Profile
Elizabeth Tan Seok Hui
(34) 3B2
This is a physics blog :D



Chapters

Chapter 7- Kinetic Model Of Matter
Chapter 8- Transfer of Thermal Energy
Chapter 9- Thermal Properties of Matter
Links and Credits
3B2 Physics

Tay Shuying
Eileen Chia
Lim Ern Huei
Dawn Chiam
Fu Jinting
Samantha Yeap
Yap Xin Ying
Jovee Gan
Deborah Loo
Vani
Hilary Fong
Victoria Lee
Amanda Teo
Janice Yoong
Phyllis Quan
Wilma Heng
Aiswary


3B1 Physics

Sheryl Tan
Stephanie Loo
Jolene Ek


Layout: hearteditorials
Codes: -ambulance
Icon: biconcave

Wednesday, September 3, 2008 Transfer of Thermal Energy.

Thermal energy is transferred only when there is a difference in temperature. Thermal energy always flows from a region of higher temperature to a region of lower temperature.

Thermal energy is transferred by any of these three processes;

• Conduction
• Convection
• Radiation

1. Conduction

Conduction is the transfer of energy from one molecule to another. This transfer occurs when molecules hit against each other, similar to a game of pool where one moving ball strikes another, causing the second to move. Conduction takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other. For example, metal is a better conductor than wood or plastic.

Conduction is the process of thermal energy transfer without any flow of the material medium.

2. Convection

Convection is the movement of heat by a liquid such as water or a gas such as air. The liquid or gas moves from one location to another, carrying heat along with it. This movement of a mass of heated water or air is called a current.

Convection currents occur only in fluids such as liquids and gases but not in solids. This is because convection involves the bulk movements of the fluid 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.

Convection is the transfer of thermal energy by means of currents in a fluid (liquid of gases)

3. Radiation

Heat travels from the sun by a process called radiation. Radiation is the transfer of heat by When infrared rays strike a material, the molecules in that material move faster. In addition to the sun, light bulbs, irons, and toasters radiate heat. When we feel heat around these items, however, we are feeling convection heat (warmed air molecules) rather than radiated heat since the heat waves strike and energize surrounding air molecules.

Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.

Unlike conduction and convection, radiation does not require a medium for energy transfer. This means that radiation can take place in a vacuum.

Factors affecting rate of infrared radiation

1. Colour and texture of the surface
2. Surface temperature
3. Surface area

Applications of Thermal Energy Transfer

Common applications of conduction

Uses of good conductors of heat.
If thermal energy has to be transferred quickly through a substance, good conductors of heat such as metals are used. Some examples of the uses of metals are:

1. Cooking Utensils like kettles, saucepans and boilers are usually made of aluminium or stainless steel where direct heating is involved.
2. Soldering iron rods are made of iron with the tip made of copper, as copper is a much better conductor of heat than iron

3.Heat exchanges, such as those used in a large laundry facility, help save energy.


Uses of bad conductors of heat (Insulators)
Insulators are very useful if we want to minimise loss of thermal energy, or prevent thermal energy from being transferred quickly. Some common uses of insulators are:

• Handles of appliances and utensils like saucepans, kettles, teapots, irons and soldering iron rods are made of wood or plastics which are poor conductors of heat. In this way, the hot utensil or iron can be picked up without scorching out hands.
  
• Tables mats are usually made of cork
• Sawdust is used to cover ice blocks to reduce melting because of its good insulating property.
• Wooden ladles are very useful for stirring or scooping hot soup and also for scooping rice that has just been cooked
• Woolen clothes are used to keep people warm on cold days.
  
• Fiberglass, felt and expanded polystyrene foam which trap large amounts of air are employed as insulators in the walls of houses, ice boxes and refrigerators.
  

Common applications of convection

• Electric kettles
• Household hot water systems
• Air conditioners
• Refrigerators

Common applications of radiation

• teapots
• The greenhouse
• Vacuum flasks

END OF CHAPTER 8.