CBSE Class 11 Physics Revision Notes Chapter 11

Class 11 Physics Revision Notes for Chapter 11 – Thermal Properties of Matter

The CBSE suggests utilising the Class 11 Physics Chapter 11 Notes for students to build a strong conceptual knowledge of the subject. It is clear that taking classes through the CBSE board increases the likelihood of students doing well on the AIEEE, IIT-JEE, NEET and AIPMT exams. Students can expect one question from Chapter 11 Thermal Properties of Matter with a weightage of 2% in the NEET exam as well as three questions from the Heat & Thermodynamics topic in the JEE Main exam.

1. Thermal Properties of Matter:

This topic covers a variety of thermal phenomena and how a substance responds to the transfer of thermal energy. We have a particular interest in

• Thermal Expansion.
• Heat and Calorimetry
• Transfer of Heat

1.1 Temperature and Heat

Temperature 

• Temperature is a measure of how hot or cold a body is relative to other bodies.
• SI Unit: Kelvin (K) 
• Commonly used unit: 
•  ∘ C or ∘ F ∘ C or ∘ F
• Conversion: 
• t(K) = t( ∘ C) + 273.15

Heat: 

• Heat is a kind of energy flow that happens when there is a temperature difference between 
  • two bodies, or 
  • between a body and its surroundings.
• SI unit: Joule (J) 
• Commonly used unit: Calorie (Cal) 
• Conversion: 1 cal = 4.186 J 
• A system with a higher temperature always transfers heat to a system with a lower temperature. 

1.2 Measurement of Temperature:

Principle: 

Temperature changes are used to observe thermometric properties, which are then compared to a set of reference conditions.

Usually, the ice point or steam point is the reference condition.

1.2.1 Celsius and Fahrenheit Temperature Scale:

It suggests that 100 degrees Celsius is similar to 180 degrees Fahrenheit.

tf-32180 = tc100

1.2.2 Absolute Temperature Scale:

• It is kelvin scale 

Ice point → 273.15 K

Steam point → 373.15 K

• The number of scale divisions in both systems is the same when compared to the Celsius scale.

tc − 0∘C100 = tk− 273.15100

tc − 0∘C100 = tk− 273.15100

• Due to the fact that it is practically impossible to get below 0 K on the negative side, the Kelvin scale is regarded as an absolute scale.

Thermometers

The temperature of any system can be measured with a thermometer. Examples include Constant Volume Gas Thermometers, Platinum Resistance Thermometers and Liquid in Glass Thermometers.

The readings from the Liquid in Glass thermometer and the Platinum Resistance thermometer are consistent for the ice point and steam point, but they differ for various liquids and substances.

No matter what gas is used, a constant-volume gas thermometer gives the same readings. Its foundation is the observation that Px T at low pressures and constant volume to get a gas

All gases collide to absolute zero at zero pressure. 

1.3 Thermal Expansion

It has been found that most materials expand when heated and contract when cooled. This growth has multiple dimensions.

The relationship between a change in temperature and a fractional change in any dimension has been proved through experimentation.

Coefficient of Volume Expansion of Cu as a Function of Temperature:

For ideal gases, γ is inversely related to the temperature at constant pressure.

V = nRT/P

⇒ΔV/V = ΔT/T

⇒ γ = ⊥/T 

On the other hand, water compresses when heated from 0о to 4о C, increasing its density from 0о to 4о C. The term “anomalous expansion” refers to this.

1.4 Heat and Calorimetry:

When two systems with different temperatures are connected, heat transfers from one system to the other until the systems’ temperatures are equal.

According to the calorimetry principle, heat loss from a body at a higher temperature is equal to heat gain from a body at a lower temperature, with heat loss to the surroundings being disregarded.

The body that receives heat either changes in temperature or state.

1.4.1 Change in Temperature:

During heating, if the temperature shifts, then

Heat supplied ∝ change in temp (ΔT)

 ∝amount of substance (m / n)

 ∝nature of substance (s / C)

 Δ H = ms Δ T

Where,

m – the mass of the body, 

S – specific heat capacity per kg, and

ΔT – change in temp 

Or, Δ H = nC Δ T

Here, 

n – the number of moles, 

C – the Specific/Molar heat Capacity per mole, 

Δ T – the change in temp. 

Specific Heat Capacity:

• It refers to the amount of heat required to raise a substance’s unit mass temperature by one degree.
• Units:

SI → J / KgK SH2O(e) = 1 cal / g∘C 

Common → Cal / g∘C S H2O(ice) = 0.5 cal / g∘C

Molar Heat Capacity:

• It is the amount of heat needed to increase a substance’s temperature by one degree for each unit mole.
• Units: SI → J / mol K

Common → Cal / gc∘

 Heat Capacity:

• It is the quantity of heat required to increase a system’s temperature by one degree.

⇒ΔH = SΔT

where S is heat capacity. Units 

SI → J / K  

Common → Cal / C ∘ 

• The difference in the specific heat capacity of H2O is minimal.
• Higher specific heat capacity materials need more energy to reach a given temperature.

1.4.2 Change in State:

The amount of material that changes the state (M) and the nature of the substance (L) is directly correlated with the heat delivered during phase transitions during heating.

⇒ΔH = mL

Where L is the latent Heat of process

1.4.3 Pressure dependence on melting point and boiling point:

With increased pressure, some substances’ melting points drop while others go up.

As the temperature rises, the melting point rises as well.

Triple Point

All three states of matter (solids, liquids, and gases) coexist at a given pressure and temperature. 273.16 K and 0.006 atm for water are its values.

The critical point is the range of pressure and temperature beyond which vapour cannot become a liquid. The critical temperature and critical pressure are the corresponding temperatures and pressures.

The phasor diagram demonstrates that as pressure rises, the melting point of water falls. The idea of regulation is built on this.

Regulation: The process whereby pressure causes water that has melted below its normal melting point to refreeze is called Regulation. Due to the pressure’s impact on the melting point, cooking on mountains is challenging, whereas cooking in a pressure cooker is simpler.

1.5 Heat Transfer:

There are three modes of heat transfer:

• Conduction 
• Convection 
• Radiation 

1.5.1 Conduction

The process of thermal conduction involves the transmission of thermal energy from a body’s hotter to cooler regions or from a hot body to a cold body that is in close proximity to it without the movement of any material particles. At a steady state, the rate of heat energy flowing through the rod stabilises.

Q = kA(TC – TD)L

The rate for rods with a uniform cross-section is shown here.

Here, 

Q – the rate of heat energy flow, 

A – the cross-sectional area, 

TC – the hot end temperature,

TD – the cold end temperature, 

L – the rod’s length, and

K – the thermal conductivity coefficient.

Being burned after touching a stove is one example of conduction that we encounter every day. Ice is used to cool the hand. The water is brought to a boil by dropping a piece of iron into it that is extremely hot.

Coefficient of Thermal Conductivity:

It is described as the quantity of heat transferred over a unit area of any cross-section of a substance in a unit of time during a steady state, with the heat flow being normal to the area.

SI units: SI → J / mSk or W/mK

For a given temperature difference, thermal energy flows more quickly the higher the thermal conductivity.

Compared to nonmetals, metals have a higher thermal conductivity.

Thermal conductivity is very low for insulators. Heat energy cannot thus be easily transferred through the air.

Combinations of rods between two ends kept at different temperatures can use the concept of equivalent thermal conductivity of the composite rod.

Keq is the equivalent thermal conductivity of the composite. 

Temperature Gradient:

The term “temperature gradient” describes the drop in temperature over a given length when heat energy is flowing in that direction.

Units: SI → K/m

The term Q is the rate of heat energy flow, which is also known as heat current.

Any conducting rod’s thermal resistance is denoted by the symbol (L / KA).

Thermal Resistance:

It is characterised as the restriction of the heat current by the medium.

Units: SI → K/W

1.5.2 Convection:

The movement of heated material particles from a higher temperature location to a lower temperature location is known as thermal convection, which is the process by which heat is transferred from one point to another.

• When the medium is made to move by a fan or a pump, forced convection occurs. The material moves because of variations in the medium’s density, which is known as natural or free convection.
• The circulatory, cooling and heating systems of an automobile are examples of forced convections.
• Trade winds, sea breeze/land breeze, monsoons and tea burning are a few examples of natural convections.

1.5.3 Radiation:

It is a technique for transmitting heat in which heat moves directly between two points without passing through a middle medium.

EM waves are the primary form of heat radiation.

Like a red-hot iron or a filament lamp, these radiators emit light as a result of their temperature.

Everyone emits energy into the environment and also absorbs energy from it. The body’s colour has an impact on how much energy is absorbed.

The thermal electromagnetic radiation from a black body inside of or surrounding a body in thermodynamic equilibrium with its surroundings is known as black-body radiation (an idealised opaque, non-reflective body). It has a particular spectrum of wavelengths that are inversely proportional to the intensity and are only influenced by the body’s temperature, which is taken for calculations and theoretical purposes to be uniform and constant.

Newton’s Law of Cooling:

Newton’s law of cooling explains how quickly a body cools. The rate of cooling of an object is directly influenced by the temperature differential between the body and its surroundings. Newton’s Law of Cooling is explained in length in Class 11 Physics Chapter 11 Notes.

Approximation:

If a body cools from Ta to Tb in t times in a medium with a temperature of T0, then

(Ta − Tb) = K − Ta+Tb2 −T0

Newton’s law of cooling can be studied experimentally.

Configuration:

A double-walled container (v) with water between the two walls.

The copper calorimeter (c), which is enclosed in the double-walled vessel, is filled with hot water.

T2 of H2O is measured using two thermometers that are threaded through the carbs between the two walls of the water calorimeter T.

Experiment:

The hot water temperature of the calorimeter is recorded at regular intervals.

Consequently, a line graph between log (T2 T1) and time is created (t).

As predicted by Newton’s law of cooling, a straight line can be seen to be drawn on the graph.