Physics Notes for Class 9 Chapter 8 Thermal Properties of matter

Physics Notes for Class 9 Chapter 8 Thermal Properties of matter

Physics Notes for Class 9 Chapter 8 Thermal Properties of matter.

Free Download in PDF Format. Physics Notes for Class 9 Chapter 8 Thermal Properties of matter.

Thermal Properties of matter

Comprehensive Questions

Q1: Explain the term internal energy and temperature. Use kinetic theory to distinguish between heat, internal energy and temperature.

Ans: Internal Energy:

Internal energy is the sum of the kinetic and potential energies associated with the motion of the atoms of the substance.

Explanation:

When we touch a hot body, internal energy flows in the form of heat into our body, so it appears to be hot. On the other hand, when we touch a cold object internal energy flows as heat from our body into the cold object, so it appears to be cold.

Temperature:

The measure of the degree of hotness or coldness of a body with respect to some standard is called temperature.

Or

The temperature can also be defined as: “The average kinetic energy of molecules of a body.”

Explanation:

Temperature is a measure of the average kinetic energy of particles. The kinetic energy may be in the form of translational, vibrational and rotational kinetic energy. As atoms or molecules of the material are in constant motion, at high temperature the kinetic energy of molecules is more and at lower temperatures, it is less.

Temperature also affects the physical states (shape, size) of material. For example, water at low temperature is ice (a solid), at a high temperature it is water (a liquid) and still at higher temperature it is steam (a gas).

Distinguishing Temperature, Heat & 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 total energy of all the molecules within the object.

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 because of a difference in temperature.

Q2: How do we measure temperature? Explain liquid in glass thermometer.

Ans: Measurement of temperature:

Temperature could be measured in a simple way by using our hand to sense the hotness or coldness of an object. However, the range of temperatures that our hand can bear is very small and our hand is not precise enough to measure temperature correctly.

The branch of physics which deals with the measurements of temperature is called thermometry. For scientific work, we need some reliable device or instrument to measure temperature accurately. Such an instrument is called thermometer.

Liquid in glass thermometer:

The liquid in glass thermometer utilizes the variation in volume of a liquid in temperature.

Construction or working:

The fluid is contained in a sealed glass bulb, and its expansion is measured using a scale etched in the stem of the thermometer. The thermometer utilizes the variation of length of liquid with temperature. In this type the liquid in a glass bulb expands up a capillary tube when the bulb is heated. The liquid must be easily seen and must expand (or contract) rapidly and by a large amount over a wide range of temperature.

The tube has a constriction just beyond the bulb. When the thermometer is removed, the liquid in the bulb cools and contracts breaking the liquid (mercury) thread at the constriction. The liquid beyond the constriction stays in the tube and shows the temperature. It must not stick to the inside of the tube. Liquids commonly used include mercury and alcohol.

Q3: What are various temperature scales? Derive mathematical expressions to convert between various scales of temperature.

Ans: Temperature Scales:

The scale which is made for the measurement of temperature is called temperature scale or thermometric scale. The scale comprises of two reference points, called fixed points. There are freezing point (ice point) and boiling point (steam point). The interval between these point is divided arbitrarily into equal divisions. There are three scales of temperature which are the following.

2. Fahrenheit scale.
3. Kelvin or absolute scale.
2. This scale was introduced by a Swedish astronomer Anders Celsius.
3. It is denoted by ‘oC’.
4. Its ice point is marked as 0oC.
5.  Its steam point is marked as 100oC.
6. The interval between ice point and steam point is divided into 100 equal parts (divisions).
7. Each part is called degree centigrade.
• Fahrenheit Scale:
• This scale was introduced by German physicist Daniel Gabriel Fahrenheit.
• It is denoted by oF.
• Its ice point is marked as 32oF.
• Its steam point is marked as 212oF.
• The interval between ice point and steam point is divided into 180 equal parts (divisions).
• Each part (division) is called degree Fahrenheit.
• Kelvin or absolute scale:
• This scale was introduced by William Thomson, (Lord Kelvin). He named this scale as absolute scale.
• It is denoted by K.
• Its ice point is marked as 273 K.
• Its steam point is marked as 373 K.
• The interval between ice point and steam point is divided into 100 equal parts.
• The lowest temperature at which the molecular movement of matter ceases is called Kelvin zero or absolute zero. Its magnitude on the Celsius scale is -273 oC or (0K).
• Kelvin is the S.I unit of temperature.

Relationship between Different Scales of temperature:

A Temperature measured on one scale sometimes, needs conversion to another scale. A − ℎ − general relation for the conversion of temperature= from one scale to the other is.

Q4: What is meant by linear thermal expansion and volume thermal expansion of solids?

Ans: Linear Thermal Expansion of Solids:

Definition:

The increase in length of a substance due to rise in temperature is called linear thermal expansion.

Mathematical Derivation:

Consider a metal rod having an original length “lo” at temperature “To”. After heating metal rod to temperature “T”, the rod expands to its new length “lT”. This means for the change in temperature ΔT (where ΔT = T – To) there is corresponding change in length Δl (where Δl = lT – lo).

The change in length Δl of almost all solids is directly proportional to the change in temperature ΔT as long as is not too large. This means by changing temperature the length also changes, more the change in temperature more is the change in length and vice versa.

Δl                    ΔT………(i)

The change in length Δl∝is also directly proportional to original length lo of the object.

i.e.                               l                       lo …..(ii)

(ii), we get

Combining eq (i) and eq∝

Were “ ” the               Δl                    lo ΔT

Changing proportionality into equality.

∝

Δl =                 lo         T …..(iii)

Proportionality constant is called the coefficient of linear thermal expansion for the         particular material.         lT = lo +          lo ΔT

Taking lo common

Since Δl = lT – lo , we can write eq(iii) as

lT – lo =          lo ΔT

lT = lo (1+       ΔT)

If the temperature change ΔT = T – To is negative, then Δl = lT – lo is also negative; the length shortens as the temperature decreases.

Coefficient of linear thermal expansion:

From eq (iii), we can define coefficient of linear thermal expansion “ ” of a substance as the increase in length per unit length of the solid per Kelvin “K” rise in temperature = In simple words,  is numerically the increase in 1m long wire for 1 degree rise of temperature.

The value of depends upon the nature of material and is different for different materials.

Unit:

The coefficient of linear thermal expansion has units of oC-1 and in SI as K-1.

Volume (cubical) Thermal Expansion of Solids:

Definition:

The increase in volume of a substance due to rise in temperature is called volume thermal expansion.

Explanation:

Consider a metal block having an original volume “Vo” at temperature “To”. After heating metal block to temperature “T”, the block expands to its new volume “VT”. This means for the change in temperature ΔT (where ΔT = T – To), there is corresponding change in volume ΔV (where ΔV = VT – Vo).

The increase in volume of a metal block on heating is directly proportional to original volume of the metal block and rise ∝ in temperature.

Mathematically,

ΔV       ΔT …..(i) and ΔV ∝               Vo …..(ii) combining eq (i) & eq (ii), we get∝

ΔV       Vo ΔT

Changing proportionality in equality

ΔV = γ Vo ΔT …….(iii)

Where “γ” is the proportionally constant is called the coefficient of volume thermal expansion for the particular material.

Since ΔV = VT – Vo we can write eq(iii) as

VT – Vo = γ Vo ΔT

VT = Vo + γ VoΔT

Taking Vo common

VT = Vo (1 + γ           T) …..(iv)

Eq (iv) represents the final volume of the object after expansion.

Coefficient of volume thermal expansion:

From eq(iii), we can define coefficient of volume thermal expansion (γ) of a substance as the change in volume per unit volume per Kelvin change in temperature.

γ =

The volume of γ depends upon the nature of material and is different for different materials.

Unit:

The coefficient of volume thermal expansion has unit of oC-1 and in SI as K-1.

This is general rule for solids that they expand to the same extent in three directions. It can be proved that all the coefficient of volume thermal expansion of solids γ is about three times the coefficient of linear thermal expansion ‘α’ of solids i.e.

γ = 3 α

so, eq 3 becomes

ΔV = 3 α Vo ΔT

Q5: What is thermal expansion of liquid? Why we have real and apparent thermal expansion in liquids. Illustrate with the help of an experiment.

Ans: Thermal Expansion of Liquids:

The increase in the volume of a liquid due to the thermal effect of heating is called thermal expansion of liquids. Since heat affects both the liquid and the container the real expansion of a liquid cannot be detected directly. In case of liquids, we have two kinds of thermal expansion.

1. Real expansion
2. Apparent expansion
1. Real expansion of liquid:

A real increase in the volume of a liquid that take place due to increase of temperature is called real expansion ( VR ) of liquid. This expansion is independent of the expansion of the container.

• Apparent expansion of liquid:

An apparent increase in the column of a liquid that takes place due to increase of temperature is called apparent expansion (VA ) of liquid. When a liquid is taken in a container and heated, both the liquid and the container expand at same time.

The difference of these expansions is called apparent expansion. If VR is the expansion in the volume of the liquid (called real expansion) and VC is the expansion in the volume of container on heating, then the apparent expansion VA is given by as;

VA = VR – VC

Experiment:

Let a vessel has water up to level A. If heat is applied, the vessel will first expand which will produce an illusion that the water has fallen. This is due to the expansion of the vessel and is given by the levels i.e. AB. On further heating the heat energy will start reaching the liquid.

The liquid will then start expanding rapidly, according to its nature exceeding its previous level to reach up to level C. So the measurement of BC gives the true (real) expansion of the liquid only. An observer presents at the start and at the end will see the whole process as just the expansion of the liquid from A to C. So AC measures the apparent expansion of the liquid.

Mathematically:

BC = AC + AB

Real expansion of liquid = Apparent expansion of liquid + Vessel Expansion.

Since there are two different types of expansion of liquids their coefficients of expansion should also be defined differently.

Coefficient of real expansion “γR”:

It is defined as the apparent increase in volume of liquid per unit original volume per unit degree rise in temperature=. Its unit is per degree rise in temperature i.e. oC-1 or K-1

Coefficient of apparent expansion “γA”:

It is defined as the apparent increase in volume of liquid per unit original volume per unit degree rise in temperature=. Its unit is per degree rise in temperature i.e. oC-1 or K-1.

Q6: Define heat capacity and specific heat capacity of a substance. Explain the importance of high specific heat capacity of water.

Ans: Heat Capacity (Thermal Capacity):

The quantity of heat required raising the temperature of a substance of mass (m) by 1 OC or 1 K is called the heat capacity (cm) of that substance.

Mathematically:

If ΔQ is the change in heat and ΔT is the change in temperature, then

cm =

The value of “cm” depends upon.

1. The nature of the material of the substance.
2. The mass of the material of the substance.
3. The rise in temperature.

Unit:

The S.I unit of heat capacity is joule per Kelvin which is expressed as JK-1.

Specific heat capacity (specific heat):

The quantity of heat required to raise the temperature of unit mass (1.0 kg) of the substance     by 1oC or 1K is called specific heat capacity of that substance.

Mathematically:

C =                              ∴          cm = ΔQ

ΔT

Putting value of cm

C =

Unit:

The S.I unit of specific heat capacity or specific heat is joule per kilogram per Kelvin which is expressed as JKg-1 K-1.

Importance of the high specific heat capacity of water:

The specific heat capacity of water is equal to 4190 JKg-1 K-1. It has some important implications.

1. Moderate climate of sea shore:

The specific heat of sand is about 800 JKg-1K-1. A certain mass of water needs five times more heat than the same mass of solid for its temperature to rise by 1oC or 1 K. Hence, the land gets heated much more easily than water.

Also it cools down much easily hence a large difference in temperature is formed that gives rise to land breeze and sea breeze. It keeps the climate of the coastal areas moderate moon soon in Pakistan is also due to the difference in temperature between the land and the surrounding sea.

• As a coolant:

Water is used as an effective coolant. By allowing water to flow in radiator pipes of the vehicles, heat energy from such part is removed. Thus, water extracts much heat without much rise in temperature.

Q7: What is meant by the latent heat of fusion and latent heat of vaporization of a substance?

Ans: 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 (melt) without any rise in temperature is called its latent heat of fusion. Liquids release the same amount of heat when they solidify (freeze).

Specific latent heat of fusion:

The amount of heat energy required to convert unit mass (1 kg) of solid at its melting point of liquid (or liquid into solid) without any change in temperature is called its specific latent heat of fusion of the solid.

Explanation:

If “ΔQ” is the amount of heat energy needed to melt mass “m” of a solid to liquid (or freeze liquid to solid), then mathematically.

ΔQ = mLf

Where Lf is the latent heat of fusion of substance and is given as

Lf =

Unit:

The S.I unit of specific latent heat of fusion is joule per kilogram which is expressed as JKg-1. Different substances have different specific latent heat of fusion.

Latent heat of vaporization:

The amount of heat energy required to convert a given mass of a substance from liquid state to the gaseous state (boil) without any rise in temperature is called its latent heat of vaporization. Gases release the same amount of heat when they liquefy (condense).

Specific latent heat of vaporization:

The amount of heat energy required to convert unit mass (1 Kg) of the liquid at it boiling point to gas, (or gas into liquid) without any change in temperature is called its specific latent heat of vaporization of the solid.

Explanation:

If “ΔQ” is the amount of heat energy needed to vaporize mass “m” of a liquid to gas (or condense gas to liquid), then mathematically.

ΔQ = m Lv

Where Lv is the latent heat of vaporization such that

Lv =

Unit:

The S.I unit of specific heat of vaporization is joule per kilogram which is expressed as JKg-1. Different substances have different specific latent heat of vaporization.

Q8: What is meant by evaporation? On what factors the evaporation of liquid depends. Explain how cooling is produced by evaporation. Differentiate between boiling and evaporation.

Ans: Evaporation of liquid:

The process by which a liquid slowly changes into its vapors at any temperature (below its boiling point) without the aid of any external source of heat is called evaporation of liquids.

Explanation:

Liquid starts to boil if they are heated to their boiling temperatures. The liquid starts to transform into vapors’ but the change of liquids into vapors goes on even when the temperature is below the boiling point. For example, a spread wet cloth on being exposed to the air becomes dry in a short time due to evaporation of water. Water left in open dish also disappears due to evaporation.

We know that the molecules of a liquid move with wide of range of instantaneous velocities and they have different kinetic energies ranging from minimum to a very high value. Some of the molecules having sufficient kinetic energy to overcome the forces of attraction leave the surface of the liquid and escape out in the form of vapors. We call this escaping of high energy molecules as evaporation.

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