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Transfer of heat energy and its effects, Applications of conduction -…
Transfer of heat energy and its effects
What does a change in temperature indicate
Temperature
Is a measure of how hot an object is
Is measured using a thermometer
The SI unit for temperature is kelvin
When an object is heated
The particles in it will move faster
(kinetic energy increases)
The higher the temperature, the faster the particles move
When two objects of different temperatures are in contact
The hotter object will transfer energy to the colder object
The energy being transferred is called heat
The hotter object gives out heat energy and its temperature decreases
Heat energy is transferred from the water in the basin to thee water in the beaker as indicated by the arrows. the temperature of the water in the beaker increases
The colder object absorbs heat energy and its temperature increases
Heat energy is transferred from the water to the beaker to the ice cubes as indicated by the arrows, the temperature of the water decreases
The temperature change will continue until both objects are at the same temperature`
What effects does the transfer of heat energy have on an object
Expansion and contraction in solids
On very hot days. Rail tracks will buckle as there is no space for them to expand
Expansion gaps in the tracks provide space for the tracks to expand on hot days
Bridges will expand in hot weather and contract and allowance is needed to make sure the structure remains intact
Bridges have a free end resting on rollers
This allows the metal structures to expand without causing any damage on hot days
Expansion in liquids
Heat trapped in the earth's atmosphere caused global warming
As the oceans absorb majority of this heat, it becomes warmer and expands
This results in sea level rising and caused erosion and flooding to coastal areas
To reduce global warming, we have to reduce the production of greenhouse gases such carbon dioxide
This can be done by using public transport instead of driving, plant trees, switch to energy efficient appliances and conserve electricity
Pipes that carry liquids or gases will expand or contract due to weather or the temperature of the substance they carry
The contraction or expansion can cause the pipes to burst
Pipes are built with flexible bends that allow the pipes to expand or contract
application of expansion and contraction
The effects of expansion and contraction due to the transfer of heat energy can be applied in daily life
Placing the lid of a jar in hot water can make it easier to unscrew the lid
The metal expands more than the glass jar
This makes the lid less tight and hence easier to unscrew
Bimetallic strip
A bimetallic strip consists of two different metals that expands at different rates when heated
As a result, the strip will bend when cooled or heated
When heated, brass contracts more than the iron.
Hence the heated strip bends with the brass on the outside of the curve
When cooled, the brass contracts more than the iron.
Hence, the strip bends with the iron on the outside of the curve
application of bimetallic strips
Thermostat
A thermostat is a device that is used to keep The temperature of appliances constant. Examples of appliances with thermostat are electric iron, refrigerators and ovens
iron
When the iron is first switched on, the bimetallic strip is straight and touching the contact This closes the circuit and enables the electricity to flow and the iron stops heating up
When the set temperature is reached, the bimetallic strip will bend upwards and away from the contact. The circuit is broken, electricity stops flowing and the iron stops heating up.
When the iron cools down, the bimetallic strip straightens, closes the contact and the iron heats up again. This process repeats and keeps the iron at a constant temperature
Bimetallic thermometer
A bimetallic strip is made into a long coil in a bimetallic thermometer.
One end is fixed and the other end is attached to a pointer
The coil unwinds (expands) as the temperature increases and tightens (contracts) as the temperature decreases
The pointer mover over a scale that marks the different temperatures
Liquid in glass thermometer
A liquid in glass thermometer is a narrow glass tube sealed at both ends with a bulb at one end
The bulb is partially filled with a liquid such as mercury or alcohol
When the temperature around the bulb increases, The liquid in the bulb heats up, expands and rises up the narrow glass tube
When temperature decreases, the liquid in the light bulb cools, contracts and falls back into the bulb
Hot air balloon
When air in a hot air balloon is heated, its volume increases
the mass of the air remains constant as the number of particles of air does not change
Since the mass of air in the balloon remains constant while its volume increases, the density of air decreases
The hotter and less dense air rises, causing the hot air balloon to float
How does the transfer of Heat Energy occur?
Heat energy can be transferred from one place to another in 3 ways:
Convection
Convection is the main heat transfer process in liquids and gases.
In convection, heat is transmitted by movement of the medium.
Convection takes place due to changes in density when gas or liquid is heated or cooled.
when liquid is heated, its volume increases. Since density is mass/volume and the mass of the liquid is constant, its density decreases. The hot liquid will rise as it is less dense while the cooler and denser liquid at the top sinks to take its place.
Conduction
Conduction is the process by which heat energy is transferred from one particle to another without the physical movement of the medium
when an object is heated, particles in the object gain energy from the heat source and vibrate faster.
They collide with their neighbouring particles and transfer some of their energy to them. Hence causing them to vibrate faster..
The process continues until the colder end of an object.
Radiation
Radiation is the transfer of heat, in the form of infrared waves, from one place to another without the need of a medium.
Radiation is the only method of heat transfer that can take place in the vacuum. This is how heat from the sun reaches the earth. The space between the sun and earth is a vacuum.
The surface of the earth and its atmosphere can absorb or reflect the heat radiated from the sun. Greenhouse gases such as carbon dioxide will retain the heat energy and keeps the atmosphere warm. This is known as the greenhouse effect. When there is too much greenhouse gases, the atmosphere will get too warm and this is known as global warming
Factors affecting the rate of radiation
Surface area of object
The larger the surface area of the object, the higher the rate of radiation
The cooler coil of a refrigerator is coiled to increase its surface area so that heat can be emitted faster
type of surface
A darker and rougher the surface, the higher the rate of absorption
A dark and rough surface is a good conductor of heat. While a white and shiny surface is a poor radiator and absorber of heat
Temperature of object
The hotter the object, the higher the rate of radiation from the object
A cup of water at 100 degrees radiates heat faster than a cup of water at 40 degrees
Emitter/ Radiator of heat
An object that is hotter than its surrounding will emit radiation to cool down
An object cannot act as both an emitter and an absorber at the same time
Absorber of heat
An object that is cooler than its surrounding will absorb heat from its surrounding to heat up
good absorber of heat/
good emitter(radiator of heat)
dark colour/ dull; matt; rough surface/ large surface area
poor absorber of heat/
poor emitter (radiator) of heat
light colour/ shiny; polished; smooth surface/ small surface area
Applications of convection
Air conditioner is placed near the ceiling so that the room can be cooled more effectively
The cold air from the air conditioner will sink as it is denser.
The hot air that is less dense rises and will be cooled by the air conditioner
The process repeats and forms a convection current to cool the room
In cold countries, a heater is used to warm a room.
It is usually installed close to the ground.
Cool air at the bottom of aa room gets warmed by the heater and rises towards the ceiling
This forms convection currents in the air, which warms up the entire room
A heating element is placed at the bottom of a kettle
The water at the bottom of the kettle heats up
The density of the water decreases and the heated water rises
The cooler water sinks and in turn gets heated
A convection current is set up and eventually bring the water to a boil
Sea breeze and land breeze
Day
During the day the land is hotter than the sea
Hot air rises from the land and a cool sea breeze blows in to take its place
The land absorbs heat from the sun and warms up faster than the sea
The air above the land is heated up and rises
Cooler, denser air above the sea moves in to replace the warm air above the land that has risen
night
At night, the land cools faster while the sea retains its heat
Air above the sea rises
A land breeze blows out to sea to take its place
The land loses heat faster than the sea and cools down faster
Air above the sea is now warmer and rises
Cooler, denser air above the land moves in to replace the warm air above the sea
Caused by convection
Application of radiation
poor absorbers of heat are used when we do not want the object to absorb too much heat
Buildings in tropical countries are painted white. White surfaces are poor absorbers of heat and it reduces the rate of heat being absorbed by the building by radiation thus keeping it cool. Another example will be clothing
poor emitters of heat are used when we do not want the object to emit too much light
Shiny and polished surface of metal pots reduces the rate of heat loss by radiation and keep food inside warmer. Another example will be emergency blanket
Good absorbers of heat are used when heat needs to be absorbed quickly
The black pipes on solar water heaters are good absorbers of radiation from the sun. It heats up the water flowing in the pipes
Good emitters of heat are used when an object needs to emit heat fast
The cooling fins ( the pipes at the back of refrigerator) of a refrigerator help to give out heat from the refrigerator to the surrounding. It is painted black because black surface is a good emitter of heat. This helps the pipes to lose heat quickly to the surroundings by radiation.
The vacuum flask reduces heat transfer by conduction convection and radiation. When it is holding hot food, it reduces heat loss from the food. When it is holding cold food, it reduces heat gain by the food. Hence it can be used to keep hot food hot and cold food cold
the support prevents the inner flask from having contact with the outer case so less heat is conducted out of the container. It is made of insulator such as styrofoam
The mouth of the flask is stoppered using a poor conductor of heat to reduce heat loss by conduction. The stopper also prevents heat loss through convection into the surroundings
The walls are painted silver on the vacuum side so heat loss by radiation is reduced
The vacuum is created between the glass walls by removing air. A vacuum does not allow heat transfer by conduction and convection
The cap is usually made of a poor conductor of heat. such as plastic to reduce heat loss by conduction
Applications of conduction
conductors are used in applications where heat needs to be conducted fast through the material.
Cooking utensils are made with good conductors like metal so that heat can be conducted from the heat source to the food fast
Good insulators are used in applications where heat needs to be conducted slowly through the material.
Handle of cooking pots are made of insulators like plastic so that heat from the pot will be conducted slowly to the hand holding it so that the user does not get burnt
Walls of buildings are made of insulating materials such as bricks to
Reduce the rate of heat conducted out of the house in winter, thus keeping the house warm
Reduce the rate of heat conducted into the house in summer, thus keeping the house cool
Wool is used to make winter clothing as it traps air and air is a good insulator of heat. This reduces the rate of heat conducted out the body