Heat is a form of energy that produces a sensation in our body by way of which we make out whether a body is cold, warm or hot.
Hence it can do work. For example, the steam
engine pulls a train converting heat into mechanical energy.
Heat can also be converted to other forms of energy. For
example, the heat in a firecracker produces both sound and light.
Effects of Heat energy
a)
Heat
energy brings about a temperature change conversely, if a hot body gives out
heat energy, its temperature falls.
b)
Heat
energy brings about a change in dimensions when a material body is heated, its
length, area and volume increase.
c)
Heat
energy brings about state change. When heat is supplied to the solid, at a
particular constant temperature it changes into the liquid state. Similarly
liquid changes into a gaseous state. During the change of state, the temperature
of the body remains constant.
d)
Heat
energy affects living things.
e)
Heat
energy brings about chemical change when calcium carbonate is heated strongly;
it changes into calcium oxide and carbon dioxide.
Sources of Heat Energy
The sun is the Earth’s main source of heat energy. Other sources
of heat include the burning of fuels, for instance, wood, coal, kerosene and
liquefied petroleum gas(LPG), rubbing of two surfaces against each other, or
from electricity. Heat always flows from a body of higher temperature to a
body at a lower temperature.
A body that is losing heat is feeling the other body to be
cold, and a body that is gaining heat is feeling the other body to be hot.
S.I unit of heat energy is joule (J). Another commonly used
unit of heat is calorie (cal). One calorie is the quantity of heat energy
required to raise the temperature of 1g of water through 10C.
1 cal = 4.2 J 1 kcal = 1000 calories
Thermometry
Thermometry is the branch of heat dealing with the
measurement of temperature. The degree of hotness or coldness of the body is
called temperature. S.I unit of temperature is Kelvin (k). Other units of
temperature are degrees Celsius (0C) and degrees Fahrenheit (0F).
The device used for measuring the temperature of a substance
is called a Thermometer.
“Thermo” is a Latin word that means heat and matter means a
measuring device. Thermometer works on the principle that matter expands when
heated and contracts when cooled.
There are different thermometric substances like mercury,
alcohol etc., used in thermometers for different ranges of temperatures to be
measured. Mercury is used in thermometers as a thermometric substance because of
the following advantages.
a)
It
expands evenly as the temperature rises.
b)
It
is a good conductor of heat.
c)
Its
density is higher.
d)
It
is very sensitive to expansion.
e)
It
does not stick on the wall of a glass tube.
f)
It
has a very low freezing point and a very high boiling point.
The calibration of the thermometer involves fixing two points
on it one lower fixed point (L.F.P) and another upper fixed point (U.F.P).
The melting point of pure ice at normal atmospheric pressure
is taken as a lower fixed point (L.F.P). The boiling point of pure water at
normal atmospheric pressure is taken as the upper fixed point (U.F.P).
The constant temperature at which, the solid substance
changes into liquid is called its melting point. The constant temperature at
which, a liquid changes into a gaseous state is called the boiling point.
Scales of Temperature
Celsius scale (or)
centigrade scale
It was devised by Anders Celsius in 1710. On this scale, the ice
point (L.F.P is taken as 00C and the steam point is taken as 1000C.
The fundamental interval (interval between L.F.P and U.F.P) is divided into 100
equal parts (divisions). Each division corresponds to a difference in
temperature of 10C.
Fahrenheit scale
It was devised by Fahrenheit in 1717. Here ice point (L.F.P)
is taken as 320F and steam point (U.F.P) is taken as 2120F.
The fundamental interval is divided into 180 equal parts. Each part corresponds
to a difference of temperature of 10F.
Kelvin scale
It was given by Lord Kelvin (1824-1907). Here, the ice point
(L.F.P) is taken as 273 K and the steam point. Steam point (U.F.P) is taken as 373
K. The fundamental interval is divided into 100 equal parts. Each division
corresponds to 1 K.
Thermal Expansion
The increase in size of an object on heating is called
thermal expansion. Thermal expansion takes place in all bodies and in all three
states of matter.
Expansion of solids
A solid has a definite shape, so it shows all three types of expansions i.e., on heating solids show an increase in length (linear expansion), in area (superficial expansion) and in volume (cubical expansion).
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Reviewed by knowledge people creators
on
October 30, 2021
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