Today let’s see
few little facts of Physics. Physics on the whole is quite a big subject, I
will try to gather everything in detail and I am picking up “Heat”. I have
taken due care to ensure that the information provided below are correct.
However, in any case, if it is found incorrect, please refer your books. To
make thing easy I am giving in small paragraph points.
I had a very
hard time remembering Physics even though I was a science student. I wrote UGC -
NET exam but fail to pass because of all the common terms not by the formulae.
Formulae is much easier than the common terms which I am trying to get down
from different books. This are common terms which may sound easy but forgets
very easily.
Heat is the
energy that spontaneously passes between a system and its surroundings in some
way other than through work or the transfer of matter. Below are few points
which is I thought worth reading. Give me feedback so I can improve the next
time.
HEAT
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 (°C) and the Fahrenheit temperature (°F) are related by: °F
= (9/5) °C + 32
In principle,
there is no upper limit to temperature but there is a definite lower limit- the
absolute zero. This limiting temperature is 273.16° 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, aluminum, 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-colored
objects absorb radiation better than the light-colored objects. That is the
reason we feel more comfortable in light-colored clothes in the summer. Woolen
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 fibers.
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. Its S.I. Unit is
(J/K).
Calorimetry
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 vapor (or gas) is called vaporization. It is observed that
the temperature remains constant until the entire amount of the liquid is
converted into vapor. That is, both the liquid and vapor states of the
substance coexist in thermal equilibrium, during the change of state from
liquid to vapor.
The temperature
at which the liquid and the vapor 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 vapor state
directly and vice versa. The change from solid state to vapor 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 vapor 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 vaporization.
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.
