PHYSICS NOTES
YEAR 11
Digitized Notes
WEEK 6 AND 7
UNIT 2.3: TRANSFER OF THERMAL ENERGY
Resources/References: - Save my exams.
Cambridge IGCSE Physics Textbook by Pople et al
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OBJECTIVES
• Describe experiments to demonstrate the properties of good thermal conductors and bad thermal conductors (thermal insulators)
• Explain convection in liquids and gases in terms of density changes and describe experiments to illustrate convection.
• Describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission, absorption and reflection of
infrared radiation.
• Describe how the rate of emission of radiation depends on the surface temperature and surface area of an object
• Explain some of the basic everyday applications and consequences of conduction, convection and radiation.
• Explain some of the complex applications and consequences of conduction, convection and radiation where more than one type
of thermal energy transfer is significant
THERMAL TRANSFER
There are 3 modes of Heat Transfer:
1.Conduction
2. Convection
3. Radiation.
UNIT 2.3.1 THERMAL CONDUCTION.
Conduction is the main method of thermal energy transfer in solids
Conduction occurs when:
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Two solids of different temperatures come in contact with one another, thermal energy is transferred from the hotter object to the
cooler object
Metals are the best thermal conductors
This is because they have a high number of free electrons
Conduction: the atoms in a solid vibrate and bump into each other
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Conduction can occur through two mechanisms:
1.Atomic vibrations
2. electron collisions
When a substance is heated, the atoms, or ions, start to move around (vibrate) more
The atoms at the hotter end of the solid will vibrate more than the atoms at the cooler end
As they do so they bump into each other, transferring energy from atom to atom
These collisions transfer internal energy until thermal equilibrium is achieved throughout the substance
This occurs in all solids, metals and non-metals alike.
Thermal Conduction in Liquids & Gases
For thermal conduction to occur the particles need to be close together so that when they vibrate the vibrations are passed along
This does not happen easily in fluids.
In liquids particles are close, but slide past each other.
In gases particles are widely spread apart and will not 'nudge' each other.
Both types of fluid, liquids and gases, are poor conductors of heat.
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Experiments Demonstrating Thermal Conductors
Good thermal conductors are solids which easily transfer heat e.g a metal pan or a ceramic tea cup
Bad thermal conductors (also called insulators) are solids which do not transfer heat well e.g a woolen blanket or layers of
cardboard or paper.
The table below shows some of the good and poor conductors in decreasing order of thermal conductivity.
Good Conductors Poor Conductors
Silver Concrete
Copper Glass
Aluminium Brick
Brass Asbestos paper
Zinc Rubber
Lead Wood
Mercury Water
1.Comparing Conduction in Tiles and Textiles
This demonstration shows why homes use rugs and carpets
• Find a tiled or stone area of floor
o In the same room leave a rug or bath towel (not a thin cloth, it must be thick)
o The textile must stay in place on the floor for several hours to ensure they are at thermal equilibrium (the same
temperature)
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• Stand with bare feet (hands can be used)
o Place one foot on the tiles or stone area, and the other on the textile (towel or rug)
o Observe the apparent temperature of the two materials as felt through the feet
o It will feel as though the tiles are cold while the rug is warm, however, they are at exactly the same temperature
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Energy is transferred by heating from the hotter foot to the cooler tiles by conduction
Explanation
• Tiles and stone are good conductors of heat
o Where the foot touches the tiles, heat is transferred away from the foot, making it feel cold
o The foot has become colder since it lost heat to the tiles
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• Textiles such as rugs are good insulators, meaning they are poor conductors of heat
o Where the foot touches the rug, heat is not transferred away from the foot
o This foot feels relatively warmer than the one which has lost heat to the tiles
o The foot has stayed at its starting temperature
2.Comparing Conduction in Wood and Paper
• A cylindrical rod made of half wood and half metal is wrapped tightly in paper
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• Using a gentle flame, and holding the rod clear of the top of the flame, gently heat the paper at the join of the wood and metal
o Turn the rod so that the paper is well-heated all around the circumference of the rod
o Stop when the paper is clearly discoloured
• Remove the rod from the flame, gently unwrap the paper and observe the burn pattern
o A distinct pattern is seen;
▪ Where the paper touched the metal surface it is undamaged
▪ Where the paper touched the wood surface it is charred
Explanation
• Metal is a good conductor of heat
o Where the paper touched the metal, heat was transferred from the paper into the metal and along the length of the metal
o This prevented the paper getting hot
• Wood is a good insulator, meaning it is a poor conductor of heat
o Where the paper touched the wood, heat was not transferred from the paper
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o This meant that the paper did get hot enough to start to burn
3.Demonstrating Different Rates of Thermal Conduction in Metals
• A simple experiment to demonstrate the relative conducting properties of different materials can be carried out using apparatus
similar to that shown in the diagram below
The above apparatus consists of 4 different metal strips of equal width and length arrange around an insulated circle
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• Ball bearings can be stuck to each of the strips and equal distance from the centre, using a small amount of wax
• The strips should then be turned upside down and the centre heated gently using a candle, so that each of the strips is heated at
the point where they meet
• When the heat is conducted along to the ball bearing, the wax will melt and the ball bearing will drop
• By timing how long this takes for each of the strips, their relative thermal conductivities can be determined.
Observation
The ball bearing in copper will fall first, followed by aluminium, brass and iron in that order. Showing that Copper is the best
thermal conductor.
Applications of good and poor conductors
1. Cooking utensils, soldering irons and boilers are made of metals which conduct heat rapidly. For cooking utensils, the handles
are made of insulators such as wood or plastic. Metal pipes carrying hot water from boilers are lagged with cloth soaked in a
plaster of paris to prevent heat losses.
2. Overheating of integrated circuits (ICs) and transistors in electronic devices can drastically affect their performance such
components are fixed to a heat sink (a metal plate with fins) to conduct away undesired heat. The fins increase the surface area
of heat sink and conduct more heat away to the surrounding.
3. Fire fighters put on suits made of asbestos material to keep them safe while putting out fire.
4. Birds flap their wings after getting wet as a means of introducing air pockets in their feathers. Air being a poor conductor
reduces heat loss from their bodies.
5. In modern buildings where desired inside temperatures is to be stabilised, double walls are constructed. Materials that are good
insulators of heat and can trap air put between the walls. Examples of such materials are glass, wool (fibre glass) and foam
plastic. Air on its own may not effectively give the desired insulation because it undergoes convection. Double glazed windows
used for the same purpose have air trapped between two glass sheets.
6. In experiment involving heating water or liquid, the beaker is placed on the wire gauze. The gauze is heated and spreads the
heat to a large area of the beaker. If the gauze is not used, heat from the Bunsen burner may concentrate on a small area and may
make the beaker crack.
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UNIT 2.3.2 CONVECTION
Convection is the main way that heat travels through liquids and gases
Convection only occurs in fluids.
Convection cannot happen in solids.
Descriptions of convection currents always need to refer to changes in temperature causing changes in density.
The temperature may fall or rise, both can create a convection current.
When a liquid (or gas) is heated (for example by a radiator near the floor):
The molecules push each other apart, making the liquid/gas expand
This makes the hot liquid/gas less dense than the surroundings.
The hot liquid/gas rises, and the cooler (surrounding) liquid/gas moves in to take its place.
Eventually the hot liquid/gas cools, contracts and sinks back down again.
The resulting motion is called a convection current.
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When a liquid or gas is heated, it becomes less dense and rises
When a liquid (or gas) is cooled (for example by an A.C. unit high up on a wall):
The molecules move together, making the liquid/gas contract
This makes the hot liquid/gas more dense than the surroundings
The cold liquid/gas falls, so that warmer liquid or gas can move into the space created.
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The warmer liquid or gas gets cooled and also contracts and falls down.
The resulting motion is called a convection current.
Demonstrating Convection Currents
• A simple demonstration of convection in liquids involves taking a beaker of water and placing a few crystals of potassium
permanganate in it, to one side, as shown in the diagram above
• When the water is heated at that side, the potassium permanganate will dissolve in the heated water and rise along with the
warmed water, revealing the convection current
Diagram showing an experiment with potassium permanganate to demonstrate convection
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Application of Convection in Fluids
1. Domestic hot water system
Initially, the two beakers A and B have cold water. Water in beaker A is coloured to distinguish it from that in beaker B. When the water
in beaker A is heated, it is observed to rise up through tube X and emerges on top of cold water in beaker B. The cold water flows down
from beaker B to beaker A.
As long as heating continues, there will be movement of hot water into beaker B and cold water will flow down into beaker A.
Thermometer will show increase in temperature for water in beaker B.
The commercial domestic hot water system utilises the same principle of operation. The hot water rises up because of the effective
lowering of density.
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The force of gravity helps the cold water to flow down from the cold tank.
The hot water tap and expansion pipe are connected to the upper region of the cylinder. The expansion pipe is an outlet for excess
water that could have resulted from overheating.
Once the cold water flows down the cylinder, the main pipe allows more cold water to flow into the tank. When filled to capacity, the
ball cork floating on water closes a valve in the main pipe, stopping further in flow of cold water.
An overflow pipe lets out water from the cold tank when the valve is not sufficiently functional.
Lagging is done on the pipe that conveys hot water to minimise heat losses.
2. VENTILATION
This is the supply of fresh air into the room. Air expelled by the room occupants is warm and less dense. It rises up and escapes through
the ventilation holes.
Cold fresh air flows into the room to replace the rising warm air. The room gets continuous flow of fresh air.
NOTE: Some devices are fitted with air conditioning devices which cause forced convection of air, giving out cold dry air and absorbing
warm moist air.
3. Car Engine Cooling System
Heat conduction and convection play a very crucial role of taking away heat from a car engine that would reduce its efficiency.
The engine is surrounded by a metal water jacket that is connected to the radiator. The metal surface conducts heat away from the
engine. This heats up the water, setting up convection currents. The hot water is pumped into the radiator which has thin copper fins
that conduct away heat from water.
Fast flowing air past the fins speeds up the cooling process.
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4. Land and Sea Breezes
This is a natural convection of air, and occurs at sea shores because of temperature difference between the mass of water and the land.
The mass of water takes longer time than land nearby land by the same temperature from the sun. Water also takes a longer time to
cool than the land after being raised at the same temperature.
During the day, the land heats up much faster than the sea. The air just above the land gets heated up and rises because of reduced
density. Cold air above the sea blows towards the land to replace the void created by warm air rising. This is called sea breeze.
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In the evening, temperature of the sea water is higher than that of the land. The air above the sea gets heated up and rises. Cold air from
the land blows to the sea. This is called land breeze.
UNIT 2.3.3.THERMAL RADIATION
All objects give off thermal radiation
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The hotter an object is, the more thermal radiation it emits
Thermal radiation is the part of the electromagnetic spectrum called infrared
Thermal radiation is the only way in which heat can travel through a vacuum
It is the way in which heat reaches us from the Sun through the vacuum of space
The colour of an object affects how good it is at emitting and absorbing thermal radiation:
Thermal Equilibrium
As an object absorbs thermal radiation it will become hotter
As it gets hotter it will also emit more thermal radiation
The temperature of a body increases when the body absorbs radiation faster than it emits radiation
Eventually, an object will reach a point of constant temperature where it is absorbing radiation at the same rate as it
is emitting radiation
At this point, the object will be in thermal equilibrium
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An object will remain at a constant temperature if it absorbs heat at the same rate as it loses heat
• If the rate at which an object receives energy is less than the rate at which it transfers energy away then the object will cool
down
• If the rate at which an object transfers energy away is less than the rate at which it receives energy then the object will heat up
• The process will always move towards thermal equilibrium
Effects of Different Surfaces
• The amount of thermal radiation emitted by an object depends on a number of factors:
1.The surface colour of the object (black = more radiation)
2.The texture of the surface (shiny surfaces = more radiation)
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3.The surface area of the object (greater surface area = more area for radiation to be emitted from)
Black objects are very good at absorbing thermal radiation, for example black clothes make you feel hotter in sunny weather.
Black objects are also very good at emitting thermal radiation, which is the reason that chargers for laptops, and radiators in cars are
coloured black - it helps them to cool down
Shiny objects reflect thermal radiation and so absorb very little
They also emit very little, though, and so take longer to cool down
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An image of a hot object taken in both Infrared and visible light. The black surface emits more thermal radiation (infrared) than
the shiny surface
Applications of radiation
1.Kettles, cooking pan and iron boxes have polished surfaces to reduce heat lose through radiation.
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2.Clouds reflect radiation back to the earth hence cloudy nights are warmer than clear nights
3.Petrol tanks are painted silvery bright to reflect away as much heat as possible.
4.Houses in hot areas have their walls and roofs painted with bright colours to reflect away heat, while those in cold areas have walls
and roofs painted with dull colours.
4.In solar concentrators, the electromagnetic waves in form of radiant heat are reflected to a common point (focus) by a concave
reflector. The temperature at this point can be sufficiently high to boil water as shown
4.The green house effect
A green house has a glass roof through which radiant heat energy from the sun passes. This heat is absorbed by objects in the house,
which then emit radiation of lower energy that cannot penetrate through glass.
The cumulative effect is that temperature of the houses increases substantially. Greenhouses are used in providing appropriate
conditions for plants in cold regions.
NOTE: Carbon dioxide (CO2) and other air pollutants in the lower layers of the atmosphere show the same properties of glass, raising
the temperature on earth to dangerous levels.
5.Solar heater
The solar heater uses solar energy to heat water. The figure below shows the solar heater.
The solar heater consists of a coiled blackened copper pipe on an insulating surface. Radiant heat from the sun passes through glass and
is absorbed by black copper pipes that contain water, which is heated up.
Copper pipes are used because they are good conductors and they are painted black to increase their absorbing power. Lower energy
emitted after absorption of radiant energy does not escape because it cannot penetrate the glass. The temperature of the air above the
pipe thus increases boosting the heating of water. A good insulating material is used at the base.
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6.Thermos flask( vacuum flask)
A thermos flask is designed such that heat transfer by conduction, convection and radiation between the contents of the flask and its
surrounding is reduced to a minimum.
The vacuum is a double walled glass vessel with a vacuum in the space between the walls. This minimises the transfer of heat by
conduction and convection.
The inside of glass walls, in the vacuum side, is silvered to reduce heat losses by radiation (Poor emitter and absorber).
The felt pads on the sides and at the bottom support the vessel vertically. The heat loss by evaporation from the liquid surface is
prevented by a well fitting cork.
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