HEAT
HEATMODES OF HEAT TRANSFERRIFFAT SHAHEEN FFMS MALIR CAMPUS
Modes ConductionConvection Radiation
The process, in which heat energy is transferred from one system
to another system without visible motion of particles, is called
conduction.CONDUCTION
Pair discussion
Conduction is the mode of transfer of heat that requires a
medium. It takes place in all the three states of matter. But it is
much dominant in solids only. Heat energy transfers from one place
to another through molecular vibrations in solids. These vibrations
travel in the form of heat waves of very high frequency.
METALS ARE THE GOOD THERMAL CONDUCTORS because heat transfer in
metals take place due to free electrons inside metals which move
very fast and transfer heat from one place to another.On the basis
of heat conduction, the materials are divided into two
groups:Thermal Conductors (Good Conductors)Thermal Insulators (Poor
Conductors)THERMAL CONDUCTORSThe materials, which can conduct heat
at an appreciably high rate, are called thermal conductors.
Examples are metals like copper, iron, aluminum, steel etc.THERMAL
INSULATORSThe materials, which cannot conduct heat at an
appreciably high rate, are called insulators or poor conductors.
Examples are non-metals like wood, rubber, plastic, glass etc.
USES OF CONDUCTORS AND INSULATORSCooking pots and pans are made
of metals, the conductors to ensure quick and fast supply of
heat.Potholders or handles are made up of wood or plastic, the
non-conductors, to avoid burning of hands.Tablemats are made from
insulators like wood or cloth to avoid burning of tabletops.Plastic
foam and fiberglass insulators are used in the walls and ceilings
of homes to keep them cooler in summer and warmer in winters
It was invented in1815by Sir Humphry Davy. It originally burned
a heavy vegetable oil. It was created for use in coal mines, to
reduce the danger of explosions due to the presence of methane and
other flammable gases, called firedamp or mine damp.
WORKING OF DAVYS LAMPA cylindrical metal guaze of high thermal
conductivity surround the flame. When this lamp is taken inside the
mine, the explosive gases present in the mine are not ignited
because the wire gauze in the form of cylinder conducts away the
heat of the flame of the lamp. The result is that the temperature
outside the guaze remains below the ignition point of the gas. In
absence of wire gauze the gases outside can explode.
CONVECTIONDEFINITIONThe process, in which heat energy is
transferred from one system to another system due to movement of
particles through large distances, is called convection.
EXPLANATION
In fluids (liquids or gases) heat transfer from one place to
another inside a system takes place through convection. This mode
of transfer of heat requires a material medium. The molecules take
heat the heat energy and their kinetic energy increases and they
carry heat through large distances.
VENTILATORS: Ventilators are provided in the walls of a room
near the ceiling, which help to keep the room temperature moderate
by continuous circulation of air. The air inside the room gets
impure and heated due to our breathing. This hot air rises up and
passes out through ventilators, thus allowing space for currents of
fresh air from outside windows or doors.
As air warmed at the Equator rises towards the poles, the
rotation of the Earth causes it to deflect and flow back towards
the Equator. In the northern hemisphere, the winds blow from the
east to the west, while they blow in the opposite direction in the
southern hemisphere. These winds tend to be much stronger over open
water than they are across land, which has made them ideal for
sailors.
LAND BREEZES AND SEA BREEZES:
Land is better conductor of heat than water. hence in day time,
the land gets hotter than in the sea. The air above the land
becomes warm and rises up being lighter and some what cold air
above sea surface moves towards the sea shore. This is known as sea
breeze.
During night land becomes cooler than water and so warm air over
the water in sea rises up. The air on the land near the sea shore
begins to move towards sea side and is called land breeze
RADIATIONDEFINITIONThe process in which heat energy travels from
one system to another in the form of electromagnetic waves with no
need of material medium is called Radiation.EXPLAINATION: In
radiation heat travels from one system to another in the form of
electromagnetic radiations (photons). Thus no medium is required
for this mode of transfer of heat.
Heat from the sun reaches earth by radiation
EXAMPLES OF HEAT TRANSFER A good example would be heating a tin
can of water using a Bunsen burner. Initially the flame produces
radiation which heats the tin can. The tin can then transfers heat
to the water through conduction. The hot water then rises to the
top, in the convection process.
The atmosphere would be another example. The atmosphere is
heated by radiation from the Sun, the atmosphere exhibits
convection as hot air near the equator rises producing winds, and
finally there is conduction between air molecules, and small
amounts of air-land conduction.
APPLICATIONS OF RADIATION
A thermo scope is a device for detecting and displaying
temperature changes.It consists of identical glass bulbs A and B
which are connected to narrow U-shaped glass tube. The tube is
filled with sulfuric acid and the space above acid levels in both
the arms is filled with air. Bulb A is coated with lamp black so
that it may completely absorb the heat radiation falling on it.When
the temperature of the two bulbs is same, there is no difference in
the acid level.When bulb A is exposed to heat, it absorbs heat and
air in this arm expands resulting in difference of acid level.This
thermo scope is very delicate and can detect very weak radiations
even DIFFERENTIAL AIR THERMOSCOPE
BOYS RADIOMICROMETER
BOYS RADIOMICROMETER
It is a combination of moving coil galvanometer and a
thermocouple. It consists of a single loop of silver or copper wire
A. the lower ends of the wire are soldered to a copper disc which
is coated with lamp black.When disc is exposed to heat radiations,
a thermo-electric current is produced in the couple made of Bismuth
and Antimony and begins to flow in the wire A. Hence we get a
current in the galvanometer. The deflection shows the amount of
radiation.
THERMO FLASK DEWARS BOTTLE
THERMO FLASK DEWARS BOTTLEAvacuum flask(also known as aDewar
flask,Dewar bottleorThermos) is aninsulatingstorage vessel that
keeps its contents hotter or cooler than its surroundings. Invented
bySir James Dewarin 1892. USE:Vacuum flasks are used domestically
to keepbeverageshot or cold for extended periods, and for many
purposes in industry.
The vacuum flask consists of twoflasks, placed one within the
other and joined at the neck. The gap between the two flasks is
partially evacuated of air, creating a near-vacuumwhich prevents
heat transfer byconductionorconvection. Heat transfer bythermal
radiationmay be minimized bysilvering flask surfaces facing the
gap. Most heat transfer occurs through the flask's neck and
opening, where there is no vacuum. Vacuum flasks are commonly made
ofmetal,glass,foam, orplastic, and have their
openingstopperwithcorkor plastic
How does a thermo flask work????
