Living Science 2020 2021 for Class 7 Science Chapter 6 - Flow Of Heat

Answer:

The difference in temperature of the bodies determines the direction of flow of heat. Heat flows from a body at higher temperature to one at a lower temperature. 

Answer:

Heat travels via process of conduction from the hot end to the cold.

Answer:

Iron and copper are the two good conductors of heat. Whereas wood and plastic are the two insulators of heat. 

Answer:

Most liquids are bad conductors of heat.

Answer:

In the process of convection, heated molecules of a liquid or gas move themselves to carry heat from the hot to the cold region. 

Answer:

In solid molecules are tightly bound with each other, they can vibrate but can not move to carry heat from one region to another region. That's why convection currents are seen in liquids and gases but not in solids. 

Answer:

Sea and land breezes in coastal areas are actually convection currents. The flow of breeze from the sea to the land or from the land to the see occurs due to the process of convection.

Answer:

Radiation does not require any medium. Through the process of radiation, the heat can be transferred even in the vacuum. 

Answer:

In radiation, heat is transferred by electromagnetic waves traveling at the speed of light. Hence, radiation is the fastest method of heat transfer. 

Answer:

Dark-coloured objects absorb and radiate more heat than light-coloured objects. 

Answer:

Dark coloured objects are better absorber and better radiator than light-coloured objects. 

Answer:

A thermos flask is a household device which does not allow heat to flow in or out of its system. Hence, it prevents loss of energy by conduction, convection or radiation. 

Answer:

(a) solid only
In conduction, the molecules vibrate and hit each other to transfer the heat energy between them. The molecules need not to travel from one end to other. Therefore, it is only possible in solids in which the molecules do not move but vibrate on heating.

Answer:

(b) Convection
In the convection method, molecules travel from the hotter region to the colder region. The case is the same when water is heated.

Answer:

(b) does not conduct heat well
Because, if handles conduct heat from the utensils, they will become too hot and make it impossible for us to hold the utensils.

Answer:

(d) liquids and gases only
In the process of convection, molecules move from a hotter place to a colder region. Therefore, convection is impossible in solids because the molecules are rigid and vibrate when heated. In liquids and gases, molecules are comparatively less rigid and can move easily, thereby making convection possible in them.

Answer:

(b) are caused by the currents build in air due to convection
Due to convection, the hot air from the land moves towards the sea during the day and vice versa during the night. This causes land and sea breezes.

Answer:

(d) all of the above
The empty space between the two glass walls prevent the loss of heat through conduction and convection, as both of these processes need a medium to transfer heat. Also, the silvery wall of flask reflects back the heat and prevents its loss through radiation.

Answer:

(a) iron
The molecules of iron transfer heat from the hotter region to the colder ones by hitting the surrounding molecules through vibration, without moving themselves from one end to the other. But water and alcohol are liquids, and nitrogen a gas; therefore, they cannot conduct heat.

Answer:

(c) nitrogen

Gases are bad conductors to heat, and nitrogen being a gas can be considered as the best insulator among the given options.

Answer:

(a) Conduction
In conduction, heat is transferred from one place to another without the movement of molecules. However, in convection and radiation, heat energy is transferred through the movement of molecules.

Answer:

In solids, heat is transferred through the process of conduction. This is because the molecules in solids are rigid and only vibrate on heating rather than moving from one place to the other. The vibrating molecules of the hotter region hit those molecules in the colder region, causing the transfer of heat.

Answer:

Most metals are good conductors of heat. This is because the molecules in metals vibrate and hit the nearby molecules to transfer the heat energy, rather than moving from one place to the other.

Answer:

True
Gases are insulators of heat because their molecules are loosely bonded; hence, they do not conduct heat.

Answer:

Heat transfer in liquids takes place through the process of convection. In this case, the molecules of liquids move from one place to other to transfer the heat energy.

Answer:

Mercury is a liquid which is a good conductor of heat. It is an exception, as all liquids are good insulator of heat.

Answer:

When we heat water in a pan, the heat travels throughout the water through the process of convection.

Answer:

Radiation is a process in which the molecules don't transfer or carry heat from the hotter to colder regions, but the material itself radiates the heat energy. Therefore, it requires no medium to transfer the heat. Higher the temperature of the body, the more it radiates heat.

Answer:

False.
In a thermos flask, the space between the two glass walls is empty. This vacuum prevents heat loss through conduction and convection, as there is no medium present to transfer the heat by these two processes.

Answer:

Radiant heat falling on a body is partly absorbed and partly reflected. Some of it may also get transmitted.

Some part of the radiation are absorbed by the object, while some will strike its surface and reflect like a ball bouncing back on striking a wall. The remaining part of the radiation will pass through the object like a light passing through a glass, i.e., transmitted.

Answer:

Conditions necessary for the transfer of heat from one body to the other through conduction are as follows:

1. The two objects must be in contact with each other.
2. The temperatures of both the objects must be different.

Answer:

Answer:

Woollen fibre comprises spaces between them that traps the air. Since air is a bad conductor of heat, it will not allow the body heat to escape, thereby keeping the body warm in winters. However, in a cotton fibre, such spaces are absent; hence, it can not keep us warmer as compared to the  woollen clothes.    

Answer:

Ventilators are installed on the top most part of the walls because the air we breathe out is warmer and lighter, and moves upwards. This stale air escapes through the ventilators, and fresh and cooler air enters the room through the doors and windows to take its place.

Answer:

Heat from the Sun reaches us through the process of radiation. There is millions of miles of empty space between the Earth and the Sun; therefore, only through radiation can heat be transferred to the Earth from the Sun, as it does not require any medium. The radiant heat travels similar to the speed of light.

Answer:

The polished curved surface at the back of the heating rod in a room heater reflects the radiant heat from the heating rod. Thus, the radiant heat spreads across the room, thereby making the room warmer.

Answer:

Heat is conducted from the heated end of a rod to its cold end through the effective vibration of its molecules. Molecules of the rod starts vibrating as heat is supplied to it. When one end of the rod heats up, the molecules of that end starts vibrating and colliding with the nearby molecules to make them unstable. These molecules further hit their neighbouring molecules to transfer the heat. In this way, heat is transferred from one end of the rod to its other end.

Answer:

Good conductors of heat are used where we want to transfer heat faster from one point to the other. For example, cooking utensils are made of metals like brass, steel and aluminium, which are good conductors of heat. Though silver and copper are best conductors of heat but not usually used in cooking utensils as they are expensive.

Bad conductors of heat are used where we want to stop the heat from getting transferred or where we do not want the heat to be lost or gained from the surroundings. For example, handles of cooking utensils are made of wood or plastic, which are bad conductors of heat. Hence, the handles don't heat up during cooking, thereby allowing us to hold the utensils comfortably.

Answer:

The process of transference of heat through a fluid (liquid or gas) is called convection.

It can be observed that when water is heated, water near the flame gets hot and starts rising up. On the other hand, cold water moves down from the sides of the flask, towards the source of heat. Then, this water becomes hot and rises up. This process continues till all the water present in the flask gets heated. This process of heat transfer is called convection.

Answer:

During the day, land gets heated up faster than the sea. So, the air above the land becomes warmer and lighter and moves upward. The air above the sea, which is relatively cooler, rushes to take its place. This results in to sea breeze during the day.


During the night, the land cools faster than the sea. Therefore, the air above the land is cooler and denser, while the air above sea is warmer and lighter. The air above the sea will move upwards and the cooler air from the land will rush to take its place. This causes land breeze during the night.

Answer:

Answer:

Black objects radiate more heat than white objects. Two everyday uses of this principle are:

1. Refrigerator pipes are painted with a dull black colour so that they can radiate the heat to the surrounding and cool themselves down.
2. Hot plates are coloured black so that it can radiate more heat energy to serve its purpose efficiently.

Answer:

Thermos flask consists of double glass or metal walls with a space in between them. Air is completely sucked out of this space, and after that the walls are sealed at the neck or at the bottom of the flask. This vacuum is created to prevent heat loss due to conduction and convection, as both these methods require a medium to transfer heat. Then, the walls are silvered. This silver polish prevents the heat loss through radiation, as heat from inside the flask is radiated back to it and the external heat is reflected back to the surrounding. Finally, the vessel is put inside a metallic case with a plastic or cork support on both sides so as to protect the glass from breaking. At last, the mouth of the vessel is closed by a stopper made from an insulating material like plastic or cork.

Answer:

The test tube which is heated from below will heat up faster. When we heat water from below, the molecules at the bottom of the water start heating and expanding due to which they become light and move upwards. On moving upwards, they transfer the heat energy to other molecules. After transferring their energy, they contract and return to the bottom again. This cycle continues till the entire water gets heated through the convection mode. If water is heated from above, the heated molecules at the top will change their state to form steam, as they can hardly transfer the heat energy to the bottom layer due to the convection currents.

Answer:

When we heat water from below, the molecules at the lower layer of the water start heating and expanding due to which they become light and move upwards. On moving upwards, they transfer the heat energy to other molecules. After transferring their energy, they contract and return to the bottom again. This cycle continues till the entire water gets heated-up. However, if the water is heated from above, the heated molecules at the upper layer will either change their state to form steam, or remain in their liquid state to transfer heat to the nearby molecules. Thus, heat takes much time to travel downwards than upwards.

Answer:

The colour white reflects the radiated heat to the surroundings; therefore, the outer walls of the houses, in the hotter regions of the Earth, are painted in white to keep them cool. 

Answer:

For conduction of heat from one object to other, it must satisfy two conditions. Firstly, the objects should be in contact with each other. Secondly, they should have different temperature. But in this case, only the first condition is satisfied; hence, the heat energy will not be transferred.

Answer:

A room heater warms the air, thereby expanding it and making it light. This warm air then moves upwards and spreads across the room. Thus, a room heater must be placed on the floor to heat up the room in an effective manner. An air conditioner should be placed near the ceiling because it cools the air near the ceiling, thereby making it heavier enough to move downwards and cool the entire room.

Answer:

Cloudy nights are warmer than clear nights because clouds prevent the radiating heat from the Earth to escape into the outer space.

Answer:

Feathers of birds have spaces in them which are filled with air. This mechanism acts as an insulator of heat and protects the birds from the cold.

Answer:

Inverting the coat makes it more effective because it traps the body heat inside and does not let it go out. By preserving our body heat, it makes us feel warmer. 

Answer:

Mohini cared for the stray dog and understood that the dog must be feeling cold, that's why she brought a blanket and wrapped the dog. She also fed the dog, that shows she is caring and understands the emotions of the animals.