time.
4. Observe the pins without touching the spoons or the hot water.

Observation: The wax on the metal spoon softens first, so its pin falls earlier. The pin on the wooden spoon remains in place for a longer time.

Conclusion: Heat travels faster through metal than through wood. Therefore, metal is a good conductor of heat, whereas wood is a poor conductor of heat.

1. Metal feels colder than wood on a winter day because metal is a good conductor of heat. When we touch it, heat is conducted away from our hand quickly, and this rapid loss of heat makes the metal feel cold. Wood is a bad conductor, so it does not draw heat away from our hand and feels warmer.

2. Satvik feels the heat from the fire by radiation. Heat travels from the fire in the form of waves and reaches him without requiring any medium or heating the intermediate space.

3. The story tells us that different materials conduct heat differently. Good conductors like metal allow heat to pass through them quickly, so they feel hotter or colder, while bad conductors like wood do not allow heat to pass easily.

1. The vacuum between the double walls reduces conduction and convection, the shiny silvered surface reduces radiation, and the tight insulating lid reduces heat transfer through the mouth. These parts help keep the drink warm.

2. The ordinary steel bottle has a single metal wall and no vacuum layer. Since steel is a good conductor of heat, heat passes rapidly through its wall and is then lost to the surroundings. A thermos flask minimises heat transfer, so the water in it stays warm for longer.

3. We can conclude that a thermos flask keeps a drink warm for much longer than an ordinary steel bottle, because its special design — a vacuum, a shiny surface and a tight lid — minimises heat loss by conduction, convection and radiation.

1. They observed that on heating, the metal wire expanded and became slightly longer, which caused the weight hung at its centre to move downward. On cooling, the wire contracted and the weight moved upward again.

2. When the wire was heated, it expanded and became longer, so it sagged. Because of this increase in length, the weight hanging at its centre moved downward.

3. This experiment shows that metals expand when heated and contract when cooled. This expansion and contraction of solids on heating and cooling is called thermal expansion.

1. They derived the ideas of using double walls with air (or a vacuum) in between, a tight lid and an inner shiny layer, all of which help to reduce heat loss, just like in a thermos flask.

2. The sealed layer of trapped air between the double walls reduces conduction and restricts convection. The tight lid reduces heat transfer through the opening, and the shiny inner layer reduces heat loss by radiation. These features make the lunchbox more effective in keeping food hot.

3. One improvement could be to keep a vacuum between the double walls instead of air, as a vacuum prevents conduction and convection even better than air.

1. The energy transferred between two bodies in contact due to the difference in their temperatures is called heat.
2. The instrument used to measure temperature is called a thermometer.
3. Conduction is the process by which heat travels through solids.
4. When water changes to steam at 100°C, the process is called boiling.
5. The process of transfer of heat without any medium is known as radiation.

1. False
   Corrected Statement — The boiling point of water is 100°C.
2. True
3. False
   Corrected Statement — Heat flows from a hotter object to a colder object.
4. True
5. False
   Corrected Statement — A thermometer is used to measure temperature.

A capillary tube is an extremely thin glass tube with a very narrow bore through which a liquid can rise or fall. In a thermometer, the thermometric liquid rises or falls in the capillary tube as the temperature changes.

Temperature is the measure of the degree of hotness or coldness of a body.

Heat is transferred from the sun to the Earth by the process of radiation. In radiation, heat travels in the form of waves and does not need any medium, so it can travel through the vacuum present between the sun and the Earth.

Change of state means the change of a substance from one physical state into another (solid, liquid or gas) on heating or cooling. For example, on heating, ice (solid) changes into water (liquid) and water changes into steam (gas).

Sea breeze is an application of convection in nature. The land gets heated faster than the sea during the day, the warm air over the land rises up, and the cool air from the sea rushes in to take its place, forming the sea breeze.

The three modes of heat transfer are conduction, convection and radiation.

1. Conduction — It is the transfer of heat from a body at a higher temperature to a body at a lower temperature when both are in direct contact, without the actual movement of molecules. For example, when one end of a metal rod is heated, the other end also becomes hot after some time.
2. Convection — It is the transfer of heat by the actual movement of molecules in liquids and gases. For example, water in a vessel gets heated due to the convection currents set up in it.
3. Radiation — It is the transfer of heat from a hot body to a cold body without any medium. For example, heat from the sun reaches the Earth by radiation.

In conduction, heat travels from the hotter part of a body to its colder part without the molecules moving from their places. The molecules vibrate about their fixed positions and pass on heat to the neighbouring molecules. For example, when a metal spoon is placed in a cup of hot tea, the heat travels from the hot end of the spoon to its other end by conduction, and after some time the upper end of the spoon also becomes hot.

Gaps are left in bridges and railway tracks to allow for the thermal expansion of metal. Metals expand in summer and contract in winter. If no gaps are left, the rails or the bridge may bend or get damaged due to expansion. For this reason, gaps are left between successive rail tracks, and one end of a metal bridge is kept on rollers so that it is free to expand and contract.

A thermos flask keeps hot things hot and cold things cold by minimising heat transfer through conduction, convection and radiation.

1. It has a double-walled glass vessel with a vacuum between the walls, which greatly reduces heat transfer by conduction and convection.
2. Its inner walls are silvered (made shiny), which reduces heat transfer by radiation.
3. Its mouth is closed with an insulating cork or plastic lid, which reduces heat transfer through the opening.

Since heat can neither escape from nor enter the flask easily, hot things remain hot and cold things remain cold for a long time.