Unit 8: Transfer of Thermal Energy

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Unit 8 Transfer of Thermal Energy
Discover PHYSICS
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8.1 Transfer of Thermal Energy

• Learning Outcomes
• In this section, youll be able to
• Understand that thermal energy is transferred
  from a region of higher temperature to a region
  of lower temperature

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8.1 Transfer of Thermal Energy

• 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 of
  higher temperature to a region of lower
  temperature.
• There is no transfer of heat at thermal
  equilibrium.

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8.1 Transfer of Thermal Energy

• How is thermal energy transferred?
• Thermal energy is transferred by
• Conduction
• Convection
• Radiation

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8.1 Transfer of Thermal Energy

• Key Ideas
• Transfer of thermal energy takes place when there
  is a temperature difference. Thermal energy is
  always transferred from a hotter region to a
  colder region.
• When thermal equilibrium is reached between two
  bodies (i.e. both bodies are at the same
  temperature), there is no net flow of thermal
  energy between them.
• There are three different processes of thermal
  energy transfer conduction, convection and
  radiation.

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8.1 Transfer of Thermal Energy

• Test Yourself
• 1. During winter, it is common for people to say
  keep the cold out of the house. Is this
  statement correct? Comment.
• Answer
• The statement keep the cold out of the house
  seems to suggest that the cold tends to move
  into the house which is not true.
• In fact, it is the transfer of heat energy from
  the inside of the house to the outside that
  causes the temperature in the house to drop.

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8.2 Conduction

• Learning Outcomes
• In this section, youll be able to
• Describe how energy transfer occurs in solid.

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8.2 Conduction

• What is conduction?

Definition Conduction is the process of thermal
energy transfer without any flow of the material
medium.
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8.2 Conduction
Experiment 8.1
Objective To investigate the transfer of thermal
energy through solids Apparatus bath, rods of
the same dimensions but of different materials,
stopwatch Procedure 1. Coat the parts of the
rods that are on the outside of the tank evenly
with melted wax (see figure). 2. Pour boiling
water into the bath, so that the ends of the rods
are submerged. 3. Record the length of wax that
melts in a given interval of time for each of the
four rods.
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ObservationThe wax melts the furthest along the
copper rod, followed by iron, glass and wood.
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8.2 Conduction

• Experiment 8.1
• Two important conclusions can be drawn
• 1. Thermal energy flows through the material of
  the rods without 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 poor conductors (e.g. wood).
• Note
• Poor conductors are also known as insulators.

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8.2 Conduction

• How does conduction work?
• Conductors and insulators have different
  mechanisms to transfer of thermal energy.
• All solids are made up of tiny particles called
  atoms or molecules.
• Metals contain free electrons which move randomly
  between the atoms and molecules.
• Non-metals do not have free electrons.

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8.2 Conduction

• How does conduction work?
• 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 to neighbouring particles.

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8.2 Conduction
How does conduction work?

• Good Conductor
• In metals, another much faster mechanism of
  thermal energy 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 parts of the metal.

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8.2 Conduction
How does conduction work?

• 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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8.2 Conduction

• Conduction in liquids and gases
• Thermal energy can be conducted from a hotter to
  a cooler region.
• Process of conduction is inefficient.
• Liquid particles are further apart and collisions
  of particles are less frequent and even lesser in
  gases.
• Thus, transfer of kinetic energy from fast-moving
  molecules 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 most solids.

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8.2 Conduction
Experiment 8.1
Objective To test conduction of thermal energy
in water Apparatus test-tube, ice, metal gauze,
Bunsen burner,water Procedure 1. Wrap a piece
of ice with metal gauze and place it at the
bottom of a test-tube. 2. Fill the test-tube with
tap water till it is almost full.
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8.2 Conduction

• 3. Heat the test-tube at the upper end, as shown
  in the figure.
• 4. Observe the water being heated and the ice
  below it.

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8.2 Conduction

• Key Ideas
• Conduction is the transfer of thermal energy
  without any flow of the material medium.
• The two mechanisms for conduction are atomic or
  molecular vibrations (for both metals and
  non-metals) and free electron diffusion (for
  metals only).
• Liquids and gases are poor conductors of heat
  compared to solids.

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8.2 Conduction

• Test Yourself
• 1. Give an account of thermal energy conduction
  in metals and non-metals.
• Answer
• In metals, conduction of heat is due mainly to
  the diffusion of free electrons from a hotter
  region to a colder region. Conduction of heat
  can also take place with molecular vibrations.
• In non-metals, conduction of heat only takes
  place due to molecular vibrations, where the K.E.
  of the vibrating molecules at the hot end is
  transferred to the neighbouring molecules.

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8.2 Conduction

• Test Yourself
• 2. Why are good conductors of thermal energy also
  good conductor of electricity?
• Answer
• Good conductors such as metals have free
  electrons. It is the presence of free electrons
  that enable metals to conduct both thermal energy
  as well as electricity.
• Conduction of electric current is the flow of
  electric charges such as electrons.

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8.2 Conduction

• Test Yourself
• 3. Is the heat transferred from a barbecue fire
  to a person standing in front of it a good
  example of heat transfer by conduction? Explain.
• Answer
• A person standing in front of a barbecue fire and
  feeling hot is not a good example of conduction
  since air is a poor conductor of heat.
• In fact, we will learn later that we feel the
  hotness of the barbecue fire due to radiation of
  the heat energy.

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8.3 Convection

• Learning Outcomes
• In this section, youll be able to
• Describe how energy transfer occurs in fluids.

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8.3 Convection

• What is convection?

Definition Convection is the transfer of thermal
energy by means of currents in a fluid (liquids
or gases).
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8.3 Convection
Experiment 8.3

• Objective
• To show convection in water
• Apparatus
• Large, round-bottomed flask, potassium
  permanganate crystals, Bunsen burner
• Procedure
• Fill the flask with water. Carefully place some
  potassium permanganate crystals at the bottom of
  the flask.
• Place a Bunsen burner with a small flame under
  the flask and observe the crystals.

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Unit 8.3 Convection
Experiment 8.4

• Objective
• To show convection in air
• Apparatus
• large box with two chimneys on top, a piece of
  clear glass on one side, candle, matches
• Procedure
• Place the candle below one of the chimneys. Light
  the candle.
• Introduce smoke into the other chimney by placing
  a piece of smouldering paper over it and observe
  the movement of the smoke.

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8.3 Convection

• How does convection work?
• When fluids (liquids and gases) are heated, they
  expand and become less dense.
• The less dense fluids tend to rise from the
  heating source.
• Cooler fluids, being more dense, sink to replace
  the less dense fluids.
• This movement of fluid due to a difference in its
  density sets up a convection current.

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8.3 Convection

• How does convection work?
• Convection currents occur only in fluids such as
  liquids and gases but not in solids.
• Convection involves the bulk movement of the
  fluids which carry with them thermal energy.

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8.3 Convection

• Key Ideas
• Convection is the transfer of thermal energy by
  means of currents in a fluid (liquid or gas).
• A convection current is the movement of fluid
  caused by the change in density in various parts
  of the fluid.

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8.3 Convection

• Test Yourself
• 1. Why does it feel hot when you put your hands
  above a small burning candle?
• Answer
• The hand feels hot because of convection. The
  air around the flame is being heated and becomes
  less dense and rises.

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8.3 Convection

• Test Yourself
• 2. Describe briefly the mechanism for the
  transfer of thermal energy in fluids.
• Answer
• When fluids are heated, they expand and become
  less dense. The less dense fluid rises.
• The cooler, denser fluids will replace the less
  dense fluids.
• This sets up a convection current.

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8.4 Radiation

• Learning Outcomes
• In this section, youll be able to
• Explain energy transfer of a body by radiation.
• State the factors affecting the rate of energy
  transfer by radiation.

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8.4 Radiation

• What is radiation?

Definition Radiation is the continual emission
of infrared waves from the surface of all bodies,
transmitted without the aid of a medium.
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8.4 Radiation

• What is radiation?
• Radiation does not require a medium for energy
  transfer.
• It can take place in vacuum.
• For example, the Sun is a major source of
  radiant heat.

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8.4 Radiation

• What is radiation?
• 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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8.4 Radiation

• Absorption of infrared radiation
• Infrared radiation is absorbed by all objects and
  surfaces.
• The absorption of radiant heat causes a
  temperature rise.

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8.4 Radiation

• Emission of infrared radiation
• Infrared radiation is emitted by all objects and
  surfaces.
• This emission causes the temperature of the
  objects themselves to fall.
• In general, good emitter of radiant heat is also
  a good absorber of radiant heat.
• Conversely, poor emitter of radiant heat is also
  a poor absorber of radiant heat.

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Unit 8.4 Radiation
Experiment 8.6

• Objective
• To investigate the emission of infrared radiation
• Apparatus
• Two temperature sensors, data logger, two
  identical tins (one black and one shiny), boiling
  water from two electric kettles
• Procedure
• Connect the temperature sensors A and B to the
  data logger
• Set the sampling rate to ten seconds
• Pour boiling water into both tins at the same
  time until both are filled to the brim.

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Experiment 8.6

• 4. Place the lid and the temperature sensors
  onto the tins. Temperature sensor A will monitor
  the temperature of the black tin, while
  temperature sensor B records the temperature of
  the shiny tin.
• 5. Start recording the temperature. Observe the
  temperature time graph of both sensors.
• 6. Stop recording after ten minutes.

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8.4 Radiation

• Factors affecting rate of infrared radiation
• Colour and texture of the surface
• Dull, black surfaces are good absorbers of
  infrared radiation than shiny, white surfaces
• Dull, black surfaces are better emitters of
  infrared radiation.

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Unit 8.4 Radiation

• Factors affecting rate of infrared radiation
• 2. Surface temperature
• Rate of infrared radiation also depends on
  surface temperature
• The higher the temperature of the surface of the
  object relative to the surrounding temperature,
  the higher the rate of infrared radiation.

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Unit 8.4 Radiation

• Factors affecting rate of infrared radiation
• 3. Surface area
• The larger surface area will emit infrared
  radiation at a higher rate.

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Unit 8.4 Radiation

• Key Ideas
• Radiation is the continual emission of thermal
  energy in the form of infrared waves.
• Radiation is emitted from the surface of all
  bodies and does not require a medium of thermal
  transfer.
• Dull, black surfaces are better emitters of
  infrared radiation than shiny, white surface.
• 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 8.4 Radiation

• Test Yourself
• Give two everyday examples of thermal energy
  transfer by radiation.
• Answer
• Feeling the hotness of Suns radiation Suns
  thermal energy reaches earth by radiation.
• Standing next the a BBQ fire will make you feel
  hot. The thermal energy reaches you by radiation.
• Placing your hand next to a hot object such a jar
  of hot water. Thermal energy reaches your hand by
  radiation.

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Unit 8.4 Radiation

• Test Yourself
• State briefly how thermal energy is transferred
  by radiation.
• Answer
• Hot objects emit thermal energy in the form of
  infrared radiation, which is a type of
  electromagnetic waves.
• The hotter the object, the higher the rate of
  radiation.

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Unit 8.4 Radiation

• Test Yourself
• State three factors that affect the rate of
  transfer of thermal energy by radiation.
• Answer
• Colour and texture of the surface.
• Surface temperature.
• Surface area.

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Unit 8.5 Applications of Thermal Energy Transfer

• Learning Outcomes
• In this section, youll be able to
• Understand and identify how thermal energy is
  transferred by conduction, convection and
  radiation in everyday life.

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of conduction
• Uses of good conductors of heat
• 1. Cooking utensils made of metals eg.
  Stainless steel or aluminium

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of conduction
• Uses of good conductors of heat
• 2. Soldering iron rods the tip is made of copper

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of conduction
• Uses of good conductors of heat
• 3. Heat exchanges.

A heat exchanger transfers thermal energy from
hot dirty water to cold clean water. Copper
tubes are used to aid rapid transfer of thermal
energy from the hot dirty water to the cold clean
water.
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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of conduction
• Uses of bad conductors of heat (insulators)
• 1. Handles of appliances and utensils made of
  plastics or wood

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of conduction
• Uses of bad conductors of heat (insulators)
• 2. Table mats made of cork

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of conduction
• Uses of bad conductors of heat (insulators)
• 3. Sawdust.
• Used to cover ice blocks because of its
  insulating property
• Wooden ladles
• Useful for stirring or scooping hot soup.
• Woolen clothes
• Used to keep body warm

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of conduction
• Uses of bad conductors of heat (insulators)
• 6. Fiberglass, felt and expanded polystyrene foam.

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of convection
• 1. Heating water in Electric kettles the
  heating coil is placed at the bottom to aid the
  heating of water by convection.

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of convection
• 2. Household hot water system the heater is
  located at the bottom of the system.

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of convection
• 3. Air conditioners.
• Air conditioners are installed near to the
  ceiling of rooms to facilitate setting up
  convection currents as cooler air sinks

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of convection
• 4. Refrigerators
• Freezing unit is placed at top to cool the air
  and facilitate the setting up of convection
  currents.

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of radiation
• 1. Teapots
• Shiny teapots can keep tea warm for a longer time
  than black teapots.
• It can also keep cold liquids cool for a longer
  time than black containers.

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of radiation
• 2. Greenhouses infrared radiation emitted by
  the contents in the greenhouse is trapped in the
  greenhouse.

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Unit 8.5 Applications of Thermal Energy Transfer

• Common applications of radiation
• 3. Vacuum flasks

• Stopper is made of poor conductor.

• The vacuum between the double-glass wall
  minimises conduction and convection.

• The glass walls are silvered and highly
  reflective to minimise heat loss due to radiation.

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Unit 8.5 Applications of Thermal Energy Transfer
Worked Example 8.2 The figure shows a typical
vacuum flask designed to keep liquids hot. Part
of the vacuum flask is enlarged. State and
explain the function of each of the parts
labelled A to C Solution A Thin silvering wall
to minimise thermal energy loss by
radiation Since shiny surfaces are poor
absorbers 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 of heat, minimising thermal energy
loss by conduction.
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Unit 8.5 Applications of Thermal Energy Transfer

• Key Ideas
• Some everyday applications of thermal energy
  transfer involving conduction include cooking
  utensils and table mats.
• Some everyday applications of thermal energy
  transfer involving convection include household
  hot water systems and electric kettles.
• Some everyday applications of thermal energy
  transfer involving radiation include vacuum
  flasks and greenhouses.

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Unit 8.5 Applications of Thermal Energy Transfer

• Test Yourself
• 1. A saucepan with a thick copper base contains
  water and is placed on a flat electric hot plate.
• a. State the process by which energy is
• i. transferred from the hot plate to the
  water,
• ii. spread through the water.
• b. The sides of a saucepan are often polished.
  How does this reduce energy loss?
• Answer
• a(i) Conduction.
• Convection.
• b. The sides are polished to reduce heat loss
  due to radiation. Polished and shiny surfaces are
  poor emitters of radiation

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Unit 8 Transfer of Thermal Energy