MODES OF HEAT TRANSFER

Amaljo Joju E s3 EEE

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EEE Dept. SCET

2How ‘Heat’ Moves

• Define “Heat”: Heat is the movement of thermal energy from a substance at a higher temperature to another substance at a lower temperature.

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3How ‘Heat’ Moves

Thermal energy in the form of heat can move in three ways.

ConductionConvection Radiation

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4ConductionCONDUCTION: • The transfer of heat from one

particle of matter to another by direct particle to particle contact.

– Conduction occurs primarily in solids because the particles are tightly packed together.

– The particles themselves DO NOT change positions.

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Metals are different

The outer e______ of metal atoms drift, and are free to move.

When the metal is heated, this ‘sea of electrons’ gain k_____ energy and transfer it throughout the metal.

Insulators, such as w___ and p____, do not have this ‘sea of electrons’ which is why they do not conduct heat as well as metals.

lectrons

inetic

ood lastic

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Why does metal feel colder than wood, if they are both at the same temperature?

Metal is a conductor, wood is an insulator. Metal conducts the heat away from your hands. Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

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7ConvectionConvection: the transfer of

thermal energy (heat) through the bulk movement of matter.

– Convection occurs in FLUIDS (liquids and gases).

– Convection produces CURRENTS in both gases and liquids.

– Thermal Energy heat is carried by the particles as they move from one location to another.

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Convection

What happens to the particles in a liquid or a gas when you heat them?

The particles spread out and become less dense.

This effects fluid movement.What is a fluid?A liquid or gas.EEE Dept. SCET

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Fluid movement

Cooler, more d____, fluids sink through w_____, less dense fluids.

In effect, warmer liquids and gases r___ up.

Cooler liquids and gases s___.

ensearmer

ise

ink

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Why is it windy at the seaside?

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11Radiation• Radiation: the transfer of

(thermal) energy by electromagnetic waves.

– Radiation does not require matter to transfer thermal energy. • All the sun’s energy that reaches Earth

travels through millions of kilometers of empty space (a vacuum).

– All matter can radiate energy.• You feel the radiation of thermal energy

from a bonfire, a heat lamp and a light bulb. EEE Dept. SCET

The third method of heat transfer

How does heat energy get from the Sun to the Earth? There are no particles

between the Sun and the Earth so it CANNOT travel by conduction or by convection.

?RADIATION

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Absorption experiment

Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes?

The __________ container would be the warmest after ten minutes because its surface absorbs heat _______ the best. The _________ container would be the coolest because it is the poorest at __________ heat radiation.

dull blackradiation

shiny metalabsorbing

Shiny metal

Dull metal

Dull black

Shiny black

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14Conduction, Convection & Radiation

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15Conduction

• Direct contact of particles

• Solids/liquids/gases

• The handle of a cooking utensil

• Transfer of energy by waves

• Only radiant energy that is absorbed becomes thermal energy

• Lightbulb• Fireplace

• Transfer of energy by bulk movement of matter (fluids)

• Currents (wind,water)

• Hot air balloon

Radiation Convection

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