Applied Physics: Chapter 4 : Heat and Temperature
What is heat?
Heat is the form of energy that is transferred
between two substances at different temperatures.
Write Units of Heat.
The SI unit of heat is Joule. CGS unit of heat
is erg. Practical unit of heat is calorie.
Define 1 calorie.
1 calorie is the amount of heat required to
raise 1 Celsius temperature of 1 gram of water.
Convert Mechanical work to heat.
W = Mechanical work
Q = Heat
J = 4.186 joule/calorie
W = J X Q
1
calorie = 4.186 Joule
Define
Temperature.
The degree of heat present in a substance or
object is defined by temperature.
Give difference between heat and temperature.
Temperature Scales
1. 1. Celsius Scale
The Celsius scale, scale based on 0° for the freezing point of water and
100° for the boiling point of water, the interval between the two
being divided into 100 equal parts. It is denoted by °C after value of
temperature.
2. 2. Fahrenheit Scale
Fahrenheit
temperature scale, scale based on 32° for
the freezing point of water and
212° for the boiling point of water, the interval between the two
being divided into 180 equal parts. It is denoted by °F after value of
temperature.
3 Absolute
temperature scale/ Kelvin scale
An absolute temperature
scale is a scale, where its zero coincides with the Absolute Zero. At the
absolute zero temperature, the volume and pressure of all gases become zero. It
is the lowest temperature possible of anybody in the universe. It is denoted by K after
value of temperature. Here 0 K = -273.15 °C
1. Convert 25oC into Kelvin.
Solution:
Temperature in K = Temperature in oC + 273
= 25 + 273
= 293 K
2. Convert 104oF into Kelvin.
Solution:
= (104-32)5/9 +273
= 72 x 5/9 + 273
=40+273 = 313K
3. A temperature x is recorded both on Celsius and Fahrenheit
scale. What is the value of x?
Solution:
We know
9x = 5x – 160
4x = -160
Therefore, x = -40
What are the different modes of heat transfer?
There are primarily three modes of heat transfer:
Conduction, Convection and Radiation.
What is heat conduction?
Heat conduction is a process in which heat is transferred
from the hotter part to the colder part of a body without involving any actual
movement of the body’s molecules.
Example: When frying vegetables in a pan. Heat transfer occurs from the flame
to the pan and the vegetables.
What is heat convection?
Heat convection is a process in which heat is transferred
in the liquid and gases from a higher temperature region to a lower temperature
region. Convection heat transfer occurs partly due to the actual movement of
molecules or due to the mass transfer.
Example: Heating of milk in a pan.
What is heat radiation?
Heat radiation is a process in which heat is transferred
from one body to another without involving the medium’s molecules. Radiation
heat transfer does not depend on the medium.
Example: In a microwave, the substances are heated directly without any heating
medium.
What is the cause of heat transfer?
The difference in temperature is the primary cause of
heat transfer.
What is Heat Capacity?
The heat capacity of a substance can be
defined as the amount of heat required to change its temperature by one degree.
Heat capacity for a given matter depends on its size or quantity (mass) and hence it is an extensive property. The unit of heat capacity is
What is Specific Heat?
Heat
capacity of body per unit mass is known as specific heat of material. Its value
depends only on type of material. It is not depend on mass or quantity.
Specific heat is
The unit of specific heat is
In the
case of copper coin, we can talk about heat capacity of the coin but specific
heat is of the copper.
Write
name of different thermometers.
Thermal conductivity
Thermal conductivity refers to the ability of a
given material to conduct/transfer heat. It is generally denoted by the symbol
‘k’. The reciprocal of this quantity is known as thermal resistivity.
The law of heat conduction states that the rate of heat
is transferred
through a material is directly proportional to the temperature difference ΔT
and cross sectional area A of the hot face, and inversely proportional to the
distance between two faces Δx = d,
Where, the proportionality constant k is
called the coefficient of thermal conductivity of the solid. In above equation 
is known as
temperature gradient, that represent the rate of fall of temperature with
distance in the direction of the heat flow.
The coefficient of thermal conductivity of a
solid is equal to the rate of flow of heat per unit area per unit temperature
gradient across the solid.
SI
unit of coefficient of thermal conductivity k is Wm-1K-1
CGS unit of coefficient of thermal conductivity
k is cal s-1 °C-1
Value
of k is large for good conductors and small for insulators.
Expansion of solids, coefficient of linear
expansion.
All three states of matter (solid, liquid and
gas) expand when heated. The atoms themselves do not expand, but the volume
they take up does. When a solid is heated, its atoms vibrate faster about their
fixed points. The relative increase in the size of solids when heated is
therefore small.
Linear Expansion means, change or increase in
length. If the change in length is along one dimension (length) over the
volume, then it is called linear expansion.
Suppose L0 is original length of solid rod. ΔL is a small
increase in length of the rod after ΔT temperature raised.
Coefficient of Linear Expansion is the rate of
change of unit length per unit degree change in temperature. It depends on nature
of the material only. The SI unit of coefficient of linear expansion can be expressed as
°C-1 or °K-1.
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