HEAT
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| HEAT | PHYSICS |
Heat is a form of energy
which produces in us the sensation of warmth.Air is a non-conductor of
heat.Heat is due to the kinetic energy of the molecules constituting the
body.Heat's units are calorie,kilo calorie or joule.
Calorie It is the amount of heat required to raise
the temperature of 1 g of water through
10 C
(from 100 C to 110 C).It is represented by cal.
1 cal = 4.18 J
TEMPERATURE
An instrument used to
measure the temperature of a body is called a thermometer.
Mercury is generally
used as thermometric substance,because it is not easily obtained in nature in
pure state and clearly visible in capillary tube but has fairly uniform
expansion with increase of temperature.
Celsius scale The melting point of ice at standard atmospheric
pressure is regarded as 00 C and the boiling point of
water as 1000 C.This scale was designed by Anders Celsius, in
the year 1710.
Fahrenheit scale The melting point of ice is regarded as 320 F
and the boiling point of water as 2120 F.This scale was designed
by Gabriel Fahrenheit in the year 1717.
Reaumer scale The melting point of ice is regarded as 00 R
and the boiling point of water as 800 R. This scale was
introduced by RA Reaumer in year 1730.
The normal temperature
of a human body is 370 C or 98.4 F.
-400 is
the temperature at which Celsius and Fahrenheit thermometers read same.
TOTAL RADIATION PYROMETER
Radiation pyrometer
measures the temperature of a body by measuring the radiation emitted by the
body.It cannot measure temperature below 8000 C because at low
temperature,emission of radiation is very small and cannot be detected.Sun's
temperature is measured by it.
SPECIFIC HEAT
It is the amount of heat
required to raise the temperature of a unit mass of the substance by 10 C.
It is usually represented
by C.
SI unit of specific heat
is joule per kilogram per degree Celsius.It may be written J/kg0 C.
Specific heat of gold =
130 J/kg0 C, specific heat of water = 4180 J/kg0 C.
THERMAL CAPACITY
It is the amount of heat
required to raise the temperature of a body through 10 C.Its
units are J/0 C or cal/0 C or kcal/0 C.
WATER EQUIVALENT
It is the mass of water,
which absorbs or emits the same amount of heat as is done by the given body for
the same rise or fall in temperature.
The water equivalent of
a body is denoted by W and its unit is kg in SI system and g in CGS system.
PRINCIPLE OF CALORIMETRY
Calorimeter is a
cylindrical vessel made of copper.
The heat lost by the hot
body must be equal to the heat gained by the cold body.
i.e., Heat gained = Heat
lost
Work and heat are two
equivalent forms of energy.
THERMAL EXPANSION
It is the increase in
size on heating.
A solid can undergo
three types of expansions.
Linear expansion
(expansion in length)
Superficial
expansion (expansion in
area)
Cubical expansion
(expansion
in volume)
SOME PRACTICAL APPLICATIONS
When hot tea is poured
into a glass tumbler made of soft glass,it cracks because of thermal expansion
of glass.
Telegraph wires are
given enough gap to allow the wires for contraction in winter.
An ordinary pendulum
clock runs faster in winter but slower in summer,because in summer the length
of pendulum increases while in winter it decreases.
A metallic ball before
heating can pass easily through the ring, but after heating the ball does not
pass through the ring because of the thermal expansion of ball.
A gap is provided
between the iron rails of the railway track so that rails can easily expand
during summer and do not bend.In the construction of bridges,ends of steel
gerders are not fixed but placed on rollers to allow free expansion and
contraction in summer and winter respectively to avoid any damage to the
bridge.
A tightly fitted cork
can be removed from a glass bottle without breaking it on heating its mouth
carefully.The glass wall expand and the cork will come out easily.
EXPANSION OF LIQUIDS
Liquids do not have
their definite length and surface area.
Liquids always need a
container.When they are heated,they expand and at the same time the container
also expands.
The expansion of a
liquid,when the expansion of its container has not been taken into account is
called apparent-expansion.
The expansion of a
liquid when the expansion of its container is taken into account, is called
real expansion.
ANOMALOUS EXPANSION OF WATER
Almost every liquid
expands with the increase in temperature.But when temperature of water is
increased from 00 C to 40 C its volume
decreases if the temperature is increased above 40 C its volume
starts increasing, clearly density of water is maximum at 40 C.
CHANGE OF STATE(FUSION AND VAPORIZATION)
The process in which a
solid changes into a liquid on heating is called fusion (melting).
Change of state takes
place i.e., ice changes into water.
The process in which a
liquid changes into a vapour on heating is called vaporization.
Change of state takes
place i.e., water changes into water vapour or steam.
LATENT HEAT
It is the amount of heat
absorbed by a unit mass of the substance to change its state without change in
temperature.
SI unit of latent heat
is joule per kilogram.
Latent heat of fusion is
80 cal/g.
Latent heat of
vaporization is 540 cal/g.
Ice at 00 C
is more effective in cooling a substance that water at 00 C.
If Q is quantity of heat
absorbed ,then Q= mL.
where, m = mass of
substance
L = Latent heat of substance
Steam contains more
heat,in the form of latent heat than boiling water.
The burns caused by
steam are much more severe than those caused by boiling water though both of
them are at the same temperature of 1000 C.
EVAPORATION
It is the slow process
of conversion of a liquid into its vapour even below its boiling temperature.
Cooking is caused by
evaporation due to expansion.
Factors affecting the
evaporation of a liquid are
- The area of the liquid surface.
- The nature of the liquid.
- Temperature of the liquid.
- Dryness of air.
- The movement of the air.
- Temperature of air or surrounding.
RELATIVE HUMIDITY
The amount of water
vapour in air is called as humidity.
The amount of water
vapour in the air varies as ir depends of the rate of evaporation.
The amount of water
vapour present in 1 m3 air is called absolute humidity.
The ratio of
amount of water vapour (m) actually present in a certain volume of air a given
temperature to the amount of water vapour (M) required to saturate it, is
called relative humidity,RH.
i.e.,
RH =
m/M✕100%
TRANSMISSION OF HEAT
Transfer of heat from
one place to other place is called transmission of heat.
These are of three types
a. Conduction
b. Convection
c. Radiation
In solids transmission
of heat takes by conduction process.
In liquids and gases
heat takes place by convection process.
Heat from the sun
reaches the earth by radiation.
NEWTON'S LAW OF COOLING
The rate of loss of heat
by a body is directly proportional to the difference in temperature between the
body and the surrounding.
KIRCHHOFF'S LAW
Kirchhoff's law
signifies that good absorbers are good emitters.If a shining metal ball with
some black spot on its surface is heated to a high temperature and seen in
dark,the shining ball becomes dull but the black spots shines brilliantly
because black spot absorbs radiation during heating and emit in dark.
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