This document discusses different modes of heat transfer - conduction, convection, and radiation. It provides definitions and examples of each:
- Conduction is the transfer of heat through direct contact of particles. An example given is a metal wire gauze conducting heat away from a mine lamp flame.
- Convection is the transfer of heat by the movement of fluids like gases and liquids. Examples of convection currents in the atmosphere and from land/sea breezes are described.
- Radiation is the transfer of heat through electromagnetic waves without a medium. Heat from the sun reaches Earth through radiation. Devices like thermoscope and pyrgeometer that detect radiation are also outlined.
11. A 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.
12. DEFINITION
The process, in which heat energy is
transferred from one system to
another system due to movement of
particles through large distances, is
called convection.
13. 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.
14. •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.
15. 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.
16. 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
17.
18. RADIATION
DEFINITION
“The 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.
20. 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.
22. 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
23.
24. 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.
26. THERMO FLASK – DEWAR’S
BOTTLE
A vacuum flask (also known as a Dewar flask, Dewar
bottle or Thermos) is an insulating storage vessel that
keeps its contents hotter or cooler than its surroundings.
Invented by Sir James Dewar in 1892.
USE:Vacuum flasks are used domestically to
keep beverages hot or cold for extended periods, and for
many purposes in industry.
27. The vacuum flask consists of two flasks, placed one within the
other and joined at the neck. The gap between the two flasks
is partially evacuated of air, creating a near-vacuum which
prevents heat transfer by conduction or convection. Heat
transfer by thermal radiation may be minimized by silvering
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
of metal, glass, foam, or plastic, and have their
opening stopper with cork or plastic
