Modes of transfer of heat

WHAT IS HEAT? A form of energy associated with the motion

of atoms or molecules. Transferred from objects at a higher

temperature to objects at a lower temperature.

Hot objects in a cooler room will cool to room temperature.

Cold objects in a warmer room will heat up to room temperature.

There are three ways in which heat can be transferred:

Types of Heat Transfer:

TYPES OF HEAT TRANSFER:

Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. It occurs through direct contact.

If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones.

As long as the objects are in contact, transfer of heat will continue until the temperature of the objects is the same.

Heat Conduction

CONDUCTION = CONTACT

Conduction

As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on and so on, the vibrations are passed along the metal and

so is the heat. We call this CONDUCTION

Heating a metal strip:When you heat a metal strip at one end, the heat travels all the way to the other

end as well!

Think of a metal spoon in a pot of water being heated.

The fast-moving particles of the fire collide with the slow-moving particles of the cool pot.

Because of these collisions, the slower particles move faster and heat is transferred.

Then the particles of the pot collide with the particles in the water, which collide with the particles at one end of the spoon.

As the particles move faster, the metal spoon gets hotter. This process of conduction is repeated all along the metal until the entire spoon is hot.

Example of Conduction

A piece of cheese melts as heat is transferred from the meat to the

cheese

Another Example

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

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

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

Conduction in metals

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.

Why does metal feel colder?

Heat Convection

Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it.

Convection that occurs above a hot surface occurs because hot air expands, becomes less dense and rises and is called natural convection. Convection assisted by breeze, pump or fan is called forced convection.

Hot water is likewise less dense than cold water and rises, causing convection currents which transport energy.

Convection currents

When particles in the air spread out, they become less dense and generally rise above the unheated, more dense particles around them.

The denser masses of the gas or liquid move in to fill the space left by the heated particles.

The particles that move away from the source of heat become cooler and more dense.

Example of Convection

When the water at the bottom of a pot is heated, its particles move faster, and they also move farther apart. As a result, the heated water becomes less dense. A less dense fluid will float on top of a more dense one. Therefore, the heated water rises in the pot. The surrounding cooler water flows into its place. This flow creates the circular motion, known as convection currents.

Sea Breeze and Land breeze

Convection currents cause sea breeze and land breeze

Where is the freezer

compartment put in a fridge?

Freezer compartment

It is put at the top, because cool air sinks, so it cools the food on the

way down.

It is warmer at the bottom, so this warmer air

rises and a convection

current is set up.

Application

Convection currents in the earth

• There is a lot of heat within the earth.• The surface of the earth is cooler than the lower mantle.• Heat is transferred to the upper layers.• Spots where the material conducts more heat become

hotter and less dense, so they rise towards the surface.• Denser material falls.

Heat Radiation

• Energy transferred in the form of rays or waves or particles.

• Radiation is the transfer of energy by electromagnetic waves

• Radiation does NOT require a medium to transfer thermal energy

Radiation from the sun

You can feel the sun warm your skin on a sunny day.This is because the energy causes the particles in

your skin to move faster = more heat energy

Radiation from other sources

We all have experience that lovely warm feeling sitting by the bonfire. The heat

that we receive from the fire comes to us by RADIATION.

Radiators

Radiators are heat exchangers designed to transfer thermal energy from one medium to another for the purpose of space

heating. The heating radiator was invented by Franz San Galli in 1855, a Prussian-born Russian businessman living in

St. Petersburg.

Applications:

Buildings which are white-washed or painted in light colors keep cooler in summer, since the light surfaces reflect radiant heat from the sun.

Many factory roofs are aluminium-painted. The bright surface reduces the heat lost in winter, and keeps the interior cool in summer.

We ourselves choose light-colored clothing in summer for the same reason.

Glass in greenhouses is often painted white in summer.Artificial satellites are highly polished.You can control your TV with a remote controller.Firemen search for people in smoke filled buildings with infra red

detectors.