Living Science Solutions for Class 7 Science Chapter 6 Flow Of Heat are provided here with simple step-by-step explanations. These solutions for Flow Of Heat are extremely popular among Class 7 students for Science Flow Of Heat Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science Book of Class 7 Science Chapter 6 are provided here for you for free. You will also love the ad-free experience on Meritnationâ€™s Living Science Solutions. All Living Science Solutions for class Class 7 Science are prepared by experts and are 100% accurate.

#### Question 1:

Conduction takes place in

(a) solid only
(b) liquids only
(c) gases only
(d) in solids, liquids and gases

(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.

#### Question 2:

In which method of transfer of heat do the molecules travel from the hot the cold protion?

(a) conduction
(b) convection
(d) all of the above

(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.

#### Question 3:

Handles of cooking utensils should be made of a material that

(a) conducts heat well
(b) does not conduct heat well
(c) radiates heat well
(d) does not radiate heat well

(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.

#### Question 4:

Convection occurs in

(a) solids only
(b) liquids only
(c) gases only
(d) liquids and gases only

(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.

#### Question 1:

What conditions are necessary for transfer of heat from one body to another by conduction?

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.

#### Question 2:

Differentiate between good and bad conductors of heat.

 Good conductors of heat Bad conductors of heat Materials that can easily conduct heat through them are good conductors of heat. Materials that cannot conduct heat through them are insulators of heat or bad conductors of heat. For example, iron, copper and silver For example, glass, wood and plastic

#### Question 3:

Why do woolen clothes keep us warmer in winters than cotton clothes?

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.

#### Question 4:

Why are ventilators in houses provided at the top of walls?

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.

#### Question 5:

How does heat from the sun reach us? At what speed does this transfer of heat take place?

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.

#### Question 6:

What is the function of the polished curved surface at the back of the heating rod in a room heater?

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.

#### Question 1:

Explain how heat is conducted from the heated end of a rod to its cold end.

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.

#### Question 2:

With an example of each, explain the uses of good and bad conductors of heat.

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.

#### Question 5:

Sea and land breezes

(a) are caused by currents set up in air due to conduction.
(b) are caused by currents set up in air due to convection
(c) are caused by currents set up in air due to radiation
(d) have no relation to conduction, convection or radiation

(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.

#### Question 6:

A thermos flask prevents loss or gain of heat by

(a) conduction only
(b) convection only
(d) all of the above

(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.

#### Question 7:

Which of these is the best conductor of heat?

(a) iron
(b) water
(c) nitrogen
(d) alcohol

(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.

#### Question 8:

Which of these is the best insulator of heat?

(a) iron
(b) water
(c) nitrogen
(d) alcohol

(c) nitrogen

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

#### Question 9:

The transfer of heat from molecule to molecule, without movement of molecules from one place to another is

(a) conduction
(b) convection
(d) either of these

(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.

#### Question 1:

By which mode is heat transferred in solids?

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.

#### Question 2:

Are most metals good or bad conductors of heat?

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.

#### Question 3:

Gases are insulators of heat. True of false?

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

#### Question 4:

By which mode is heat transferred in liquids?

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.

#### Question 5:

Name a liquids which is a good conductor of heat.

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

#### Question 6:

When you heat water in a pan, by which mode does most of the heat travel in water-conduction, convection or radiation?

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

#### Question 7:

In which mode of transfer of heat does the medium not get heated up?

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.

#### Question 8:

A thermos flask consists of a double-walled glass vessel with the space between them filled with air, which is a bad conductor of heat. True of flase?

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.

#### Question 9:

Radiant heat falling on a body is partly absorbed and partly __________. Some of it may also be__________

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.

#### Question 3:

Describe the process of convection of heat.

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.

#### Question 4:

Explain with the help of diagrams how sea and land breezes are set up.

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.

#### Question 5:

Black objects absorb more heat than white or polished ones. Give two daily uses of this principle.

The everyday uses of the principle that black objects absorb more heat than white or polished ones are as follows:
1)      Cooking utensils have black-polished/black-painted outer surface to absorb more heat. This helps in cooking the food in less time.
2)      During winters, we wear black or dark coloured clothes, as these can absorb more heat and keep us warm than the white or dull coloured ones.

#### Question 6:

Black objects radiate more heat than white objects. Give two everyday uses of this principle.

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.

#### Question 7:

Draw a neat labelled diagram of a thermos flask. How does it prevent transfer of heat by conduction, convection or radiation?

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.

#### Question 8:

Two test tubes full of water are heated-one from below and the other near the top. In which case will the water heat up faster and why?

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.

#### Question 1:

In water, heat travels up much faster than it travels down. Explain.

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.

#### Question 2:

In places with hot climate, it is advised that the outer walls of houses be painted white. Explain why?

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.

#### Question 3:

Two objects at the same temperature-one bigger than the other-are placed in contact.
Will heat be transferred from one object to another? Explain.

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.

#### Question 4:

In a room it is best to place a room heater on the floor but an air conditioner near the ceiling. Why?

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.

#### Question 5:

Why are cloudy nights warmer than clear nights?

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

#### Question 6:

How do the feathers of a bird protect it from the cold?