·
Temperature is a relative measure, or
indication of hotness or coldness.
·
Heat is the form of energy transferred
between two (or more) systems or a system and its surroundings by virtue of
temperature difference. The SI unit of heat energy transferred is expressed
in joule (J) while SI unit of temperature is kelvin (K), and °C
is a commonly used unit of temperature.
·
Thermometer is a device used for measuring
temperatures. The two familiar temperature scales are the Fahrenheit
temperature scale and the Celsius temperature scale. The Celsius
temperature (tC) and the Farenheit temperare (tF) are
related by: tF = (9/5) tC + 32
·
In principle, there is no upper limit to
temperature but there is a definite lower limit- the absolute zero. This
limiting temperature is 273.15° below zero on the celsius scale of
temperature.
·
Clinical thermometer is used to measure our
body temperature. The range of this thermometer is from 35°C to 42°C. For other
purposes, we use the laboratory thermometers. The range of these thermometers
is usually from –10°C to 110°C. The normal temperature of the human body
is 37°C.
·
The heat flows from a body at a higher
temperature to a body at a lower temperature.There are three ways in which heat
can flow from one object to another. These are conduction, convection
and radiation.
·
The process by which heat is transferred from
the hotter end to the colder end of an object is known as conduction. In
solids, generally, the heat is transferred by the process of conduction.
·
The materials which allow heat to pass through
them easily are conductors of heat. For examples, aluminium, iron and copper.
The materials which do not allow heat to pass through them easily are poor
conductors of heat such as plastic and wood. Poor conductors are known
as insulators.
·
In convention heat is carried from one
place to another by the actual movement of liquid and gases. In liquids and
gases the heat is transferred by convection.
·
The people living in the coastal areas
experience an interesting phenomenon. During the day, the land gets heated
faster than the water. The air over the land becomes hotter and rises up. The
cooler air from the sea rushes in towards the land to take its place. The warm air
from the land moves towards the sea to complete the cycle. The air from the sea
is called the sea breeze. At night it is exactly the reverse. The water cools
down more slowly than the land. So, the cool air from the land moves towards
the sea. This is called the land breeze.
·
The transfer of heat by radiation does
not require any medium. It can take place whether a medium is present or not.
·
Dark-coloured objects absorb radiation better
than the light-coloured objects. That is the reason we feel more comfortable in
light-coloured clothes in the summer. Woollen clothes keep us warm during
winter. It is so because wool is a poor conductor of heat and it has air
trapped in between the fibres.
·
A change in the temperature of a body causes
change in its dimensions. The increase in the dimensions of a body due to the
increase in its temperature is called thermal expansion. The expansion in
length is called linear expansion. The expansion in area is called area
expansion
The expansion in volume
is called volume expansion.
The amount of heat energy required to raise the
temperature of 1g of a substance through 1° is called specific heat
capacity of the substance. The S.I. Unit of specific heat capacity is(
J/kg) K. Water has the highest specific heat capacity which is equal
to 4200 (J/kg )K.
The specific heat capacity is the property
of the substance which determines the change in the temperature of the
substance (undergoing no phase change) when a given quantity of heat is
absorbed (or rejected) by it. It is defined as the amount of heat per unit mass
absorbed or rejected by the substance to change its temperature by one unit. It
depends on the nature of the substance and its temperature.
The amount of heat energy required to raise the
temperature of a given mass of substance through 1° is called heat
capacity or thermal capacity of the substance. It’s S.I. Unit
is (J/K).
Calorimeteric means measurement of heat.
When a body at higher temperature is brought in contact with another body at
lower temperature, the heat lost by the hot body is equal to the heat gained by
the colder body, provided no heat is allowed to escape to the surroundings. A
device in which heat measurement can be made is called a calorimeter.
CHANGE OF STATE: Matter normally exists in three
states: solid, liquid, and gas. A transition from one of these states to
another is called a change of state. Two common changes of states are solid to
liquid and liquid to gas (and vice versa). These changes can occur when the
exchange of heat takes place between the substance and its surroundings.
The change of state from solid to liquid is
called melting and from liquid to solid is called fusion. It is observed
that the temperature remains constant until the entire amount of the solid
substance melts. That is, both the solid and liquid states of the substance
coexist in thermal equilibrium during the change of states from solid to
liquid.
The temperature at which the solid and the liquid
states of the substance in thermal equilibrium with each other is called
its melting point. It is characteristic of the substance. It also depends
on pressure. The melting point of a substance at standard atmospheric pressure
is called its normal melting point.
The change of state from liquid to vapour (or gas)
is called vaporisation. It is observed that the temperature remains
constant until the entire amount of the liquid is converted into vapour. That
is, both the liquid and vapour states of the substance coexist in thermal
equilibrium, during the change of state from liquid to vapour.
The temperature at which the liquid and the
vapour states of the substance coexist is called its boiling
point. At high altitudes, atmospheric pressure is lower, reducing the
boiling point of water as compared to that at sea level. On the other hand,
boiling point is increased inside a pressure cooker by increasing the pressure.
Hence cooking is faster.
The boiling point of a substance at standard
atmospheric pressure is called its normal boiling point.
However, all substances do not pass through the
three states: solid-liquid-gas. There are certain substances which normally
pass from the solid to the vapour state directly and vice versa. The change
from solid state to vapour state without passing through the liquid state is
called sublimation, and the substance is said to sublime. Dry ice (solid CO2)
sublimes, so also iodine. During the sublimation process both the solid and
vapour states of a substance coexist in thermal equilibrium.
Certain amount of heat energy is transferred
between a substance and its surroundings when it undergoes a change of state.
The amount of heat per unit mass transferred during change of state of the
substance is called latent heat of the substance for the process.
The amount of heat energy supplied to a solid at
its melting point, such that it changes into liquid state without any rise in
temperature is called latent heat of fusion and that for a liquid-gas
state change is called the latent heat of vaporisation.
Newton’s
Law of Cooling says that the rate of cooling of a body is proportional to
the excess temperature of the body over the surroundings.
