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Transfer of Thermal Energy
Transfer of Thermal Energy
Thermal Energy Transfer
Conduction
Convection
Radiation
Transfer of Thermal Energy
Thermal Energy Transfer
Conduction
Convection
Radiation
Lesson Objectives
Understand that thermal energy is transferred from a
region of higher temperature to a region of lower
temperature.
Thermal Energy
If a hot object is placed next to a cold object, will both
objects remain at their temperatures?
Why?
Will the transfer of thermal energy between the two
bodies end?
If so, when will it end?
800oC20oC
Thermal Energy Transfer
Thermal energy is transferred when there is a
difference in temperature.
Thermal energy always flows from a region of higher
temperature to a region of lower temperature.
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.
Misconception!
Thermal energy ≠ Heat.
Heat is defined as energy in transit from a high
temperature body to a lower temperature body.
Amount of thermal energy transferred will be the
amount of heat that flowed.
Processes of Thermal Energy Transfer
When you stand
near to the pit,
you feel the heat.
Why?
When you put your
hand above the pit,
you feel the heat.
Why?
When you hold the
skewer with your hand,
you feel the heat. Why?
Processes of Thermal Energy Transfer
Thermal energy is transferred by:
Conduction
Convection
Radiation
Transfer of Thermal Energy
Thermal Energy Transfer
Conduction
Convection
Radiation
Lesson Objectives
Define conduction.
Describe some uses of conductors and insulators.
Conduction
Conduction is the process of thermal energy transfer
without any flow of the material medium.
Video Time!
Conduction
All matter are made up of tiny particles.
When thermal energy is supplied to one end of a rod,
the particles gain energy and vibrate more vigorously.
These particles collide with neighbouring particles,
making them vibrate more as well.
Thermal energy is transferred from the hot end to the
cold end.
There is no net movement of particles during the
process of conduction.
Conduction in Solids
Different materials conduct heat at different rates.
Those that conduct faster are called good conductors
and those slower are called poor conductors.
Poor conductors are also known as insulators.
Most metals are good conductors of heat.
Materials such as wood, rubber, asbestos, plastic and
glass are poor conductors of heat.
Touch Test
At room temperature, certain objects feel warmer to
the touch than others.
This is because some objects are better conductors of
heat than others.
Both the carpet and the marble
floor are at the same temperature.
The carpet is a bad conductor
of heat. It feels warm as it
does not conduct heat away
easily from the body.
Normal body temperature is about 37
°C, while room temperature is about 30
°C. Heat is conducted away from the body when it is in contact with cooler
objects in the room.
The marble floor is a good
conductor, so it is cool to the touch.
Conduction in Liquids and Gases
Thermal energy can be conducted from a hotter to a
cooler region.
Liquid particles are further apart and collisions of
particles are less frequent and even lesser in gases.
Thus, transfer of energy from particles to
neighbouring particles is slower.
Hence, air is poor conductor of heat compared to
water, which in turn, is a poor conductor of heat
compared to most solids.
Video Time!
Applications of Conduction
Transfer of Thermal Energy
Thermal Energy Transfer
Conduction
Convection
Radiation
Lesson Objectives
Describe how energy transfer occurs in fluids via
density changes during the process of convection.
Think About It…
Liquids and gases are poor conductors of heat.
How does thermal energy gets transferred in liquids
and gases?
Convection
Convection is the transfer of thermal energy by means
of currents in a fluid.
A fluid can be a liquid or gas.
Video Time!
Convection
When fluids are heated, they expand and become less
dense than the surrounding fluids.
The less dense fluids tend to rise from the heating
source.
The 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.
Put On Your Thinking Cap!
Why do the fluids become less dense when they
expand?
When they expand, volume increases.
Mass remains the same.
Given that density = mass / volume, density decreases.
Hence, they become less dense and rises.
Put On Your Thinking Cap!
Why does convection occurs only in liquids and gases,
but not solids?
Liquid and gas particles are able to move about.
Liquids and gases can flow.
Solid particles are held in fixed positions.
Solids cannot flow.
Convection
Convection involves the bulk movement of the fluids
which carry with them thermal energy.
Applications of Convection
Heating water in electric kettles: the heating coil is
placed at the bottom to aid the heating of water by
convection.
Applications of Convection
Household hot water system: the heater is located at
the bottom of the system.
Applications of Convection
Air-conditioners: they are installed near to the ceiling
of rooms to facilitate setting up convection currents as
cooler air sinks.
Applications of Convection
Refrigerators: freezing unit is placed at the top to cool
the air and facilitate the setting up of convection
currents.
Transfer of Thermal Energy
Thermal Energy Transfer
Conduction
Convection
Radiation
Lesson Objectives
Explain energy transfer of a body by radiation.
State the factors affecting the rate of energy transfer by
radiation.
Radiation
Radiation is the transfer of thermal energy in the form
of waves such as infrared radiation without the aid of a
medium.
It can take place in vacuum.
Example of Radiation
The Sun emits infrared radiation.
Thermal energy from infrared radiation makes us feel
warm.
All objects emit infrared radiation.
The hotter the object, the greater the rate of infrared
radiation emitted.
Emission of Infrared Radiation
Infrared radiation is emitted by all objects and surfaces.
This emission causes the temperature of the objects to
fall.
Absorption of Infrared Radiation
Infrared radiation is absorbed by all objects and
surfaces.
The absorption of infrared radiation causes a
temperature rise.
Emission and Absorption
In general, a good emitter of infrared radiation is also a
good absorber of infrared radiation.
Conversely, a poor emitter of infrared radiation is also a
poor absorber of infrared radiation.
Factors Affecting Rate of Radiation
Colour and texture of the surface:
Dull, black surfaces are better emitters of infrared
radiation than shiny, silver surfaces.
Dull, black surfaces are better absorbers of infrared
radiation than shiny, silver surfaces.
Factors Affecting Rate of Radiation
Surface temperature:
The higher the temperature of the surface of an object
relative to the surrounding temperature, the higher the
rate of emission of infrared radiation.
Factors Affecting Rate of Radiation
Surface area:
The larger surface area will emit or absorb infrared
radiation at a higher rate.
Who is Hotter?
Who is Cooler?
Applications of Radiation
Teapots: shiny teapots can keep tea warm for a longer
period of time as compared to black teapots. It can also
keep cold liquids cool for a longer time than black
containers.
Applications of Radiation
Greenhouses: infrared radiation emitted by the
contents in the greenhouse is trapped in the
greenhouse.
Vacuum Flask
Heat loss is minimized in four possible ways:
Conduction
Convection
Radiation
Evaporation
Vacuum Flask
Plastic stopper reduces heat transfer by conduction,
convection and evaporation.
Air is trapped in the vacuum flask. Air is also a poor
conductor of heat.
The vacuum between the double-walled glass
minimises conduction and convection.
The glass walls are silvered and highly reflective to
minimise heat transfer due to radiation.
Cork is a poor conductor of heat.
Spacesuit
Living conditions in space are very harsh. Not only is
there no air to breathe, but temperatures can be very
high in some places and very low in others.
Astronauts on space shuttle missions usually have to
work outside their shuttle. Thus, it is important that the
astronauts’ spacesuits are able to keep temperature at a
comfortable level.
Layers of insulating material
These layers are made of a special
insulating material, which prevents
heat from conducting into or out
of the spacesuit. Air trapped
between these layers also prevents
the conduction of heat.
A white outer layer
Spacesuits are coloured
white in order to
reflect radiation from
the Sun. This prevents
the astronaut from
absorbing too much
radiation that heats up
the body.
Helmet
This is made of a special insulating
material to prevent heat transfer
through conduction.
Concept Map
Transfer of
Thermal Energy
Conduction Convection Radiation
Vibration and
collision of
particles; no flow
of medium
Fluid medium flows
in currents due to
density changes
Transfer of infrared
waves; no medium
needed
Affected by:
Surface areaSurface temperatureSurface colour & texture
