Chapter 23: HEAT
a solid is heated the kinetic energy of the molecules is
increased and it overcomes the force of attraction of
molecules and the solid melts. When a solid melts the
temperature doesn’t increase, the heat energy is used up
and is hidden inside the solid and is used for changing
its state so it’s called latent heat of fusion.
system is where there is an exchange of matter and energy
between system and surroundings. Closed system has only
exchange of energy. In isolated systems both are absent.
Law of Thermodynamics
law of thermodynamics is the law of conservation of energy
can neither be created nor destroyed”.
heat capacity is the amount of heat absorbed or reject by
unit mass of a substance to raiseits temperature by 1°C.
Water has higher specific heat than land and so it heats
and cools slower. Skating is possible on snow as water is
formed belowthe skates due to low temperature. This acts
as a lubricant.
F = 9C/5 + 32; F= temp in Fahrenheit, C= temp in Celsius
Effect of Heat on substances
like camphor, ammonium chloride, naphthalene, anthracene
turn from solid to gas on heating. This is called sublimation.
on application of pressure the state of matter changes.
Pressure exerted by gases is measures using unit
atmosphere. 1 atm is pressure at sea level.It decreases
occurs as a few particles of a liquid on its surface
have higher kinetic energy and they break away from the
rest and turn into vapor.
solids, heat is transferred from higher temperature to
lower temperature by process called conduction [Copper
is coated on the bottom of the utensils as it’s a good
conductor and helps in spreading out the heat evenly].
and air are poor conductors of heat. In water, when its
heated hot water at the bottom rises up and cold water
moves down. This continues till the whole water is heated.
This process is called convection.
is the process by which sun’s heat reaches us.
Radiation doesn’t need a medium. All objects emit
radiation when heated; the frequency of radiation goes
from lower to higher as the temperature increases.
States of Matters
– fourth state of matter. This consists of super
energized and excited ionized gaseous matter. The neon
bulb has neon gas and CFL has argon or helium, this gets
energized when electricity is passed through them. This
creates plasma that glows depending on the nature of the
gas. In sun and stars plasma is created due to high
temperatures and produces light.
condensate – fifth state of matter. BEC is formed when a
gas at very low density is cooled to super low
measured as kilogram per meter cube.
gas contains methane [50-90%], ethane, propane,
butane and hydrogen sulphide [waste]. LPG uses propane and
butane. when natural gas is heated we get carbon black
used in tyre industries.
reactors work on nuclear fission [fission was discovered
by Fermi]. Atom bombs also work on the same principle. In
reactors, neutrons need to be slowed to ensure interaction
with the uranium isotope this is done using heavy water [D2O]
rods are used to control the reaction. Energy produced by
the sun and the working of the hydrogen bomb is based on
has a three stage nuclear program.
first stage, the natural uranium is used as fuel, heavy
water as moderator [pressurized heavy water reactors] and
we get plutonium.
second stage, plutonium and uranium oxide is used to give
uranium -233 in fast breeder reactors where no moderator
final stage, thorium and uranium-233 is used to get energy
[thorium based reactors].
water reactors need frequent refueling and produce less
energy compared to light water reactors that use distilled
water as coolant and fissile uranium -235 as fuel.
behavior of water
exhibits anomalous behavior below 4°C, its volume
increases and density decreases. Due to this the ponds and
lakes freeze at the top during winters as at 4°C water
becomes less dense and remains on top and solidifies. This
leaves animal and plant life intact at bottom.
of Wool and Cotton on Heat
clothes or surfaces absorb more heat and hence dark
clothes are preferred in winter. Light clothes reflect
more heat and so such clothes are preferred in summer.
is a bad conductor of heat, woolen fibers trap air and
this prevents body heat from escaping. So woolen clothes
are preferred in winters. We should use two blankets
instead of one in winters as the trapped layer of air
between the blankets prevents heat from escaping.
clothes are sweat absorbent, they expose the sweat to the
atmosphere and when it evaporates it absorbs latent heat
from the body, this heat is removed and the body cools
of Air parcel on heat
of houses is done using hollow bricks that have air
trapped in them. This prevents heating of houses during
summer and keeps flats cool. During winter the heat from
flats doesn’t escape out keeping them warm.
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