Friday, July 3, 2009

some of my pictures uploaded may be cut off,
so please click the image to enlarge it.

Also, sorry if the image is blur!

Thanks (:

Tuesday, June 30, 2009

Applications Of Thermal Energy Transfer

Common applications of conduction
(I'm sorry, it's a little blur)

Example of applications of conduction in the home

Common applications of convection
1.Electric Kettles
Ever wondered why the heating coil of an electric kettle is always placed at the bottom of the kettle? Actually, it is to facilitate the transfer of thermal energy in water by convection. When the kettle is switched on, the water near the heating coil get heats up, gets expanded and becomes less dense. The heated region of water then rises while the cooler regions descend to replace the heated water. A convection current is set up.

2. Household hot water systems
Water is heated in the boiler by gas burners. The hot water expands and becomes less dense. Hence, it rises and flows into the upper half of the cylinder. To replace the hot water, cold water from the cistern falls into the lower half of the cylinder and then into the boiler due to the difference in pressure. The overflow pipe is attached to the cylinder, in case the water temperature becomes too high and causes a large expansion of the hot water. The hot water tap which is led from the overflow pipe must be lower than the cistern so that the pressure difference between the cistern and the tap causes the water to flow out of the tap.

3.Air conditioners
An air conditioner is always installed near to the ceiling of a room, to facilitate the setting up of convection currents. The rotary fan in the air conditioner releases cold dry air into the room. As cool air is denser, it sinks and the warm air below rises. This way, air is recirculated and the air temperature will eventually fall to the desired temperature.

Common applications of radiation

As we’ve learnt before, shiny surfaced objects are bad emitters of radiation. Yes, a shiny teapot can keep tea warm for a longer time than black teapots. Likewise, shiny containers can also keep cool liquids cool for a long time.

A greenhouse is used in cold climates to help plants grow better by trapping heat. Daytime, infrared radiation from the sun passes through the glass roof of the greenhouse. As contents in greenhouse get warm, they start to emit infrared radiation.

This emitted infrared radiation is different from the infrared radiation emitted by the sun, and is unable to pass back through the glass roof. Thus, the infrared radiation emitted by the contents in the greenhouse gets trapped. The amount of infrared radiation in the greenhouse gets built up over time. This causes the temperature in the greenhouse to increase.

3.Vacuum Flask
Also known as a thermos flask, it is designed to keep liquids hot by minimising heat loss in four possible ways! They are conduction, convection, radiation and evaporation.
-The stopper is usually made of plastic which is a poor conductor of heat.
- Conduction through the trapped air above the liquid is minimal since air is a poor conductor of heat
-Conduction and convection through the sides of the flask are prevented by the vacuum between the double-glass walls of the flaskTo minimise heat loss through radiation, the walls of the glass are silvered so as to reflect radiant heat back into the hot liquid. Convection and evaporation can only occur when the plastic stopper is removed during use. Heat loss by radiation is harder to stop as radiant heat can pass through a vacuum

Friday, June 26, 2009

5. Radiation

What’s Radiation?
Radiation, is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium. Yes, radiation DOES NOT require a medium for energy transfer, meaning, it can take place in a vacuum.

A daily example of radiation is shown above. Thermal energy from the sun reaches the earth by the process of radiation. Conduction/convection is impossible because there is vacuum between the Sun and the Earth. The Sun emits infrared waves, which is a part of electromagnetic waves emitted by the Sun to keep us warm. Thermal energy from infrared waves is called radiant heat, which is heat that all objects emit. The hotter the object is, the larger the amount of radiant heat emitted.

Absorption of infrared radiation
Infrared radiation is absorbed by all objects and surfaces. The absorption of radiant heat causes a temperature to rise. Above is a simple experiment that shows the absorption of infrared radiation. The experiment is whereby two temperature sensors A and B are connected to the data logger, with their ends wrapped in aluminium foils of equal size. One of the aluminium foils was then coloured black. The sensors were then placed at equal distances from the bulb. The result is that the blackened foil absorbed radiation at a faster rate compared to the shiny aluminium foil. Thus, this proves that dull and black surfaces absorb infrared radiation faster compared to shiny, light-coloured surfaces.

Emission of infrared radiation

Infrared radiation is emitted by all objects and surfaces. This emission causes the temperature of the objects to fall. Above, is an experiment that shows us how the nature of the surface affects the rate of emission of infrared radiation. The experiment is set up with two identical tins, one black and one shiny. They are filled to the brim with boiling water at the same time, after which were covered with a lid with temperature sensors attached. Temperature sensor A monitors the temperature of the black tin, while temperature sensor B records the temperature of the shiny tin. The result will show that the temperature for the black tin is observed to fall at a faster rate than the shiny tin. The reason is that the black tin emitted thermal energy at a faster rate compared to the shiny tin. Thus, we can draw a conclusion that dull and dark surfaces are better emitters of infrared radiation than shiny and white surfaces.

In conclusion, a good emitter of radiant heat is also a good absorber of radiant heat. Likewise, a poor emitter of radiant heat is also a poor absorber of radiant heat.

Factors affecting the rate of infrared radiation
Colour and texture of the surface
As I have mentioned before,
Dark and dull surfaces = good emitter of radiant heat
Shiny and light-coloured surfaces = poor emitter of radiant heat
Surface Temperature
The rate of temperature decrease is high initially, however, as time passes, the rate of temperature decrease becomes lower. Thus, the rate of infrared radiation is also lowered. This phenomenon is also shown in the diagram above on the emission of infrared radiation. The higher the temperature of the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.
Surface area
Simply put it,
Object with large surface area = emit infrared radiation at a higher rate
Object with small surface area = emit infrared radiation at a slower rate

Thursday, June 25, 2009

4. Convection

Convection is the transfer of thermal energy by means of currents of a fluid (liquids or gases)
Well, how then, does convection work? Below is an example of convection in water. The experiment procedure is whereby the round bottomed flask is filled with water, after which potassium permanganate crystals are placed at the bottom of the flask. The flask is then placed above a flame. When the water at the bottom of the flask is heated, it expands and hence that portion is less dense than the surrounding water and therefore starts to rise. Subsequently, the cooler regions of water at the upper part of the flask, become less dense, hence sink. This is shown by the red arrows in the diagram and the movement of the liquid is due to a difference in its density sets up convection current. Potassium permanganate is used as it is purple in colour, thus can clearly show the movement of water.

Convection In Liquids
Convection In Air
Similarly, the diagram above illustrates the convection in air. Firstly, the experiment is set up using a large box with two chimneys at each end. A candle is placed below one of the chimneys, while a piece of smouldering paper is placed over the other chimney. Firstly, the air above the candle is heated and expanded. As the air is now less dense than the surrounding air, it rises out of the chimney. Subsequently, the cooler and denser surrounding air sinks through the other chimney to replace the less dense air. This movement in and out of the chimneys due to a difference in density sets up a convection current.

Convection currents occur only in fluids such as liquids and gases, but not solids. Convection, basically, is the bulk movement of fluids which posses thermal energy. However, as solids are made up of very closely packed particles, bulk movement is impossible and hence solids have to resort to molecular vibrations to transfer heat energy.

Saturday, June 20, 2009

Conduction Part 2

Conduction, how does it work?
First, let us start off by learning about atoms and molecules in solids. All solids, be it metallic or non-metallic, are made up of tiny particles, also known as atoms and molecules. Metals contain many free electrons which move randomly between the atoms or molecules, whereas non-metals do not contain such free electrons

When thermal energy is supplied to one end of the rod, the particles at the hot end vibrate vigorously. These particles then collide with neighbouring particles, making them vibrate as well. Thus, the kinetic energy of the vibrating particles at the hot end is transferred to the neighbouring particles. Take note that, there is no transfer of particles, hence it takes rather long for thermal energy to be transferred. This process (atomic/molecular vibration) takes place in metallic and non-metallic objects. In metals, free electron diffusion also occurs. When metals are heated, the free electrons present in it will gain kinetic energy and hence move faster, spreading/diffusing into cooler parts of the object and transferring their kinetic energies to them. This then shows why thermal conductors are capable of transferring heat faster than insulators.

Conduction in liquids and gases
The way particles are arranged in liqiuds and gases, are different to that of colids. Liquid and gas particles are spaced further apart, hence the rate of collisions between particles are less frequent in liquids, even more so for gases. Therefore, the conduction in liquids and gases are inefficient, hence resulting in liquids and gases becoming poor conductors of heat.

Friday, June 12, 2009

3. Conduction Part 1

Huh, conduction?
Conduction is the process of thermal energy transfer without any flow of the material medium. For example, when you heat one end of a metal rod with a flame, after a while, the other end of the rod will also get heated up. From this simple experiment, we can conclude that thermal energy is transferred from one end of the metal rod to the other. Conduction, in simple terms, is known as the transfer of thermal energy through a medium, without the medium moving.

Above, is an example of conduction. The rods that are on the outside of the tank are of the same length and are evenly coated with melted wax. Boiling water is then poured into the bath, and it was ensured that the ends in the bath were completely submerged in water. The final observation is shown above, whereby wax melts the furthest along the copper rod, followed by iron, glass and wood. The end of the rod (hotter end) in the boiling water transfers thermal energy to the other end outside the bath (cooler end), thus causing the wax to melt. The length of unmelted wax is on each four rods is different, as they are made out of different materials.

From this experiment, we also can observe that different materials conduct heat at different rates. As the copper rod has the shortest length of unmelted wax whilst the wooden rod has the longest, we can conclude that copper is a good conductor of heat whereas wood is a poor conductor of heat. Poor conductors of heat are also known as insulators. The rate of thermal energy transfer is faster in copper than in wood, is because conductors and insulators have different mechanisms to transfer thermal energy.

Generally, good conductors of heat are mainly objects that are made out of metals like copper, silver, steel and iron. On the other hand, insulators are mainly objects that are made out of non-metals, such as glass, plastic, wood and wool. Insulators can not only be in the solid form, as air and water are also heat insulators!

In the next post, the details on how conduction works will be posted, and also further information on the conduction in liquids and gases.

2. Transfer of Thermal Energy

Why is there a transfer of thermal energy?
Firstly, what is thermal energy? Thermal energy is the kind of energy that is caused by/related to heat. Thermal energy can only be transferred if there is a difference in the temperature of two regions. Additionally, thermal energy always flows from a region of higher temperature to a region of lower temperature.

Above, is an example of the transfer of thermal energy. Basin 1, 2 and 3 are filled with water at 10°C, 37°C and 70°C respectively. The girl’s right hand is placed in Basin1, while her left hand is placed in Basin 3. The hand in Basin 1 felt cold, while the hand in Basin 3 felt hot. After which, both her hands were dried and the hotness and coldness in her hands subsided. She then put both her hands in Basin 3 and her hands felt neither cold nor hot.

Her hand in Basin 1, felt hot as it had gained thermal energy from the hot water in it. On the contrary, her left hand, in Basin 3, feels cold as it had lost thermal energy to the cold water in it. Similarly, when both her hands were immersed in Basin 2, it felt neither hot nor cold as the temperature of the water is same as that of the human body.

Thus, from the experiment, we can therefore conclude that thermal energy transfers only when there is a difference in temperature between two regions. Furthermore, thermal energy flows from a hotter region to a cooler region.

What ways can thermal energy be transferred?
As I have mentioned in the introduction of this blog, thermal energy can be transferred by three processes: conduction, convection and radiation. The next few blog posts will then describe the processes in detail.

1. Introduction

Hello everyone, this is a blog on the physics chapter: Transfer of Thermal Energy. In this educational blog, I would be covering firstly, the basic information on the Transfer of Thermal Energy. After which, I would cover the different ways thermal energy is transferred, and these three processes are conduction, convection and radiation. I would further elaborate on what the processes are, how they work, and their applications. And finally, conclude this chapter.