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Energy transfer

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Lesson 2 – Heat Transfer

Aims:

•recall that energy transfer may take place by conduction, convection and radiation.

•describe the role of convection in everyday phenomena.

•describe how insulation is used to reduce energy transfers from buildings and the human body.

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Heat energy only flows when there is a temperature difference from a w______ area to a c______ area.armer ooler

Heat is a type of energy called thermal energy.

Heat can be transferred (moved) by three main processes:

1. conduction

2. convection

3. radiation

During heat transfer, thermal energy always moves in the same direction:

HOT COLD

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How are the particles arranged in a solid, a liquid and a gas?

Particles that are very close together can transfer heat energy as they vibrate. This type of heat transfer is called conduction.

Conduction is the method of heat transfer in solids but not liquids and gases. Why?

What type of solids are the best conductors?

solid liquid gas

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Conduction

Conduction is the method of heat transfer in solids. If you touch the handle of a metal saucepan that has been heated, the heat from the saucepan will conduct along the metal handle into your hand.

The heat energy conducts along the

handle to your hand

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Graphite is a non-metal that is a good conductor of heat. If you heat one end of a graphite rod, the heat travels to the other end.

As the rod is heated, the atoms vibrate. These vibrations make the adjacent/neighboured atoms vibrate, and so on. This is how the heat energy travels along the rod.

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The outer electrons of metal atoms are not attached to any particular atom. They are free to move between the atoms.

When a metal is heated, the free electrons gain kinetic energy.

This means that the free electrons move faster and transfer the energy throughout the metal.

This makes heat transfer in metals very efficient.

Insulators do not have these free electrons, which is why they do not conduct heat as well as metals.

heat

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Convection

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Liquids and gases can behave in similar ways and so are called fluids.

What happens to the particles in a fluid when it is heated?

The heated fluid particles gain energy, so they move about more and spread out. The same number of particles now take up more space so the fluid has become less dense.

heatless dense

fluid

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Cooler regions of a fluid are more dense than warmer regions of the same fluid.

The cooler regions will sink as they have the greatest mass per unit volume.

The warmer regions will rise as they have a lower density. In effect, they float on top of the denser, cooler regions.

This is how heat transfer takes place in fluids and is called convection.

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Convection currenthot water

cools

cool water sinks

hot water rises

cool water warms

This is called a convection current.

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Convection currentsThe air reaching land is heated, the air becomes hotter, the air particles move faster, the air expands, becomes less dense and rises.

The air then cools, its particles move slower, it contracts, becomes more dense and falls.

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•In sunshine, the land warms up more quickly than the sea

•Heat is conducted from the land to the air layer immediately next to it, so this air becomes warmer than that over the sea

•The warmer air over the land rises, because it is now less dense, and the cooler air above the sea moves inland, forming a convection current or breeze, blowing in from the sea - this is called a sea breeze

•In sunshine, the land warms up more quickly than the sea

•Heat is conducted from the land to the air layer immediately next to it, so this air becomes warmer than that over the sea

•The warmer air over the land rises, because it is now less dense, and the cooler air above the sea moves inland, forming a convection current or breeze, blowing in from the sea - this is called a sea breeze

Sea breeze

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•At night, the land cools more quickly than the sea

•The warmer air is now over the sea

•This air rises and the cooler air moves from the land towards the sea - this is called a land breeze

•At night, the land cools more quickly than the sea

•The warmer air is now over the sea

•This air rises and the cooler air moves from the land towards the sea - this is called a land breeze

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Why is the freezer compartment at the top of a fridge?

freezer compartment

The freezer compartment is at the top because cool air sinks.

The freezer cools the air at the top and this cold air cools the food on the way down.

It is warmer at the bottom of the fridge.

This warmer air rises and so a convection current is set up.

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Why are boilers placed beneath hot water tanks in people’s homes?

Hot water rises.

When the boiler heats the water, and the hot water rises, the water tank is filled with hot water.

Why does hot air rise and cold air sink?

Cool air is more dense than warm air.

The cool air ‘falls through’ the warm air and so the warm air rises to the top.

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The water at the top quickly boils, while the water at the bottom remains cold - the ice does not melt . This indicates that water is a poor conductor of heat. Gases are also poor conductors.

The water at the top quickly boils, while the water at the bottom remains cold - the ice does not melt . This indicates that water is a poor conductor of heat. Gases are also poor conductors.

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The air above the candle is heated, and becomes less dense, so it rises, and cooler air is drawn into the left-hand funnel, and is made visible by the smoke.

The air above the candle is heated, and becomes less dense, so it rises, and cooler air is drawn into the left-hand funnel, and is made visible by the smoke.

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Radiation

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Radiation

Radiation is the only method of heat transfer that does not require particles as a medium. This means it is the only

method of heat transfer that can travel through a vacuum.

Heat radiation is emitted by all objects above absolute zero and travels in straight lines.

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Earth is warmed by heat energy from the Sun.

How does this heat energy travel from the Sun to the Earth?

There are no particles between the Sun and the Earth so the heat cannot travel by conduction or by convection.

?infraredradiation

The heat travels to Earth by infrared waves. They are similar to light waves and are able to travel through empty space.

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Heat can move by travelling as infrared waves.

These are electromagnetic waves, like light waves, but with a longer wavelength.

This means that infrared waves act like light waves:

They can travel through a vacuum.

They travel at the same speed as light – 300,000,000 m/s.

They can be reflected and absorbed.

Infrared waves heat objects that absorb them and so can be called thermal radiation.

RadiationRadiationRadiationRadiation

Radiation

RadiationRadiationRadiationRadiation

Radiation

Greenhouses

《 physics for you 》 P5028

Greenhouses Effect

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Greenhouses Effect

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Absorption of RadiationAbsorption of RadiationAbsorption of RadiationAbsorption of Radiation

Absorption of Radiation

Absorbing & Emitting RadiationAbsorbing & Emitting RadiationAbsorbing & Emitting RadiationAbsorbing & Emitting Radiation

Absorbing & Emitting Radiation

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Infrared waves heat objects that absorb (take in) them.

Some surfaces are better at absorbing thermal radiation than others – good emitters are also good absorbers.

Matt black surfaces are the best absorbers of radiation.

Shiny surfaces are the worst emitters because they reflect most of the radiation away.

Why are solar panels that are used for heating water covered in a black outer layer?

best emitter worst emitter

best absorber worst absorber

white silvermattblack

Emitting RadiationEmitting RadiationEmitting RadiationEmitting Radiation

Emitting Radiation

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All objects emit (give out) some thermal radiation.

Some surfaces are better at emitting thermal radiation than others.

Matt black surfaces are the best emitters of radiation.

Shiny surfaces are the worst emitters of radiation.

Which type of kettle would cool down faster: a black kettle or a shiny metallic kettle?

white silvermattblack

best emitter worst emitter

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Why are houses painted white in hot countries?

White reflects heat radiation and so keeps the house cooler.

Why are shiny foil blankets wrapped around marathon runners at the end of a race?

The shiny metal reflects the heat radiation from the runner back in to their body, which stops the runner from getting cold.

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Why is black cool?

The black smock absorbs infra red radiation from the Sun and becomes hot. It heats the air inside and causes a convection current. Cool air is pushed in the bottom. Although the air and garment are hot the movement of air keeps the person cool.

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Insulation at home

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Charlie forgot to take the chicken out of the freezer last night! Will his plan to defrost the chicken in time for lunch work?

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Insulation

Insulation works because of the air space…

foam, fibre glass, double glazed windows.

Poor conductors are called insulators.

Wood, snow and gases are excellent insulators.

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How does it insulate?

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Uses of energy

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Home insulation

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Most energy is lost through the roof and walls.

In houses, energy is lost from the roof, the door, the walls, the windows and the floors.

In order to reduce electricity bills and pollution, energy losses must be kept to a minimum.

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Reducing heat loss – curtains

Curtains reduce draughts (convection currents) leaving the house. They are opaque and so don’t allow much radiated heat to pass through them.

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Reducing heat loss – walls

This reduces heat loss by conduction through the bricks.

Most outside walls have an empty space between the two layers of bricks called a cavity.

Cavity wall foam insulation is pumped in-between the bricks to prevent convection within the cavity.

convection currents in the cavity

foam prevents convection

currents

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Window insulation

• Double glazed windows trap a small amount of air between two panes of glass.

• The air cannot move very much so energy loss by conduction through the glass is reduced.

Preventing heat transfer at home.

Double glazed windows consist of two panes of glass with a layer of air trapped in between. This reduces conduction losses since there are two panes, so increasing the thickness of the window. Also, the air trapped in between is a very good insulator. This reduces conduction losses further

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Side-view of double glazing

insulating layer of air

Double glazing keeps a house warmer because there is a layer of a__ between the panes of glass.

Air is a poor c________, so it acts like an i_______, .

The trapped air r_______ heat loss by c_________ from a house.

ir

onductornsulator

educesonduction

How does double glazing keep a house warmer?

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Reducing heat loss – roof

Loft insulation contains trapped air, which forms an insulating layer between the loft and the rest of the house.

The air heated by central heating is less dense than the cooler air around it. As a result, warm air rises.

The ceilings become heated and eventually the heat escapes through the roof.

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Roof insulation

Air is an excellent insulator. Fibreglass is an excellent material for trapping small amounts of air. A two or four inch layer of fibre glass insulation in you roof will pay for itself within a year.

Preventing heat transfer at home.

Insulating your loft is one of the cheapest ways of insulating

your home and goes a long way to reducing heat

lost through the roof by conduction.

It also prevents energy from being wasted in heating the loft (with heat that is conducted through the ceiling of the rooms below!)

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Reducing heat loss – radiators

Placing shiny silver-coated card between the wall and the radiator reduces heat loss by radiation by reflecting it back into the room.

Radiators heat the wall a great deal – this wastes heat energy.

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Reducing heat loss – draughts

Draught excluders are hairy or spongy strips that close gaps and prevent the convection currents escaping.

Draughts are convection currents.

A great deal of heat energy is lost in this way but it is the easiest type of heat loss to prevent.

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Insulation and heat transfer

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Insulation and heat loss

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Multiple-choice quiz

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A model of heat transfer processes

1. CONDUCTION – book passed from student to student.

2. CONVECTION – book taken a student to the back of the room.

3. RADIATION – book thrown to the back of the room.

The structure of a vacuum flask

Screw on cover (plastic and cork are good insulators)

Vacuum. Only infra red radiation can pass through a vacuum

The inside walls of the vacuum is silver. This reflects any radiant heat that enters.

How heat transfer is stopped.

The screw cover prevents heat loss by convection as the air above the hot liquid gets warm.

Making the screw cover out of materials which are poor conductors reduces heat loss by conduction.

The inside wall of the vacuum may become hot as

heat is conducted through the glass layer, but …..

How heat transfer is stopped.

Heat cannot be lost in this region by conduction or convection since it is a

vacuum!When the glass walls of

the vacuum gets warm, it will radiate only a small

amount of heat since shiny surfaces are poor

emitters of radiation.Any heat that is radiated cannot escape however as it will be reflected by

the second silvery surface

Hot liquid on this side

Cooler air surrounding the

thermos

Examples of heat transfer by radiation

It keeps food hot or cold by heat exchange between food & environment

through conduction, convection & radiation.

The thermos or vacuum flask

cap

plastic / cork stopper

insulated support

The thermos or vacuum flask

heat loss by conduction & convection

heat loss by conduction

vacuum flask

The thermos or vacuum flask

Silvery glass wall heat loss by radiation

vacuum between the double glass walls heat loss by conduction & convection

outer case

outer pot

inner pot

The thermos or vacuum flaskVacuum cooker

It works on the same principle as thermos.

(conducting)

Food is first cooked using the inner pot put into outer pot to keep food hot without a fire

a stainless steel thermos

made of stainless steel/ aluminium

(insulating)

Vacuum flaskVacuum flaskVacuum flaskVacuum flask

Vacuum flask

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Summary – Heat transfer•We can insulate objects, including ourselves, to reduce energy transfers from hot objects to cooler objects.

•Air is often used as an insulating material. Air is a bad conductor. Air must be trapped in small bubbles to stop it moving around. If air can move around it will transfer energy by convection.

•Common ways of insulating our homes include: roof insulation, cavity wall insulation, carpets, curtains and double glazing.