Transfer of Thermal Energy

• Transfer of Thermal Energy• Conduction • Convection• Radiation• Applications of Thermal Energy Transfer

Transfer of Thermal Energy

Thermal energy always flows from a region of higher

temperature to a region of lower temperature. Net flow

of thermal energy occurs only when there is a

difference in temperature.

Types of Thermal Energy Transfer

Learning OutcomeAt the end of this section, you should be able to:

• describe how energy transfer occurs in solids at the molecular level.

Conduction

ConductionConduction is the transfer of thermal energy through a

medium without any flow of the medium.• Particles at the heated end

vibrate vigorously.• They collide with neighboring

particles and transfer their energy.

• Eventually the particles at the cooler end are also set into vigorous vibration.

Conduction

Heat from the source as causing the atoms of the solid to vibrate and gain kinetic energy.

Conduction

These atoms cause neighbouring atoms to vibrate. Kinetic energy is transferred from one atom to the next.

Conduction

Heat energy is conducted through the solid in this way. As the atoms of the solid gain kinetic energy the temperature of the

solid increases.

Conduction in Metals

• In all solids, thermal energy is transferred through the vibration and collision of particles.

• However, in metals, due to the presence of free electrons, thermal energy is also spread through electron diffusion.

• Electrons gain kinetic energy and move more rapidly. They collide with atoms in the cooler parts of the metal and pass on their energy in the process.

Conduction in Liquids and Gases

• Particles in liquids and gases are spaced farther apart those in solids

• Collisions between particles occur less frequently; slow transfer of kinetic energy

• Poor conductors of heat

Conduction in Liquids

What did we learn yesterday?

ConvectionLearning Outcome• Describe convection in fluids (liquids and gases) in

terms of density changes.

ConvectionConvection is the transfer of thermal energy by means of convection currents in a fluid (liquid or gas), due to a difference in density.

Conduction vs Convection

• In conduction, thermal energy is transferred from one particle to another.

• In convection, thermal energy is transferred through the actual movement of the heated particles from the warmer to cooler parts of the fluid.

Convection in Liquids• When the water at the bottom of the

flask is heated, it expands.

• The expanded water is less dense than the surrounding water and rises.

• Since the upper region is cooler, it is denser and therefore sinks.

• The difference in the densities of water in the different regions sets up a convection current.

• This is shown by purple streams rising from the bottom and sinking at the sides.

Convection in Gases• When the air above the candle is

heated, it expands.

• The expanded air is less dense than the surrounding air and rises out of chimney B.

• Since the surrounding air is cooler, it is denser and therefore sinks into chimney A.

• The difference in the densities of the air at the different chimneys sets up a convection current.

How are Land and Sea Breezes Formed?

Discuss with you partners!

Land & Sea Breeze• In the day, the land heats up faster than the

sea. The air above the land is heated, expands and rises.

• Cool air above the sea is denser and moves in to replace the warmer air. This sets up a sea breeze.

• At night, the land cools faster than the sea.

• The air above the land is now cooler than the air above the sea.

• A convection current is set up in the opposite direction. This is called a land breeze.

Radiation

Learning Objectives• explain energy transfer of a body by radiation• state the factors that affect the rate of energy

transfer by radiation

RadiationRadiation is the transfer of thermal energy in the form of

electromagnetic waves without the aid of a medium.

• All bodies emit infrared radiation • Infrared radiation does not require a medium to be

transmitted. This is how the Earth is warmed by the Sun

Electromagnetic Waves

Effect of Colour and Texture onEmission Rate of Infrared Radiation

Both tins are filled with water, heated to the same

temperature and connected to a data logger

Effect of Colour and Texture onEmission Rate of Infrared Absorption

The pieces of foil need to be positioned at equal distances

from the source of heat in order for the experiment to

be fair

Other Factors affecting Rate of Infrared Radiation Absorption/Emission• Surface areaFor two objects of identical mass and material, the object with the larger surface area will emit or absorb infrared radiation at a higher rate.• Surface temperature

The higher the temperature of an object relative to its surrounding temperature, the higher the rate of emission of infrared radiation.

Applications of Transfer of Thermal Energy

Learning OutcomeAt the end of this section, you should be able to:

• describe how the concept of thermal energy transfer by conduction, convection and radiation applies to everyday life.

Conductors

A material through which heat can move easily is called a conductor.

Iron, silver, aluminum, stainless steel, and copper are all examples of metals that are good conductors.

Insulators

A material that heat does not move through easily is an insulator.

Plastic, wood, rubber, and glass are all good insulators.

Many cooking utensils and handles are made out of plastic, wood, or rubber.

Air is a good insulator.

Geothermal Power Station

• Explain how water is used to generate electricity in geothermal power stations.

• How could the system be improved to make it more efficient?

Geothermal Power Station

Plastic StopperVacuum Flask

• Using the components given, explain how the vacuum flask manages to keep the water at a steady temperature. If possible, also explain how the material of the components affect the heat transfer.

Vacuum Flask

Black Chrome PlateInsulating Material

Outlet Connection

Inlet Connection Glass Cover

ss

s

sss

Collector Housing made from aluminium alloy

Flow Tubes

Solar Panel

• Using the components given, explain how heat transfer occurs in a solar panel. If possible, also explain how the material of the components affect the heat transfer.

Solar Panel

Solar Water Heater

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• Using the components given, explain how heat transfer occurs in a solar water heater. If possible, also explain how the material of the components affect the heat transfer.

Solar Water Heater

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Household Water System

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• Using the components given, explain how the water system functions.

Household Water System

A saucepan with a thick copper base contains water and is placed on a flat electric hot plate.

(a)State the process by which thermal energy is

(i) transferred from the hot plate to the water;

(ii) transferred through the water.

(b)The sides of a saucepan are often polished, but the base is usually dark-coloured. Why is this so?

Saucepan

freezing unit

Refrigerator• The freezing unit of the refrigerator is

placed at the top.

• Why this this the case? Draw arrows to show the convection current within the refrigerator

What have we learnt today?

• How conductors and insulators can aid or reduce the rate of heat trasnfer

• How conduction, convection and radiation can all play a part at the same time to affect the transfer of heat

Applications of Transfer of Thermal Energy

In your own groups, discuss how the following

appliances would work!

Plastic StopperVacuum Flask

Black Chrome PlateInsulating Material

Outlet Connection

Inlet Connection Glass Cover

ss

s

sss

Collector Housing made from aluminium alloy

Flow Tubes

Solar Panel

Solar Water Heater

f

Household Water System

f

f

freezing unit

Refrigerator Saucepan

What have we learnt today?

•Appliances can make use of all 3 methods of heat transfer simultaneously

• Both conductors and insulators play an important role in transfer of heat energy