### HEAT CHAPTER – 11

**HEAT DEFINITION:**

Total Kinetic energy of a body is known as HEAT.

OR

Transfer of energy from a hot body to a cold one is

termed as Heat.

Heat is measured by using an measurement

centimeter.

**UNITS**

Since heat is a force of energy therefore its unit is

Joule (J).

**TEMPERATURE**

DEFINITION

DEFINITION

The average kinetic energy of a body is known as

Temperature.

OR

The quantitative determination of degree of hotness

may be termed as Temperature.

**SCALES OF TEMPERATURE**

There are three main scales of temperature.

1. Celsius Scale

2. Fahrenheit Scale

3. Kelvin Scale

Celsius and Fahrenheit scales are also known as

Scales of Graduation.

1. Celsius Scale

The melting point of ice and boiling point of water at

standard pressure (76cm of Hg) taken to be two

fixed points. On the Celsius (centigrade) scale the

interval between these two fixed points is divided

into hundred equal parts. Each part thus represents

one degree Celsius (1°C). This scale was suggested

by Celsius in 1742.

Mathematically,

°C = K – 273

OR

°C = 5/9 (°F – 32)

2. Fahrenheit Scale

The melting point of ice and boiling of water at

standard pressure (76cm of Hg) are taken to be two

fixed points. On Fahrenheit scale the lower fixed

point is marked 32 and upper fixed point 212. The

interval between them is equally divided into 180

parts. Each part represents one degree Fahrenheit

(1°F).

Mathematically,

°F = 9/5 (°C + 32)

3. Kelvin Scale

The lowest temperature on Kelvin Scale is -273°C.

Thus 0° on Celsius scale will be 273 on Kelvin scale

written as 273K and 100 on Celsius scale will be

373K. The size of Celsius and Kelvin scales are

same.

Mathematically,

K = °C + 273

**THERMAL EQUILIBRIUM**

Heat flows from hot body to cold body till the

temperature of the bodies becomes same, then they

are said to be in Thermal Equilibrium

.

**THERMAL EXPANSION**

**DEFINITION**

The phenomenon due to which solid experience a

change in its length, volume or area on heating is

known as Thermal Expansion.

Explanation

If we supply some amount of heat to any substance

then size or shape of the substance will increase.

This increment is known as Thermal Expansion.

Thermal expansion is due to the increment of the

amplitudes of the molecules.

Types of Thermal Expansion

There are three types of Thermal Expansion.

1. Linear Expansion

2. Superficial Expansion

3. Volumetric Expansion.

1. Linear Expansion.

If we supply some amount of heat to any rod, then

the length of the rod, then the length of the rod will

increase. Such increment is known as Linear

Expansion.

2. Superficial Expansion.

If we apply some amount of heat to any square or

rectangle then area of the square or rectangle will

increase. Such increment is known as Superficial

Expansion.

3. Volumetric Expansion

.

If we apply some amount of heat to any cube, then

the volume of the cube will increase. Such increment

is known as Volumetric Expansion.

**COEFFICIENT OF LINEAR EXPANSION**

**CONSIDERATION**

Let Lo be the initial length of rod at t1 °C. If we

increase the temperature from t1 °C to t2 °C, then

length of the rod will increase. This increment in

length is denoted by ΔL. The increment in length

depends upon the following two factors.

1. Original Length (Lo)

2. Difference in temperature Δt

Derivation

The increment in length is directly proportional to

the original length and temperature difference.

Mathematically,

ΔL ∞ Lo —– (I)

ΔL ∞ Δt —– (II)

Combining eq (I) and (II), we get

ΔL ∞ LoΔt

piston is allowed to move upward. When we do so

temperature and pressure of the working substance

will decrease while volume will increase. In order to

keep the temperature constant, we have to supply

required amount of heat from source to cylinder.

Since in this expansion, temperature is constant

therefore it is known as Isothermal Expansion.

**ii. Isothermal Compression**

In this process, cylinder is placed on a sink and

piston is allowed to move downward. When we do

so temperature and pressure of working substance

will increase while volume will decrease. In order to

maintain the temperature, we have to reject required

amount of heat from cylinder to the sink.

Since in this compression, temperature is kept

constant therefore it is known as isothermal

compression.

**SECOND LAW OF THERMODYNAMICS**

**Introduction**

It is inherit tendency of heat that it always flows

from hot reservoir to cold reservoir. Rather than to

flow in both the directions with equal probability.

On the basis of this tendency of heat a law was

proposed that is known as Second Law of

Thermodynamics.

**Statement**

It is impossible to construct a process which reserves

the natural tendency of heat.

This law is also known as Law of heat and can also

be stated as

Efficiency of heat engine is always less than unity.

**Explanation**

Many statements of this law has been proposed to

cover similar but different point of vies in which two

are given below.

1. Lord Kelvin Statement

2. Clausius Statement

**1. Lord Kelvin Statement**

**According to this statement,**

It is impossible to construct a heat engine which

extract all heat form the source and convert it into

equal amount of work done and no heat is given to

the sink.

Mathematically,

Q1 ≠ W

Q2 ≠ O

**2. Clausius Statement**

According to Clausius Statement,

Without the performance of external work heat

cannot flow from cold reservoir towards, the hot

reservoir.

**Example**

In case of refrigerator flow of heat is unnatural but

this unnatural flow of heat is possible only when we

apply electrical power on the pump of the

refrigerator.

Qs. Define the term Entropy and Give its Uses

**ENTROPY**

**Definition**

It measures the disorderliness of any system.

**Mathematically**,

ΔS = ΔQ/T

Where Δs shows change in entropy.

**Units**

Joule per degree Kelvin – J/°K.

**Explanation**

As we know that incase of isometric process volume

is constant. In case of Isothermal process

temperature and pressure is constant, but in case of

adiabatic process neither temperature, nor pressure

or volume is constant but one thermal property is

constant which is known as Entropy.

There are two types of Entropy.

1. Positive Entropy

/

2. Negative Entropy

**1. Positive Entropy**

If heat is supplied to the system the entropy will be

positive.

**2. Negative Entropy**

When heat is rejected by the system the entropy will

be negative.

**Qs. What is carbot engine an carnot cycle?**

**CARNOT ENGINE**

**Definition**

‘Carnot engine is an ideal heat engine which

converts heat energy into mechanical energy.

Working of Carnot Engine

It consists of a cylinder and a piston. The walls of

the cylinder are non-conducting while the bottom

surface is the conducting one. The piston is also non-

conducting and friction less. It works in four steps.

Which are as follows.

1. Isothermal Expansion

2. Adiabatic Expansion

3. Isothermal Compression

4. Adiabatic Compression

**1. Isothermal Expansion**

First of all, cylinder is placed on a source and allow

to move upward as a result temperature and pressure

of the working substance decreases, while volume

increases. In order to maintain temperature we have

to supply more amount of heat from source to the

cylinder. Since in this expansion temperature is kept

constant.

**2. Adiabatic Expansion**

Secondly cylinder is placed on an insulator and

piston is allow to move downward as a result

temperature and pressure of working substance will

decrease. While volume will increase but no heat is

given or taken of the cylinder.

**3. Isothermal Compression**

In this state cylinder is placed on a sink and piston is

allow to move downward as a result temperature and

pressure of the working substance will increase

while volume will decrease. In order to maintain

temperature we have to reject extra heat from

cylinder to the sink. Since in this compression

temperature is constant.

**4. Adiabatic Compression**

Finally cylinder is placed on an insulator and piston

is a flow to move downward, when we do so neither

temperature nor pressure or volume is constant. But

no heat is given or taken out of the cylinder.

**CARNOT CYCLE**

**Definition**

By combining the four processes Isothermal

Expansion, Adiabatic Expansion, Isothermal

Compression and Adiabatic Compression which are

carried out in carnot engine, then we get a cycle

knows as Carnot cycle.

**Qs. How can we increase the efficiency of Heat**

Engine?

Engine

If we want to increase the efficiency of any heat

engine then for this purpose we have to increase

temperature of source as maximum as possible and

reduce the temperature of sink as minimum as

possible.

**Qs. Define Specific Heat and Molar Specific Heat.**

SPECIFIC HEAT

Definition

SPECIFIC HEAT

Definition

Specific heat is the amount of heat required to raise

the temperature of a unit mass of a substance by one

degree centigrade.

Different substances have different specific heat

because number of molecules in one kg is different

in different substances. It is denoted by c.

Mathematical Expression

Consider a substance having mass m at the

temperature t1. The amount of heat supplied is ΔQ,

which raises the temperature to t2. The change in

temperature is Δt.

The quantity of heat is directly proportional to the

mass of the substance.

ΔQ ∞ m

And the temperature difference

ΔQ ∞ Δt

Combining both the equations

ΔQ ∞ mΔt

=> ΔQ = cmΔt

=> c = ΔQ / mΔt —- (I)

Where c is the specific heat of the substance. Its unit

is Joules / Kg°C.

**MOLAR SPECIFIC HEAT**

Definition

Definition

Molar specific heat is the amount of heat required to

raise the temperature of one mole of a substance

through one degree Celsius.

Almost all the substances have the same amount of

molar specific heat because the numbers of

molecules in all substances are same in one mole. It

is denoted by cM.

Mathematical Expression

Mathematically,

No. of Moles = Mass / Molecular Mass

=> n = m / M

=> nM = m

=> nM = ΔQ / nΔt

Where n is the number of moles. The unit of molar

specific heat is J/Kg°C.

**Qs. Define Molar Specific Heat at Constant**

volume and at Constant Pressure.

MOLAR SPECIFIC HEAT AT CONSTANT

VOLUME

Definition

volume and at Constant Pressure.

MOLAR SPECIFIC HEAT AT CONSTANT

VOLUME

Definition

The amount of heat required to raise the

temperature of one mole of any gas through one

degree centigrade, at constant volume is known as

molar specific heat volume.

It is denoted by Cv.

Mathematical Expression

Mathematically,

ΔQv = nCvΔt

Where ΔQv is the heat supplied at constant volume.

**MOLAR SPECIFIC HEAT AT CONSTANT**

PRESSURE

PRESSURE

**Definition**

The amount of heat required to raise the

temperature of unit mass of a substance through one

degree centigrade at constant pressure is known as

Molar Specific Heat at Constant Pressure.

It is denoted by Cp.

Mathematical Expression

Mathematically,

ΔQp = nCpΔt

Where ΔQp is the heat supplied at constant volume

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