Appreciate how thermal energy is transferred form a higher temperature region to one of lower temperature

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Learning outcomes

• Appreciate how thermal energy is transferred form
  a higher temperature region to one of lower
  temperature
• Describe how energy transfer occurs through
  molecules in solids
• Describe density changes in fluids leading to
  convection
• Explain that radiation does not require a
  material medium and rate of energy transfer is
  affected by (i) colour and texture of the
  surface
• (ii) surface temperature (iii) surface area
• Apply the concept of thermal energy transfer to
  everyday applications

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9.1 Conduction

• Conduction is the process by which heat is
  transmitted through a medium from one particle to
  another

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9.1 Conduction

• How Conduction Works
• Solids conduction occurs when thermal energy
  transferred from one particle to another.
• When one end of a rod is heated, molecules there
  gain energy and vibrate faster.
• These molecules collide with less energetic
  neighbors.
• Some of their energy is transferred to
  neighboring molecules which thus gains kinetic
  energy.

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9.1 Conduction

• Thermal energy is passed along the rod by the
  vibrating molecules.
• Vibration transfers energy from molecule to
  molecule until cold end reaches same temperature
  as hot end
• No net movement of molecule during conduction.
• Solids are better conductors of heat than liquids
  and gases as molecules are closer together.
• Kinetic energy transferred more quickly.

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9.1 Conduction

• Conduction takes place through vibrating
  molecules as well as free electrons in metals.
• When heated, free electrons gain energy and move
  faster.
• They move in the spaces between atoms before
  colliding with some atoms and transferring energy
  to them.
• Much faster than conduction by vibration of
  molecules in non-metals.
• Metals conduct heat much faster then non-metals
  which have no free electrons.

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9.1 Conduction

• Examples of good and poor heat conductors
  (insulators)

Good conductors Poor conductors (insulators)
Gold Silver Copper Iron Brass Aluminium Glass Stone Water Plastics Wood Polystyrene foam Wool fibreglass
Materials containing trapped air
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9.1 Conduction

• Applications of Conduction
• Uses of Good Conductors
• Used where thermal energy has to be quickly
  transmitted
• E.g. Pans, kettles, cooking utensils are made of
  metals like aluminium, copper, steel
• Copper pipes for heat exchangers
• Heat sinks in electronic equipment

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9.1 Conduction

• Uses of Poor Conductors (Insulators)
• E.g. Cloth, plastics, wood
• Help keep unwanted heat away
• Help prevent heat loss

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9.1 Conduction

• Air is good insulator, reduces conduction of heat
• Felt material used as insulators to trap air
  pockets

Prevent?
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1. Thermal energy of a body is the total kinetic
   energy of the atoms or molecules in the body.
2. Heat is energy in transit from a high temperature
   object to a lower temperature object. It is
   recognised only as it crosses the boundary of a
   system.
3. Conduction is the process by which thermal energy
   is transmitted through a medium from one
   particle to another.
4. Conduction in metals is far better than in other
   solids because other than the vibrating atoms,
   the free electrons in a metal are also
   responsible for the conduction of thermal energy.
5. Liquids and gases are poor thermal conductors.

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9.2 Convection

• Convection is the process by which heat is
  transmitted from one place to another by the net
  movement of heated particles of a gas or liquid.
• Convection takes place in liquids and gases where
  atoms or molecules move from one place to another
• Transfer of thermal energy depends on transfer of
  heated matter.

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9.2 Convection

• Convection in Liquids
• Add potassium permanganate crystals into a flask
  of water
• Crystals sink to bottom of the flask
• Heat flask gently just below the crystals
• Purple-streaked water rise upwards and fan
  outwards.
• Water becomes uniformly purplish after a while.

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9.2 Convection

• Circulation of a liquid called a convection
  current
• Due to heating at the bottom of liquid
• When a liquid is heated, it expands, becomes less
  dense and floats upwards.
• Cold and denser liquid moves down to replace the
  rising liquid masses.
• This cold liquid, in turn, gets heated up.

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9.2 Convection

• A Hot Water System
• based on convection principle
• Consists of a boiler, water storage tank and cold
  tank interconnected by pipes
• Convection currents drive hot water from the top
  of boiler into hot water storage tank.
• Cold water from storage tank is drawn down to the
  boiler where it in turn becomes heated.

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9.2 Convection

• Car Radiator
• Car engines are kept cool by a water jacket
• When water in the water jacket gets heated, it
  flows into copper tubes which include many
  cooling fans.
• A fan causes air to flow past the tubes and cool
  water in them.
• Cooled water flows down and back into engine
  through a hose at the bottom.

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9.2 Convection

• Convection in Gases
• To show convection currents in air
• Fit two glass chimneys to the top of a wooden box
  with plane glass window
• Light a candle below one chimney
• Heat from this initiates a convection current
• Hot air flows out of this chimney
• Cold air flows in through other chimney
• Used to ventilate underground mines

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9.2 Convection

• Air-conditioners
• Cold air blown in sinks because its denser then
  warm air.
• Warm air displaced by cold air rises and is drawn
  into air-conditioner to be cooled.
• Refrigerator
• Cold air below freezer sinks
• Set up convection currents

Todays fridge, freezer below??
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9.2 Convection

• Ventilation in a Kitchen
• Hot air rises through chimney
• Sets up a convection current
• Cool air drawn into kitchen

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9.2 Convection

• Sea and Land Breezes
• In the day, land heats up faster than the sea
• Air above land expands and rises
• Cool air above sea moves inland
• Result in sea breeze

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9.2 Convection

• At night, the land cools faster than the sea
• Warm air above sea rises
• Cool air above land moves out to take its place
• Result in land breeze

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1. The process of thermal energy transfer most
   significant in fluids (liquids and gases) is
   convection.
2. Convection is the process by which heat is
   transmitted from one place to another by the
   movement of heated particles of a gas or liquid.
3. Convection currents are caused by density
   changes.
4. Hot fluids rise and cold fluids sink.

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9.3 Radiation

• Radiation is the transfer of energy by
  electromagnetic waves
• Emission of Radiation
• The rate of energy transfer by radiation is
  affected by
• (a) surface temperature
• (b) colour and texture of the surface
• (c) surface area

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9.3 Radiation

• Uses of Poor Emitters of Radiation
• Shiny surfaces are poor radiators
• E.g. Teapots and aluminum foil
• Uses of Good Emitters of Radiation
• Dull surfaces radiate quickly
• E.g. Cooling fins of refrigerators and heat sinks

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9.3 Radiation

• Absorption of Radiation
• Dull black surfaces better absorbers than shiny
  surfaces
• Good emitter is also good absorber
• Shiny surfaces good reflectors of heat

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9.3 Radiation

• Uses of Good and Poor Absorbers of Radiation
• Houses in hot countries painted white to keep
  them cooler.
• Factory roofs coated with aluminium to reduce
  radiation absorption.
• Solar control films allow light to pass through
  but keep out infrared radiation.
• Dark clothes absorb radiation readily.
• Solar panels painted black to absorb as much
  radiation from sun as possible.

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9.3 Radiation

• Vacuum Flask
• Reduces transfer of thermal energy by conduction,
  convection and radiation
• Consists of a double-walled glass container with
  a vacuum in between
• Silvered walls on vacuum side
• Container supported by foam plastic, a poor
  conductor

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9.3 Radiation

• No heat can enter or leave flask by conduction or
  convection across the vacuum
• Inner silvered surface reflects radiation from
  hot fluids back into flask
• Outer silvered surface reflects radiation in
  external surroundings away from flask
• Foam plastic support and plastic cup minimises
  heat transmitted by conduction through thin glass
  walls
• Plastic cup stops convection and evaporation

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• Radiation is the transfer of energy by infra-red
  electromagnetic waves.
• Dull black surfaces are good emitters and
  absorbers of radiation.
• The greater the surface area of the object, the
  faster is the rate of thermal transfer from it.
• The greater the temperature of an object, the
  faster is the rate of thermal energy transfer
  from it.

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