pure unit 10 transfer of thermal en

8/9/2019 Pure Unit 10 Transfer of Thermal En

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23 August 2010Copyright 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd.

10.1 Transfer of Thermal Energy

Learning Outcomes

In this section, youll be able to:

Understand that thermal energy is transferred from aregion of higher temperature to a region of lower

temperature


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What causes transfer of thermal energy? Thermal energy is transferred only when there is a

difference in temperature.

Thermal energy always flows from a region ofhighertemperature to a region oflower temperature.

There is no net transfer of heat at thermalequilibrium.


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How is thermal energy transferred?

Thermal energy is transferred by:

Conduction Convection

Radiation


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Quick check 1During winter, it is common for people to say keep thecold out of the house. Is this statement correct?Comment.

The statement keep the cold out of the house seems tosuggest that the cold tends to move into the house whichis not true.

In fact, it is the transfer of heat energy from the inside ofthe house to the outside that causes the temperature inthe house to drop.


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10.2 Conduction

Learning Outcomes


Describe how energy transfer occurs in solid.


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10.2 Conduction

What is conduction?

Definition:

Conduction is the process of thermalenergy transfer without any flow ofthe material medium.

-Burningpapers-Wiremesh


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10.2 Conduction

Objective:To investigate the transfer of thermal energy through solids

Experiment 10.1

Observation

The wax melts the furthest along thecopper rod, followed by iron, glassand wood.


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10.2 Conduction

Experiment 10.1

Two important conclusions can be drawn:

1. Thermal energy flows through the material of the rodswithout any flow of the material itself. This process is

called conduction.

2. Different materials conduct heat at different rates.Those that conduct faster are called good conductors(e.g. copper) and those slower are called poorconductors (e.g. wood).

Note:

Poor conductors are also known as insulators.


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How does conduction work? (Mechanism of conduction)

When thermal energy is supplied to one end of a rod, the particles (atoms and

molecules) at the hot end vibrate vigorously.

These particles collide with neighbouring particles, making them vibrate as well.

Kinetic energy of vibrating particles at the hot end is transferred toneighbouring particles.


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10.2 Conduction

Mechanism of conduction (Conductor vsinsulator)

Good Conductor

In metals, another much faster mechanism of thermalenergy transfer takes place at the same time -free

electron diffusion. The free electrons gain kinetic energy and move faster.

The fast-moving electrons then diffuse into cooler partsof the metal.

Insulators In insulators, the transfer of thermal energy is solely the

results of vibrating atoms and molecules.

There is no free electrons.


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10.2 Conduction

Conduction in liquids and gases


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10.2 Conduction

Conduction in liquids and gases

Thermal energy can be conducted from a hotter to acooler region.

Process of conduction is inefficient.

Liquid particles are further apart and collisions ofparticles are less frequent and even lesser in gases.

Thus, transfer of kinetic energy from fast-movingmolecules to neighbouring molecules is slower.

Hence air is poor conductor of heat compared to water,

which is in turn is a poor conductor compared to mostsolids.


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Objective:To test conduction of thermal energy in water

10.2 Conduction

Experiment 10.1

Observation

Water started to boil at thetop.

Ice remains as solid

ConclusionWater is a poor conductor ofheat energy

What is the purpose of the wiregauze?


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10.2 Conduction

Lets summarize

1. Conduction is the transfer of thermal energy withoutany flow of the material medium.

2. The two mechanisms for conduction are atomic ormolecular vibrations (for both metals and non-metals)

and free electron diffusion (for metals only).3. Liquids and gases are poor conductors of heat

compared to solids.


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Quick check 2


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10.3 Convection

Learning Outcomes


Describe how energy transfer occurs in fluids.


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10.3 Convection

What is convection?

Definition:

Convection is the transfer of thermalenergy by means ofcurrents in a fluid(liquids or gases).


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10.3 Convection

Experiments

ObjectiveTo show convection in water

ObjectiveTo show convection in air


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10.3: Convection

How does convection work? (Mechanism ofConvection Current)

When fluids (liquids and gases) are heated, they expandand become less dense.

The less dense fluids tend to rise from the heatingsource.

Cooler fluids, being more dense, sink to replace the lessdense fluids.

This movement of fluid due to a difference in its densitysets up a convection current.

Convection currents occur only in fluids such as liquidsand gases but not in solids.

Convection involves the bulk movement of the fluidswhich carry with them thermal energy.


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Lets summarize

1. Convection is the transfer of thermal energy by meansof currents in a fluid (liquid or gas).

2. A convection current is the movement of fluid causedby the change in density in various parts of the fluid.


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Unit 10.5: Applications of Thermal

Energy TransferCommon applications of convectionHousehold hot water system the heater is located at thebottom of the system.


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Unit 10.5: Applications of Thermal

Energy TransferCommon applications of convectionRefrigerators

Freezing unit is placed at top to cool the airand facilitate the setting up of convection

currents.


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Quick check 3


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10.4: Radiation

Learning OutcomesIn this section, youll be able to:

Explain energy transfer of a body by radiation.

State the factors affecting the rate of energy transferby radiation.


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10.4: Radiation

What is radiation?

Definition:

Radiation is the continual emission ofinfrared waves from the surface of allbodies, transmitted without the aid ofa medium.


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10.4: RadiationWhat is radiation?

Radiation does not require a medium for energytransfer.

It can take place in vacuum.For example, the Sun is a major source of radiant heat.

The sun emits electromagnetic waves. Part of this electromagnetic waves, called infrared

waves, make us feel warm. Thermal energy from infrared waves is called radiant

heat.

All objects emit some radiant heat. The hotter the object, the greater the radiant heat

emitted.


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10.4: RadiationAbsorption of infrared radiation

Infrared radiation is absorbed by all objects andsurfaces.

The absorption of radiant heat causes a temperaturerise.

Emission of infrared radiation

Infrared radiation is emitted by all objects and surfaces.

This emission causes the temperature of the objectsthemselves to fall.

In general, good emitter of radiant heat is also a goodabsorber of radiant heat.

Conversely, poor emitter of radiant heat is also a poor

absorber of radiant heat.


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Unit 10.4: RadiationExperiment 10.6

ObjectiveTo investigate the emission of infrared radiation

ApparatusTwo temperature sensors, data logger, two identical tins (one

black and one shiny), boiling water from two electric kettles

Procedure1. Connect the temperature sensors A and B to the data logger2. Set the sampling rate to ten seconds3. Pour boiling water into both tins at the same time until both

are filled to the brim.


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4. Place the lid and the temperature sensors onto the tins.

Temperature sensor A will monitor the temperature of theblack tin, while temperature sensor B records thetemperature of the shiny tin.

5. Start recording the temperature. Observe the temperaturetime graph of both sensors.

6. Stop recording after ten minutes.

Experiment 10.6


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Factors affecting rate of infrared radiation

1. Colour and texture of the surface

Dull, black surfaces are good absorbers of infraredradiation than shiny, white surfaces

Dull, black surfaces are better emitters of infraredradiation.

3. Surface temperature

Rate of infrared radiation alsodepends on surface temperature

The higher the temperature ofthe surface of the object relativeto the surrounding temperature,the higher the rate of infraredradiation.

2. Surface area

The larger surface area will emit infrared radiation at ahigher rate.


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Unit 10.4: Radiation

Lets summarize

1. Radiation is the continual emission of thermalenergy in the form ofinfrared waves.

2. Radiation is emitted from the surface of all bodies

and does not require a medium of thermaltransfer.

3. Dull, black surfaces are better emitters ofinfrared radiation than shiny, white surface.

4. The factors affecting rate of energy transfer by

radiation are: colour, and texture of the surface,surface temperature and surface area.


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Unit 10.4: Radiation

Quick check 4


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Unit 10.5: Applications of ThermalEnergy Transfer

Common applications of radiation

Greenhouses infrared radiation emitted by thecontents in the greenhouse is trapped in the greenhouse.


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Unit 10.5: Applications of Thermal EnergyTransfer

Common applications of Conduction, Convection andradiation. (Vacuum Flask)

Stopper is made ofpoorconductor.

The vacuum between thedouble-glass wall minimisesconduction and convection.

The glass walls are silveredand highly reflective tominimise heat loss due toradiation.


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Unit 10.5: Applications of ThermalEnergy Transfer

Quick Check 5The figure shows a typical vacuum flask designed tokeep liquids hot. Part of the vacuum flask is enlarged.State and explain the function of each of the partslabelled A to C

A: Thin silvering wall to minimisethermal energy loss by radiationSince shiny surfaces are poorabsorbers of radiant heat.B: Vacuum to prevent thermal energy

loss by conduction and convention(both require material medium for energy transfer)C: Hollow plastic stopper. Plastic is a poor conductor ofheat, minimising thermal energy loss by conduction.


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Unit 10: Transfer of Thermal Energy

pure unit 10 transfer of thermal en

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