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Evaporation and Vaporization
The conversion of a liquid into a
gaseous state is called
vaporization. Liquid into a gaseous
can happen in one of two ways.
One is the boiling that takes place
at the boiling point of a liquid
when further heat is supplied. The
other is the conversion of the
liquid into a gas gradually at
temperatures below the boiling
point. The conversion of a liquid at
a temperature below the boiling
point is known as evaporation.
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• Since the specific latent heat of vaporization has a fairly large value, in
the evaporation of water through the process nof perspiration taking
place from our skin, a large amount of heat is removed from our body.
Thermal Expansion
• Telephone wires and cables carrying electricity are loosely fitted between posts
in order to allow them to contract without breaking the wires when the
environmental temperature drops.
• Tight-fitting metallic bottle lids can be easily opened by heating them up to
expand slightly.
• In electrical appliances such as electric irons and rice cookers, bimetallic strips
consisting of two different metals that have different expansions for a given
temperature difference are used to regulate the temperature.
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• It consists of two metallic strips with inequal expansions rigidly riveted together. One
of its ends is rigidly fixed to a piece of metal while the other end remains free. When
the temperature of the bipolar strip is increased, one of the strips expands more
than the other. Then the two strips bend
Expansion of Liquids
When making thermometers, the thermal expansion of liquids is commonly
used. In mercury and alcohol thermometers, the expansion of liquid volume is
used for measuring temperature.
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Expansion of Gases
• The thermal expansion of gases is a
change in their volume in response to
a change in temperature. Heating a
gas increases the kinetic energy of
the particles, causing the gas to
expand. The thermal expansion of
gases is larger than that for solids and
liquids.
Heat Transfer
• Heat passing from one place to another in this manner is known as heat
transfer.
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The energy known as thermal energy (heat) of an object is actually present as the kinetic
energy resulting from the random motion of the particles that form the object. This energy
can be the translational, rotational or vibrational kinetic energy of the particles. Heat
transfer is the spreading of kinetic energy from a region with atoms having a high degree of
random motion (with a high temperature) to a region of atoms having a low degree of
random motion (with a low temperature)
• There are three methods of transferring heat.
• 1.
• 2.
• 3.
Conduction
• Since the atoms of a solid are tightly bound to
one another, they cannot freely move
throughout the volume of the solid.
• Heat exists as the vibrational kinetic energy of
atoms. In metallic substances, part of the
thermal energy exists as kinetic energy of
freely moving electrons (free electrons) in
addition to this.
• Conduction is the spreading of the kinetic
Conductors –
energy of atoms and electrons throughout the
substance due to collisions among these
Insulators - particles.
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Conduction through a metal rod
Convection
• When heat is supplied to liquids or gases they expand and decrease in
density and move upwards. In order to fill these gaps, liquids or gases with
lower temperatures move downwards. Due to this process, heat flows
upwards from the region where heat is supplied. This is known as convection.
• Upward motion of heated up particle streams is known as convection
currents.
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Thermal Radiation
• The propagation of heat in the form of electromagnetic radiation
from a warm body without the aid of matter is known as thermal
radiation.
• For heat transfer by radiation, a material medium is not required.
However, for heat transfer by conduction or convection, particles of a
medium are essential.
