conduction,convection and radiation

Part 3! Radiation

What is RADIATION?????

-Radiation is the process of heat transfer by electromagnetic waves

-Radiation is the only form of heat that can travel through a vacuum.

-The sun shines and warms our body. The energy is carried through the vast space between the sun and the earth by radiation. Unlike conduction and convection, radiation does not need a medium to transfer energy; the energy is carried by electromagnetic (EM) waves. Light is a kind of EM waves that is visible to naked eyes.

-A hot object gives out energy by radiation in the form of EM waves. Objects on fire give out visible light because of their high temperatures. Objects at normal temperature range give out mainly infrared radiation that is detected by our skin as warmth. Place your hand near the side, but without touching, a glass of hot water and you will feel what I mean. If the emitted radiation is absorbed by a colder object, the colder object will gain internal energy and rise in temperature.

Heat from the sun reaches us by radiation.

Absorption and emission of radiation

-The higher the temperature of the object, the higher the amount and more energetic the radiation it emits. A cool object still radiates energy but less than a hot object.Consider your hands as being warmed by the infrared radiation from a lamp for example. Though your hands also radiate energy, the lamp radiates energy to your hands at a much higher rate and thus your skin has a net gain in energy and an increase in temperature.

-The appearance of a surface determines its rates of emission and absorption of radiation. In general, shiny coloured (e.g. white or silvery) and metallic surfaces emit or absorb radiation energy slowly since they reflect the radiation. Dark coloured (e.g. black) surfaces emit or absorb radiation energy more effectively. A good emitter of radiation is also a good absorber of radiation and vice versa.

-Now you can see how people use different colours to enhance or reduce heat gain. People usually wear light-coloured clothing in hot summer to reduce radiation heat gain from the sun, and dark coloured clothing to increase heat gain in cold winter. Solar panels are usually black in colour to increase radiation gain.

The surface of this balloon is silvery white in colour. This can help to reduce radiation energy absorption.

The Hubble Space Telescope is covered by a layer of silvery material.

REMEMBER!:D

Shiny or white surfaces do not radiate heat well which is why water in a shiny jug will stay warm longer than one in a black jug.
Dark or dull surfaces absorb heat radiation much more easily than white shiny surfaces.This is why in hot countries the houses and clothing are often white because white reflects the heat better.

The vacuum flask has a double glass shell and a vacuum in between. The glass shell is coated with silvery reflective material. The shell is protected by an outer casing, usually made of plastic or metal.
The vacuum prevents any energy transfer through conduction and convection. The silver coating reflects much of the radiation and thus radiation energy transfer is also minimized. So the contents inside a vacuum flask can be kept at a more or less constant temperature for a long time

-A vacuum flask has a vacuum in a sealed double-walled container. This is to reduce heat loss or heat gain by conduction and convection.

-The walls of the double-walled container are coated with aluminium. This is designed to reduce heat loss or heat gained by radiation.

-The stopper is usually made from plastic or cork. If hot liquid is placed in the container a stopper will reduce heat losses by conduction and convection.

-Modern vacuum flasks are made from double-walled steel containers instead of double-walled glass containers. The main reason for this change is to reduce damage by impact.

Refrigerator door

A refrigerator has many features which lower conduction, radiation and convection energy transfer to reduce the consumption of electrical energy.
Refrigerators usually have a light coloured outer surface to reflect radiation, and so reduce heat entering the refrigeration compartment. The edges of a refrigerator's doors of the refrigerator have flexible seals to prevent cold air inside from mixing with hot air outside, thus reducing convection. The flexible seals are made of good insulating material to further reduce energy transfer through conduction. The thick walls and doors of the refrigerator are also well-insulated to reduce heat transfer through conduction.

Part 2.1! Convection

When the stove is first turned on heat is transferred first by conduction between the element through the bottom of the pot to the water. However, eventually the water starts bubbling - these bubbles are actually local regions of hot water rising to the surface, thereby transferring heat from the hot water at the bottom to the cooler water at the top by convection. At the same time, the cooler, more dense water at the top will sink to the bottom, where it is subsequently heated.

Hot-air balloons use convection in order to rise into the air. The air inside the balloon is heated. As the hot air rises, so does the balloon. For the balloon to descend, the air in the balloon is cooled or allowed to escape.

LAND BREEZE AND SEA BREEZE.

In a convection cycle hot air rises and cool air falls, the freezer is at the top so that air cooled by it falls down the sections of a refrigerator and this type of design helps maintain a lower temperature at the top (for ice creams and ice cubes) as compared to that at the bottom (vegetable) section.

Part 2! CONVECTION

Have ever you had a hot pot meal in the winter? You may notice that the food keeps moving up and down in the water even when the temperature is well below that of boiling. This indicates that bulk movement of water exists when it is being heated. In fact, this movement of water, known as convection, serves as an effective way of heat transfer, especially when the rate of heating is high.

WHAT IS CONVECTION?
-Convection is the transfer of thermal energy by means of currents in a fluid( liguids or gases)

-When a fluid is heated, the particles in the volume of the fluid that is heated most (closest to the heat source) becomes more energetic and move faster and move apart from each other, lowering the density. The hotter and less dense part of the fluid thus rises. On the other hand, cooler and denser fluid sinks. As a result, heat is transferred upwards by the rising fluid. The rising of the hotter portion and the sinking of the cooler portion of the fluid form a convection current.






When a beaker of water is heated from below, a convection current is set up.






Convection of water observed with a dissolving crystal





Convection of water observed in a transparent pot. Tiny food particles are seen to move up and down.





The convection of air drives the smoke of a burning candle upwards.




-Convection does not take place in solids; it only takes place in fluids.
This is because convection involves the bulk movement of the fluids which carry thermal energy with them. For solids, the thermal energy is transferred from one particle to another through vibrations, without any bulk movement of the particles themselves.

Part 1! CONDUCTION.

What is conduction??

-Conduction is the process of thermal enerygy transfer by the direct contact of molecules,without any flow of the material medium.

-Conduction takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other.

-Conduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object. As they collide, the faster molecules give up some of their energy to the slower molecules. The slower molecules gain more thermal energy and collide with other molecules in the cooler object. This process continues until heat energy from the warmer object spreads throughout the cooler object. Some substances conduct heat more easily than others. Solids are better conductor than liquids and liquids are better conductor than gases. Metals are very good conductors of heat, while air is very poor conductor of heat. You experience heat transfer by conduction whenever you touch something that is hotter or colder than your skin e.g. when you wash your hands in warm or cold water





This is a thermal infrared image of a coffee cup filled with a hot liquid. Notice the rings of color showing heat traveling from the hot liquid through the metal cup. You can see this in the metal spoon as well. This is an example of conduction.
The spoon in a cup of hot coffee becomes warmer because the heat from the coffee is conducted along the spoon.

Heat being conducted along a copper rod.The heat makes the copper atoms vibrate faster. These atoms in turn make the atoms near them vibrate faster.In this way the heat energy is gradually transferred along the rod from the hot end towards the cooler end.
We can demonstrate this by coating the rod in wax and timing how long it takes the wax to melt.With a copper rod the wax will melt quicker than an iron rod because copper is a better conductor of heat.

Good conductors of heat:

-silver
-copper
-gold
-aluminium
-iron
-lead

Good insulators of heat:

-vacuum
-styrofoam
-air
-wood
-glass
-water

One ice cube is placed on a wood board, a second one on a black sheet of aluminium. Both bases are black and have the same shape. On Aluminium, the ice melts much quicker because Aluminium has an excellent heat conductivity.At room temperature the ice on the aluminium sheet melts much quicker than the one on wood.

This experiment demonstrates that water is a poor conductor of heat. The ice at the lower part of the test tube does not melt quickly even when the water boils at the top.


Applications of good conductors of heat:

Good conductors of heat are used in heating processes to increase the rate of energy transfer. They are also used in cooling systems to facilitate heat loss. Metals are good conductors of heat and can normally endure strong heating. They are therefore used in cooking utensils, such as pans and woks. Metals are widely used in the cooling systems of cars and power plants to enhance heat loss.

Cooling fan (left) and heat sink (right) of a computer CPU.

Applications of good insulators of heat:
Good insulators of heat are used for energy retention, or for keeping a place cold against a warm environment. Materials such as plastics and polystyrene are good insulators of heat. The handles of many cooking utensils are made of plastics for safe handling. Expanded polystyrene cups and boxes are used in fast food shops to hold hot food or drinks. Materials such as fur, cotton and feathers are often used to make clothing and quilts. We wear lightweight goose-down jackets in cold weather. Cotton quilts keep us warm at night. Fur and feathers are the natural clothing of many animals; they help animals like polar bears and seals to withstand the extreme coldness in the poles. But why are furry materials so effective for heat insulation? The secret lies not in the materials themselves but in the air they trap. Air is a very poor conductor of heat. These furry materials have many tiny air spaces within them, and can thus slow down the rate of heat loss effectively

The fur of polar bears helps to keep them warm under extreme coldness.

The handle of a cooking pan is made of plastic for safe handling.

About Thermal (Heat) energy

Heat is given off whenever energy is being used. You can tell if a television has been on by feeling if it is warm. When you run up a flight of stairs you feel warm because you are burning food energy.
What exactly is heat? Heat is the transfer or flow of energy from a hot object to one that is cooler. When you feel a warm object, you are actually feeling thermal energy, which is the movement of molecules that make up the object. An object has more thermal energy when it is warm than when it is cool.
The more thermal energy an object has, the faster its molecules move. These faster moving molecules bump into each other more frequently and spread out as they require more space (decreasing the density of the molecules).
Think of people standing in an elevator. If they started moving around, they would start bumping into each other and need more space. This is essentially what happens when molecules get more energy and start moving around; they spread out.
For the most part, the volume of an object increases as the amount of thermal energy it receives increases. In other words, the molecules in warmer objects are less densely packed than the molecules in cooler objects.
You can't see thermal energy, but you can detect evidence of heat transfer. You might see the air shimmering over a radiator (convection), put your hand on a warm spoon that's been sitting in a hot bowl of soup (conduction), or notice that the sun shine feels warm on your skin (radiation). If you need evidence of thermal energy or heat in your life, just feel your arm. Your body generates heat 24 hours a day!
Therefore, Thermal Energy can be transferred by 3 ways: conduction, convection and radiation. :D