9th Class Chapter 9 Physics Short Questions

Define Radiation of Heat.

The heat transfers from a hotter plae to a colder place with or without having material medium in between is called radiation of heat.

Write Radiation of Heat Mechanism

Mechanism of Radiation of Heat: The mechanism of radiation is energy transfer by electromagnetic waves. Electromagnetic radiation comes from accelerating electric charges. Unlike conduction and convection transfer of heat by radiation, does not necessarily require a material medium. Heat energy transferred through radiation is a familiar as thelight, in fact, it is the light but not visible or barely visible.

Why white clotes are preferred wearing in summer ? Explain brefly.

The white and light coloured clothes are most suitable in summer because they reflect maximum sun radiation and absorb minimum sun radiation and thus keep our body cool. That's why we feel comfort by wearing white clothes in summer.

Why is the freezer compartment kept at the top of refrigretor ? Explain briefly.

The freezer compartment kept at the top of a refricgetor, because it cools the surrounding air and makes it denser as compare to the air at the bottom. Convectional current are used to circulate cold air in a refrigetor.The cool denser air moves down while the warmer air at the bottom moves up, where it is cooled. That is why, he freezer compartment is kept at the top of a refrigerator.

Define the Green house effect.

Green House effect: The green hous effect is the process by which radiation from a planet\s atmosphere warmsthe planet\s surface to a temperature above what it would be without its atmosphere.

Define and explain thhermal conductivity of a substance and is mathematicla description.

Thermal Conductigy: The thermal conductiviy of a substances in measure of the ability of the substances to conduct heat energy.

What are good and bad conductors?

Good Conductor: some moterials allow heat to pass through them easily. They are called heat conductor or good conductors.

Write importance of Green house effect.

Green house effect is important for the survival of life on earth. Earth atmosphere containign naturally occuring greenhouse gases causes aire temperature near teh surface to be about 33 oC warmer. Without the Earth's atmosphere, the Earth's average temperatue would be well below the freezing temperature of water.

Explain conduction of heat and its mechanism.

Conduction of Heat: The process by which heat energy is trnasferred from particle to particle by collision is clled conduction of hea.

Write Mechanism of Green house Effect.

Write Concerns of Globla Warming

How woolen swaters keep us warmer in winter?

Woolen swaters have fine pores filed with air.Air and wool are bad conductors of heat.Thus heat from our body does not flow out to atmospher.Thus woolen sweater keep our boyd warm in winter.

In certain places, birds can fly for hours without flapping their wings, Explain.

In certain places , there are streams of hot air which are arising from the sun such streams of hot air is known as thermalls. Thermals are convecton currents. The birds can fly for hours on thermals without flapping their wings.And this phenomenon occurs due to the convection of heat.

Good quality thermos bottle is double walled and evacuated between these walls, and the internal surfaces are like mirrors with a silver coating. How does this configuration combat heat loss for all three transfer methods and keep teh bottle's contents your coffee bot?

A Themos bottle consists of a double walled glass vessel silversed on the inside. The silvered surface reflects all the radiant heat trying to enter or leave the vessel by radiation. The space between the walls is highly evacuated to prevent convection. The glass being a poor conductor minimizes the conduction of heat. That is why a liquid placed in the thermos bottle will remain bot for a long time.

A piece of wood lying in the sun absorbs more heat than a piece of shiny metal. Yet the metal feels hotter than the wood when you pick it up. Explain.

Why a tile Floor feels colder to bare feet than a carpeted floor?

A carpet is a bad conductor of heat as compared to tle floor. When bare feet are put on the tile floor more heat is lost by the feet which is absorbed b the tile floor and as a result we feel cool. If we put the feet on a carpetd floor, we feel less coldness because in this case our feet losses no heat or very less amount of heat to the carpet. That is why a tile floor feels colder to bare feet than a carpeted floor.

When sunlight warmstheland beside a cool body of water, a breeze begins to blow from the water towards the land.Explain.

: Convection causes coastal breeze. During the day the sun shines equally on land and sea. The land heats up more quickly than the sea. The hot air over the land rises and the cold air from the sea blows to replace it. Thus there is a sea breeze during the day time.

A black car standing in the sun warms up more quickly than any other. Why?

A black car standing in the sun warms up more quickly than any other car because the black surfae is a good absorber or heat radiaton than any other surface.

Some pot handlesremain cool during cooking while others become unpleasantly hot. What determine which handles remain ool and which become hot?

:-The thermal conductivity of the material of handles determines which handle remain cool and which become hot. When a pot handle is made of material which has high value of thermal conductivity such as metal, then it becomes hot quickly. On the other hand, if the pot handle is made of material which has low thermal conductivity such as wood, then it does not get hot. Thus the handle of good conductors becomes hot and the handles of bad conductors remain cool