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Heat
Chapter 11
= Question No.1-A =
Define and explain the term Heat and Temperature.
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 instrument Calorimeter.
Units
Since heat is a form of energy therefore its unit is Joule (J).
Temperature
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:
Celsius and Fahrenheit scales are also known as Scales of Graduation.
The melting point of ice and boiling point of water at standard pressure (76cm of Hg) are 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
The melting point of ice and boiling point 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:
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
= Question No. 1-B =
Define the term Thermal Equilibrium
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.’’
= Question No. 2-A =
What do you mean by thermal expansion? Give its types.
Thermal Expansion
Definition
"The phenomenon due to which solid experiences 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:
If we supply some amount of heat to any rod, then the length of the rod will increase. Such increment is known as Linear 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.
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.
= Question No. 2-B =
Derive Coefficient of Linear and 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 D L. The increment in length depends upon the following two factors.
Derivation
The increment in length is directly proportional to the original length and temperature difference.
Mathematically,
D L µ Lo ----- (I)
D L µ D t ---- (II)
Combining eq (I) and (II), we get:
D L µ LoD t
Þ D L = a LoD t
Where a is the constant of proportionality and it is known as coefficient of Linear Expansion. It is defined as;
"It is the increment in length per unit length per degree rise in temperature.’’
Its unit is 1/° C or ° C-1. If LT is the total length, then:
LT = Lo + D L
Þ LT = Lo + a LoD t
Þ LT = Lo(1 + a D t)
Coefficient of Volumetric Expansion
Consideration
Let Vo 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 D V.
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