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Science - Effects of Heat and its Transmission

group 3... lol.
by

Johanne Duroy

on 12 July 2013

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Transcript of Science - Effects of Heat and its Transmission

Effects of Heat and its Transmission
Effects of heat in our daily necessities

Effects of Expansion and Contraction in:
●Riveting
Definitions
SOLID
LIQUID
GAS
Opening a Tight Container
Overhead Electrical Power Cables
due to the heat, these wires expand and thus, there is greater length of power line extending from pole to pole than under lower temperature conditions.
when the temperature is lower (loses heat), the wires contract, causing them to be taut.
Engine Coolant
an increase in temperature (heat gained) in the car's radiator could cause the engine coolant to
expand
until it overflows, if the radiator is "topped off" with coolant on a day where the temperature is low.
as the engine cools down (heat lost) again and
contracts
, the container (used to collect the excess fluid from the volume expansion) returns the excess fluid to the radiator.
Mercury in Thermometers
When the temperature rises (heat gained), the mercury is pushed up, which is constricted by the force of expansion. When the temperature falls (heat lost), the mercury contracts and the column of mercury breaks at the constriction and cannot return to the bulb, thus remaining stationary in the tube.
Rising of Dough
Hot Air Balloon
when the air inside a hot air balloon is heated (heat gained), the hot air balloon will rise because the air inside it becomes less dense than the surrounding air.
this is because thermal expansion or contraction in a gas causes its density to change.
when the air inside is cooled (heat lost), the air's density changes again and the hot air balloon will fall.
● Bridges, gaps in pavements, MRT lines
●Thermostats
Riveting
everyone has had their experience of trying unsuccessfully to open a tight metal lid on a glass container.
the solution to that is simple: run the lid over hot water, and the lid will give way and open fast.
why?
the hot water which has a high temperature causes the lid to expand (heat gained). also, glass has a low coefficient of expansion so we are able to open the lid.
when we bake bread/cake, it rises because of the gases inside the dough which expand.
the gases inside the dough gains heat when placed in the oven, thus they expand.
Bridges
when bridges (concrete and steel) are built, they are given allowance in case they
contract
or
expand
. they also include expansion joints.
when the bridge expands due to the heat of the sun (heat gained), the two sides of the expansion joints move one another;
then, when the bridge contracts (loses heat) at night, they begin to gradually retract.
thus, the bridge is given allowance for "safety zone"; otherwise the bridge would have no room for expansion or contraction due to the temperature change.
Gaps in Pavements
concrete pavements/road surfaces are built with expansion joints or gaps between them.
this is because the concrete pavement/road surface can gain heat or lose heat, depending on the temperature. when there is high temperature, the concrete gains heat and
expands
; when there is low temperature, the concrete loses heat and
contracts
.
if there is no room for expansion, these concrete pavements/road surfaces will crack.
Gaps in MRT lines
on hot days, the MRT lines will gain heat and
expand
.
MRT lines are built with gaps or fitted with sliding joints to allow for small changes in the length of the rails from thermal expansion.
without these gaps or sliding joints, the metal would buckle or develop stress fractures as it expanded and contracted. the rails might push themselves, resulting in a disastrous misalignment.
Railway tracks that have bent
Thermostats
thermostats use a bimetallic strip, which is made of two different metals; one metal expands or contracts more than the other for the same rise in temperature.
that causes it to bend breaking the electrical connection when a certain temperature was reached.
Compressed gas inside the thermostat thermal element expands when the surrounding temperature is above or lower its temperature setting.
This causes a thermal expansion valve to open and hot oil flows to the condenser.
Thermostat
CONDUCTION:
the process of thermal energy transfer without any flow of the material medium.
CONVECTION:
the transfer of thermal energy by means of currents in a fluid (liquids or gases).
RADIATION:
the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.
Radiation
Factors affecting rate of infrared radiation:
1. Colour and texture of the surface
Dull, black surfaces are better absorbers of infrared radiation rather than shiny, white surfaces. Also, dull, black surfaces are better emitters of infrared radiation.
2. Surface temperature
The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.
3. Surface area
The object with the larger surface area will emit infrared radiation at a higher rate.
Applications
- Conduction
- Convection
- Radiation
Conduction
when we place a metal spoon in a cup of coffee, initially the spoon is cool. After awhile it grows warm, and eventually becomes hot. This is because the heat from the coffee travels to the spoon. Evidently, conduction is the process that we can see in this illustration.
Convection
Radiation
The heating coil of an electric kettle is always placed at the bottom of the kettle to aid transfer of thermal energy in water by convection.
When the power is switched on, the water near the heating coil is heated up, expands and becomes less dense.
The heated water therefore rises while the cooler regions in the upper part of the body of water descend to replace the heated water. A convection current is set up.
Greenhouses is used in cold climates to help plants grow better by trapping heat.
During the day, infrared radiation from the Sun passes through the glass roof of the greenhouse. This warms up the soil and plants in the greenhouse.
As the contents in the greenhouse get warm, they start to emit infrared radiation.
The infrared radiation emitted by the contents in the greenhouse is slightly different compared to the infrared radiation emitted by the Sun and is unable to pass through the glass roof.
Therefore, the infrared radiation emitted by the contents in the greenhouse gets trapped. The amount of infrared radiation gets built up over time. This causes the temperature in the greenhouse to increase.
Vacuum Flask
A practical vacuum flask is a bottle made of glass, metal, or plastic with hollow walls; the narrow region between the inner and outer wall is evacuated of air. It can also be considered to be two thin-walled bottles nested one inside the other, and sealed together at their necks.
Using vacuum as an insulator avoids heat transfer by conduction or convection. Radiative heat loss can be minimized by applying a reflective coating to surfaces; silver is common.
The contents of the flask reach thermal equilibrium with the inner wall; the wall is thin, with low thermal capacity, so does not exchange much heat with the contents, affecting their temperature little. At the temperatures for which vacuum flasks are used (usually below the boiling point of water), and with the use of reflective coatings, there is little infrared (radiative) transfer.
The flask must, in practice, have an opening for contents to be added and removed. A vacuum cannot be maintained at the opening; therefore, a stopper made of insulating material must be used, originally cork, later plastics. Inevitably, most heat loss takes place through the stopper.
Metal Tyre and Wooden Wheel
Since the metal tyre is already heated up, it will expand.
The expansion in the tyre enables the metal tyre to be slipped easily over the wooden wheel. However, it would be loose.
Eventually, the metal tyre loses heat to the surroundings and contracts. The contraction would cause the metal tyre to make a tight fit.
Bimetallic strip in Alarm Systems
The bimetallic strip in a fire alarm is made of two different metals with different expansion rates bonded together to form one piece of metal.
The strip is electrically energized with a low-voltage current. When the strip is heated with fire, the high-expansion metal causes the bimetallic strip to bend toward an electric contact.
When the strip touches that contact, an electrical circuit is switched on and triggers the alarm to sound. The width of the gap between the contact determines the temperature that will set off the alarm.
How a circuit looks like
Lava Lamps
The bottles of lava lamps are filled with water and a wax substance that is similar to water.
The base of the lamp has a bulb and a heating element; the bulb gives out light and heat energy. The heating element conducts it and causes the denser wax to increase in volume (the principle of fluids expanding as their particles grow further apart as they are heated)
Hence, with a larger volume, the density of the wax is changed.
The density of the wax is now less dense compared to the water within the bottle and thus, it'll rise to top of the bottle and float. As soon as it makes its way to the top, the wax's temperature begins to drop.
And hence the wax particles, when it reaches the top of the bottle, they are cooler and draw closer to one another. Therefore, the volume of the wax is now smaller, and the density now becomes higher. Thus, becoming much more dense than water.
Experiment
Metals are better heat conductors as compared to non-metals. Metals have electrons in the outer shell of the metal atoms and these electrons are delocalised and are able to conduct the heat energy throughout the metal. When heated, the electrons also vibrate vigorously and bump into the neighbouring metal atoms, conducting heat evenly and the transfer of kinetic energy throughout the metal.
Non-metals are not good conductors of heat and most times are used as insulators as they are unable transfer the heat energy from one atom to another since they do not have free electrons. There are strong covalent bonds between atoms and are only able to vibrate among their fixed positions. Hence, the heat energy is converted and loss as kinetic energy.
Heat energy from the Bunsen burner goes through convection via the medium of fluids, such like air. Heat is also radiated from Bunsen burner to the rods.
The best conductor of heat being “1)” and the worse conductor of heat being “4)” (among the 4 rods of different materials used in this experiment).
1) Aluminium Rod
The aluminium rod is the best heat conductor among the four rods. Since its element, Aluminium (Al) is in the 3rd group of the periodic table, it denotes that its aluminium atoms have 3 electron shells. Hence having 3 electron shells and 3 valence electrons, the forces of attraction between the 3 valence electrons and the protons within the nucleus is smaller than that of Copper (Cu) and Iron (Fe) respectively and hence, more valence electrons are loss as compared to Cu & Fe.
Therefore, Al has more delocalised electrons which would mean the Aluminium rod is able to conduct heat away the fastest. With the heat being able to be conducted from one end to the other the fastest; the moist piece of cobalt chloride paper would be the least moist as compared to those on the other rods.
This is because the Aluminium rod has the most efficient conduction of heat energy (as compared to the other 3 rods) caused the most water molecules on the cobalt chloride to evaporate.
2) Copper Rod
The copper rod is the second best heat conductor among the four rods. Although the element Cu and Fe are both in group 2 and period 4, indicating that both elements have 4 electron shells, the Cu atom has 5 valence electrons to lose while the Fe atom only has 2 valence electrons to lose.
Hence, the Cu rod has more delocalised electrons as compared to that of the Fe rod. Therefore, with more delocalised electrons moving and vibrating vigorously and conducting both kinetic and heat energy, heat is able to be conducted from one end to the other much faster than the Fe rod as the Fe metal has less delocalised electrons to conduct heat energy through the rod.
Thus, the heat energy also caused the cobalt chloride paper to be less moist due to evaporation. However, it is moister than the one on the Aluminium rod.
3) Iron Rod
The iron rod is the 3rd in standard among these 4 rods to conduct heat. The Fe atoms within the Fe rod have the least number of delocalised electrons (2 electrons) to conduct the heat energy throughout the Fe rod itself. Hence, it is less efficient in conducting away heat; and thus less heat energy causes the water molecules in the moist cobalt chloride paper to evaporate.
The piece of cobalt chloride paper on the Fe rod should be the third moistest.
4) Glass Rod
The glass rod has no free electrons to conduct the heat energy. The particles within the glass rod have strong covalent bonds and require a massive force to overcome this. The heat energy is unable to overcome these strong forces of attraction between particles.
Hence, the particles are only able to vibrate among their fixed positions. This then causes the poor conduction of heat and heat energy may not be evenly distributed and may concentrate in a certain point nearer to the Bunsen burner, within the glass rod and eventually cause it to crack and shatter at higher temperatures.
With the poor conduction of heat energy, the cobalt chloride piece of paper would be the moistest because not much heat energy caused the water molecules from the piece of paper to evaporate.
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