Ch.9: Heat

Heat | Class 7 | Science | Chapter 9 | Maharashtra State Board

Welcome to our latest educational video on heat transfer! In this lesson, we’ll dive into the fascinating world of heat, exploring its transfer mechanisms, the behavior of different materials, and its effects on solids, liquids, and gases.

🔥 Lesson Highlights 🔥

1️⃣ Transfer of Heat: We’ll begin by unraveling the concept of heat transfer, understanding how heat flows from one object to another. Discover the fundamental principles governing this intriguing process.

2️⃣ Modes of Heat Transfer: Explore the three primary modes of heat transfer - Conduction, Convection, and Radiation. Learn how each mode works and their real-life applications.

3️⃣ Good and Bad Conductors of Heat: Uncover the mystery behind materials that efficiently conduct heat (good conductors) and those that resist heat flow (bad conductors). The answers may surprise you!

4️⃣ Expansion and Contraction: Witness the effects of heat on solid substances, liquids, and gases. See how temperature changes cause them to expand and contract, leading to captivating phenomena.

5️⃣ Thermos Flask (Dewar Flask): We’ll conclude our lesson by exploring the ingenious design of the thermos flask (Dewar Flask) and how it utilizes heat transfer principles to keep your beverages hot or cold for extended periods.

Join us in this captivating journey through the world of heat transfer and gain a deeper understanding of the science behind this everyday phenomenon. Whether you’re a student, a science enthusiast, or simply curious about the world around you, this video will provide valuable insights into the wonders of heat.

Don’t forget to like, subscribe, and hit the notification bell to stay updated with our latest educational content. Share this video with your friends and let’s ignite the passion for learning together!

Questions & Answers

1. Fill in the blanks.

(radiation, white, conduction, blue, convection, bad conductor, good conductor, black, reflection)

1. Maximum heat is absorbed by a black coloured object.

2. Radiation of heat does not require a medium.

3. Conduction of heat takes place through a solid substance.

4. The shining surface in a thermos flask decreases the outgoing heat by reflection .

5. Cooking utensils are made from metals due to their property of conduction.

6. The earth receives heat from the sun by radiation.

2. What will absorb heat?

Steel spoon, wooden board, glass vessel, iron griddle (lava), glass, wooden spoon, plastic plate, soil, water, wax.

Ans. Following will absorb heat fast:

Steel spoon, iron griddle, water, wax. Remaining objects also absorb heat but very slowly.

3. Write Answers to the following questions.

1. How does a fever get lowered by putting a cold compress on the forehead of a patient?

Ans. The water in the cold compress on the forehead lowers fever by cooling the blood vessels in the skin, reducing blood flow and transferring heat away from the body, helping to lower the body’s core temperature.

2. Why are the houses in Rajasthan painted white?

Ans. Houses in Rajasthan are painted white to reflect sunlight and keep interiors cooler in the hot desert climate. White reflects heat, helping to maintain a comfortable indoor temperature.

3. What are the modes of heat transfer? Ans. There are three modes of heat transfer:

1. Conduction

2. Convection

3. Radiation.

4. Explain which mode of heat transfer causes sea breezes and land breezes. Ans. Sea breezes and land breezes are primarily caused by convection, a mode of heat transfer. During the day, the land heats up faster than the sea, creating rising warm air over the land, and cooler air from the sea moves in to replace it, causing a sea breeze. At night, the land cools down faster, leading to sinking cool air over the land, and warmer air from the sea rushes in, causing a land breeze.

5. Why is the outer coat of the penguins of Antarctica black? Ans. The outer coat of Antarctica’s penguins is black because it helps them absorb and retain heat in the cold environment, allowing them to stay warm and survive in extremely low temperatures.

6. Why are heaters fitted near the floor and air conditioners near the ceiling of a room? Ans. Heaters are fitted near the floor because hot air rises, so it will naturally circulate throughout the room. Air conditioners are fitted near the ceiling because cold air sinks, so it will naturally cool the room from top to bottom ensures better cooling distribution.

4. Give scientific reasons.

1. An ordinary glass bottle cracks when boiling water is poured into it, but a borosil glass bottle does not. Ans. Ordinary glass has a higher coefficient of thermal expansion than borosilicate glass, which means that it expands more when heated. When boiling water is poured into an ordinary glass bottle, the inner surface of the bottle heats up and expands quickly, while the outer surface remains cooler and does not expand as much. This difference in expansion creates stress in the glass, which can cause it to crack.

Borosil glass can withstand such changes, so it doesn’t crack. This is why borosilicate glass is often used to make laboratory glassware and other items.

2. The telephone wires which sag in summer become straight in winter. Ans. Telephone wires are made of metal, and metals expand when heated and contract when cooled. This is known as thermal expansion.

In the summer, the telephone wires heat up and expand, causing them to sag. In the winter, the telephone wires cool down and contract, causing them to become straight again.

3. In winter, why does an iron pillar feel colder than a wooden pole. Ans. In winter, an iron pillar feels colder than a wooden pole because metal is a good conductor of heat, while wood is a poor conductor of heat. When you touch the iron pillar, it quickly conducts heat away from your hand, making it feel cold. Wood, on the other hand, doesn’t conduct heat as efficiently, so it doesn’t draw heat away from your hand as fast, and therefore feels less cold. The sensation of coldness is due to the rapid transfer of heat from your hand to the colder material.

---

E X T R A

1. Fill in the blanks.

1. Temperature is an indicator of the heat in a substance. 2. Heat flows from higher temperature to lower temperature.

3. Transfer of heat from one place to another takes place by conduction, convection or radiation.

4. During conduction, particles of the substance vibrate in their place.

5. In a convection current, warm air travels upwards.

6. A medium is necessary for the conduction of heat.

7. The speed of conduction of heat through a medium depends on the medium.

8. Convection takes place only in fluid.

9. Transfer of heat in the absence of a medium is called radiation.

10. Substances through which heat travels easily and quickly are called good conductors.

11. Substances through which heat travels very slowly are called bad conductors of heat.

12. Copper is a better conductor of heat than iron.

13. Sir James Dewar made first thermos flask in 1892.

14. Neither conduction nor convection of the heat occurs in thermos flask due to the vacuum.

2. Say whether true or false, correct and rewrite the false statements:

1. Glass is a bad conductor of heat. Ans. True

2. In a thermos flask, hot things remain hot and cold things become warm. Ans. False.

In a thermos flask, hot things remain hot and cold things remain cold.

3. Radiation of heat depends upon the colours of the outer surface of the body. Ans. True

4. During radiation, the particles of the medium vibrate. Ans. False.

Radiation is a type of heat transfer that does not require a medium.

5. Copper is better conductor of heat than iron. Ans. True

6. During conduction, particles of the conductor leave their original place. Ans. False.

During conduction, particles of the conductor do not leave their original place, but only vibrate in their own place.

7. Hot water is heavier than cold water. Ans. False.

Hot water is lighter than cold water.

8. A convection current always travels down wards. Ans. False.

A convection current always travels upwards.

9. Every warm substance continuously emits heat in all directions around itself. Ans. True

10. Air is a bad conductor of heat.

Ans. True

3. Define the following:

1. Conduction

2. convection

3. radiation

4. Transfer of heat.

Ans. 1. Conduction: Conduction is the transfer of heat through a material by the vibration of its particles. Hotter particles pass some of their energy to neighbouring cooler particles through direct contact, causing heat to flow from hot to cold regions within the material. Metals are good conductors,

2. Convection: Convection is the process of heat transfer in a fluid (liquid or gas) due to the movement of the fluid itself.

3. Radiation: Radiation is the transfer of heat energy through electromagnetic waves, such as infrared radiation, without the need for a medium or material to carry the heat.

4. Transfer of heat: Heat transfer is the movement of thermal energy from one object or system to another. It is a spontaneous process that occurs when there is a temperature difference between the two objects or systems.

4. Give scientific reasons:

1. Cricketers wear white clothes when playing in the sun. Ans. Cricketers wear white clothes when playing in the sun because white reflects heat better than other colours. This helps to keep the cricketers cooler and more comfortable in the hot sun.

2. Sawdust is a bad conductor of heat. Ans. Sawdust is a bad conductor of heat because it’s made up of tiny, insulated wood particles that don’t allow heat to pass through easily.

3. We can feel the heat of table lamp under it. Ans. We can feel the heat of a table lamp under it because the lamp emits infrared radiation, which heats the surrounding air and objects, including your skin, when it comes into contact with them.

4. Medium is necessary for conduction. Ans. Medium is necessary for conduction because it involves the transfer of heat through direct contact between particles in a material. In the absence of a medium (solid, liquid, or gas), conduction cannot occur since there are no particles to conduct the heat.

5. Explain the structure and functioning of thermos flask. Ans. A thermos flask, also known as a vacuum flask, is a container that keeps liquids hot or cold for long periods of time. It consists of two glass or stainless-steel walls with a vacuum in between. The vacuum prevents heat from being transferred by conduction and convection, and the reflective inner wall prevents heat from being transferred by radiation.

6. What is infrared camera? Write its uses. Ans. An infrared camera is a device that captures infrared radiation (heat) emitted by objects and converts it into a visible image. It allows us to see temperature variations and heat patterns.

Uses:

1. Thermography: Infrared cameras are commonly used for thermal imaging, which helps detect overheating electrical components, insulation issues in buildings, and identify heat leaks.

2. Security: They’re used in surveillance systems to detect intruders at night or in low-light conditions by sensing body heat.

3. Medical: Infrared cameras assist in medical diagnostics, like identifying feverish patients or detecting injuries by measuring skin temperature.

4. Industrial: Used for predictive maintenance in manufacturing, identifying faulty machinery through temperature variations.

5. Firefighting: Firefighters use them to locate people and hotspots in smoke-filled environments.

6. Astronomy: Infrared cameras are employed in astronomy to capture heat radiated by celestial objects, revealing hidden details.

7. Why is mercury used in a thermometer? Ans. Mercury is used in thermometers because it has a number of properties that make it ideal for this purpose:

1. High coefficient of expansion: Mercury expands and contracts very evenly as its temperature changes, making it easy to read the temperature accurately.

2. Wide liquid range: Mercury remains liquid over a wide range of temperatures, from -38.87 °C to 356.73 °C.

3. Good conductor of heat: Mercury is a good conductor of heat, which means that it quickly responds to changes in temperature.

3. Opaque and shiny: Mercury is opaque and shiny, which makes it easy to see the mercury in the thermometer tube.

8. Write the difference between Conduction, Convection and Radiation.

Property	Conduction	Convection	Radiation
Definition	Transfer of heat energy through direct contact	Transfer of heat energy through the movement of a heated fluid	Transfer of heat energy through electromagnetic waves
Medium required	Yes	Yes	No
Examples	Heating a metal spoon in a cup of hot coffee	Heating a pot of water on a stove	Feeling the heat from a distance

9. Draw and explain about the Thermos flask or Dewar flask.

Ans. A thermos flask, also known as a Dewar flask, is a container that keeps liquids hot or cold for long periods of time. It consists of two glass or stainless steel walls with a vacuum in between. The vacuum prevents heat from being transferred by conduction and convection, and the reflective inner wall prevents heat from being transferred by radiation.

Parts of a thermos flask:

• Outer vessel: The outer vessel is made of glass or stainless steel and is sealed to the inner vessel at the neck.

• Inner vessel: The inner vessel is made of glass or stainless steel and contains the liquid that you want to keep hot or cold.

• Vacuum: The space between the inner and outer vessels is a vacuum.

• Reflective inner wall: The inner wall of the thermos flask is coated with a reflective material.

How a thermos flask works:

The thermos flask works by preventing heat from being transferred from the inside of the flask to the outside of the flask. This is done by the vacuum and the reflective material on the inner wall of the flask.

• Vacuum: The vacuum prevents heat from being transferred by conduction and convection.

• Reflective inner wall: The reflective inner wall prevents heat from being transferred by radiation.

Thermos flasks are used in a variety of applications, such as:

• Keeping drinks hot or cold

• Storing food, such as soup

• Transporting medical supplies

• Scientific research

5. Answer the following questions:

1. Why does the halwai wrap up cloth around the end of his slotted spoon while stirring the boiling milk in his large kadhai? Ans. The halwai wraps a cloth around the end of his slotted spoon while stirring the boiling milk in his large kadhai to protect his hands from the heat.

Milk is a good conductor of heat, which means that it transfers heat quickly. When the halwai stirs the boiling milk with a metal spoon, the heat from the milk is transferred to the spoon very quickly. This can cause the spoon to become very hot, which can burn the halwai’s hands.

The cloth acts as an insulator, preventing the heat from the milk from being transferred to the halwai’s hands. This allows the halwai to stir the milk without burning his hands.

2. Why do we hold a steel glass in a handkerchief while drinking hot milk from it? Ans. We hold a steel glass in a handkerchief while drinking hot milk from it because steel is a good conductor of heat, and the handkerchief acts as an insulator. This means that the heat from the milk is transferred to the handkerchief, and not to our hands. This prevents us from burning ourselves.

3. Why do we wear woollen clothes in winter? Ans. We wear woolen clothes in winter because wool is a good insulator. This means that it prevents heat from our bodies from escaping to the cold air outside.

Wool fibers also trap air pockets, which act as additional insulation. When we wear woolen clothes, the air pockets trap our body heat and keep us warm.

4. Why do we use white clothes in summer and dark or black clothes in winter? Ans. We use white clothes in summer and dark or black clothes in winter because of the way they interact with light.

White clothes reflect light, which helps to keep us cool in the summer. Dark clothes absorb light, which helps to keep us warm in the winter.

5. Why is there a gap at the joints of rails and of cement concrete bridges? Ans. The gap at the joints of rails and cement concrete bridges is to allow for expansion and contraction due to temperature changes.

All materials expand when they are heated and contract when they are cooled. This is true for both steel and concrete. If there were no gaps at the joints of rails and bridges, the expanding and contracting materials would push against each other, which could cause buckling or even cracking.

The gaps at the joints allow for the materials to expand and contract without damaging each other.

6. Why hot substance does not remain as hot after two or three hours? Ans. A hot substance not remain as hot after two or three hours, because of heat transfer. The heat from the hot substance is transferred to the surrounding air and to other objects through conduction, convection, and radiation. Over time, the hot substance will cool down to the same temperature as its surroundings.

Note: If you have any additional questions or feedback, please leave them in the comment section below. We will try to answer them and update this blog accordingly as soon as possible.