9th Class Chapter 8 Physics Short Questions

Define Heat

Heat is a form of energy transferred from a hotter body to colder body.

Write Distinguishing Temperature, Heat and Internal Energy.

Using the kinetic theory, we make a clear distinction between temperature, heat and internal energy. Temperature is a measure of the average kinetic energy of individual molecules. Internal energy refers to the totla energy of all the molecules within the objet. Thus two equal mass hot ingots of iron may have the same temperature. but two of them have twice as much internal energy as one does. Heat, finally, refers to a transfer of energy from one object to another becaus eof a difference in temperaure.

Conversion between centrigrade and Fahrenheit scale.

Define Latent heat of vaporization.

The amount of heat energy requried to convet a given mass of a substance from liquid state to the gaseous state without any rise in temperature is called its latent heat of vaporization Gases release the same amount of heat when they liquify.

What are various temperature scales. Derive mathematical expressions toconvert between variious scales of temperature.

Temperatrue Scales: The scale which is made for the measurement of temperature is called temperature scale or thermometric scale. The scale comprises of two refrence points. called fixed points. there are freeing point and boiling point . The interval between these point is divided arhitarily into equal divisions. There are three scales of temperature which are the following.

Conversion between centigrade and Kelvin scale.

What is meant by the latent heat of fusion

The amount of heat energy is required to convert a given mass of a substance from the solid state to the liquid state without any rise in temperatue is called its latent heat of fusion Liquids release the same amount of heat when they solidfy.

What is meant by Evaporaton?

The process by which a liquid slowly changes into its vapors at any temperature without the aid of any external sources of heat is called evaporation of liquids.

Define Heat capacity and Specific heat capacity of substance.

Heat Capacity : The quantity of heat required to raise the temperature of a substance of mass by 1 oC or 1 k is called the heat capacity of the substance.

How do we measure temperature?

Temperature could be measure din a simple way be using our hand to sense the hotnes sor coldness of an object, However, the range of temperatures that ur hand can bear is very small and our hand is not precise enough to measure temperature correctly. The branh of physics which deals with the measurements of temperature is called thermometry. For scientific work, we need some reliable devie or instruments to measrue temperature accurately Such an instrument is called thermometer.

Write Relationship between different Scales of Tempeature.

Ordinary electric fan increases the kinetic energy of the air molecules caused by the fan blades pushingthem means the air temperature increse slightly rather than cool the air? Why use it.

For cooling we usually use electric fans But the electric fan does not actually cool the air inside the room. It increases the kinetic energy of air molecules due to which the temperature of molecules increases. Such high speed molecules touch ur body and evaporate water molecules from our body. The evporated water molecules absorb heat of vaorization from our body and as a result we feel cool.

Why are small gap left behid the girders mounted in walls?

Small gaps are left behidn the girders mounted in walls because to allow for he expansion of the girders during summer usually one end of the iron structure is fixed and the other end is allowed to expendin summer into the left out out gap. If there is nogap left, then the expansion will cause the girders to buckle.

An iron rim which is fixed arond a wooden wheel is heated before its fixture. Explain why?

An iron rim which is fixed around a wooden wheel is heated before its fixture becasue at room temperature the diameter of the iron rim is made slightly less than the diameter of wooden wheel. The rim expands on heating and can be plaed around wooden wheel. When the rim comes to room temperature, it contracts and produces tight fit.

Explain why it is advisable to add water to an overheated automobile engine only slowly, and only with the engine running.

It is advisable to add water to an overheated automobile engine slowly and only with the engine runningit is because if we add water quicely to an overhead engine, water will come into contat with the hot metal part of the engine.Some are of metal part wil cool down very rapidly, While other part wil not, some part of the water will quickly turn to steam and will rapidly expand which can result a cracked engine block or radiator.

Why is ice at 0 oC a better oolant of sof drink than water at 0 oC.

The ice when absorb heat, it melts, During melting the temperature of ice remains constant at 0 oC. When the ice is put in the soft drink, it absorbs heat from the drink So, its temperature falls and become cool. But in case of water at 0 oC when it absorbs heat its temperaure rises.Therefore, ice is the better coolant than water at 0 oC

Why we feel cool after perspiration?

We feel cool after perspiration because for evaporation threre is need of heat energy. When the sweat drops evaporate from our body it absorbs heat from our body and as a result we feel cool due to decreasse in temperature of our body.

Why you should not put a closed glass jar into a campfire.What could happenif you tossed an empty glass jar, with the lid on tight, into a fire.

When we put a closed glass jar into a campfire, the inside of theglass jar is not empty, It is filled with air, As the fire heats the air inside, its temperature rises and as a result the pressure inside the jar increses. Due to high internal pressure the jar may explode andturn into pieces. Also if we tossed an empty glass jar into a fire, then due to high internal pressure in jar may cause it crack. So we should not throw a close glass jar into a campfire.

Explain why bruns caused by steam at 100 oC on the skin are often more sever than burns caused by water at 100 oC

When the temperature of water rises at 100 oC, water is converted to steam. At this point the temperature remains constant. Although heat is being given to water hteat equal to latent heat of vaporizaton i.e.