TRANSFER OF THERMAL ENERGY

1/Transfer of Thermal Energy

ex: A cup of coffee will cool down because it is giving out heat energy into the surroundings This drink (taken out of the fridge) will warm up because it is taking in heat energy from the surroundings

=>Thermal energy is transferred only when there is a difference in temperature. =>Thermal energy is always flows from a region of higher temperature to a region of lower temperature.
=>There is no net flow of thermal energy between two bodies when thermal equilibrium is reached

*There are three mechanisms by which thermal energy is transported.

1. Convection 2. Conduction 3. Radiation

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

*Note: Conduction takes place without any movement of the material medium Different materials conduct heat at different rates.
-Conduction works by atomic collisions and free electron diffusion.

+ Atomic collisions: is a slow process.
· In a solid, particles vibrate more vigorously or have more kinetic energy in a higher temperature region than in a lower temperature region.
· Particles in a substance are always vibrating. If the substance gets hotter, the atoms vibrate more. The heat energy is given to the particles, which makes them move about faster .Every time they collide with another particles, some of their kinetic energy is transferred, the heat energy is transferred to colder regions, so the temperature increases

+ Free Electron diffusion: faster than atomic collisions
· In metal, free electrons gain kinetic energy and diffuse or spread unto the cooler parts of the metal. They collide with atoms in the cooler part and transfer their kinetic energies on them.
That is why metals are good conductors

+ Good conductors of heat: metals such as copper, silver, iron They are good because of free electrons present in them + Bad conductors of heat/insulator: Non-metal such as glass, plastic, wood, brick ,wool, air and water.
- In liquid and gas, particles are further apart with other than in solid, therefore, the collisions between particles is less frequent in liquid and lesser in gas

Rate of collision increases:

GAS -> LIQUID -> SOLID

=> Gases and liquids are poor conductor compared to solid

3/Covection

Some examples:

Heated air rises, cools, then falls. Air near heater is replaced by cooler air, and the cycle repeats.






Hot water rises, cools, and falls.

Warm land is cooled during the day, while cooler land is warmed at night

· Convection is the transfer of thermal energy by means of currents in fluid (liquid or gas)
· In water heated, the expanded water in the bottom is less dense than the surrounding water and so starts to rise. The cooler regions of the water in the upper part of the flask, is dense and sink. The movement is due to the difference in its density in various parts of the fluid.
· Similar with in water, convection also occurs in gas.
· Convection currents occur only in fluids such as liquids and gases but not in solids.
?? Why air conditioner placed high up the room?? Due to the convection in gases, the hot air ath the bottom is less dense and rise while the cool air in the upper part is dense and sink. when the air conditioner works, it produces cool air so that will sink , the whole room can feel cool.

4/ Radiation

Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium
· Radiation does not require a medium for energy transfer, it means radiation can take place in a vacuum.
· A thermal energy form infrared wave is called radiant heat.

Factor affecting the rate of infrared radiation
1/ Colour and texture of the surface
- Black ,dull surfaces are better absorbers of infrared radiation than shiny, white surfaces
2/Surface temperature
- The hotter the object the more energy it radiates
3/Surface Area
- The greater the area the more energy it radiates


5/Application of thermal Energy transfer

Uses of good conductors of heat

1/ cooking utensils: often made with metals where direct heating is involved
2/ Soldering from rods: made of iron with the tip made of copper as copper is a better conductor
3/Heat exchangers: help saving energy

Uses of bad conductors of heat ( insulator)

1/ Handles of appliances and utensils: made by bad conductor, so our hands would not be scalded
2/ Table mats: made of cork, the table would not be damaged once the hot things placed on it
3/ Sawdust: convert the ice cubes to delay melting
4/Wooden ladles: stir or scoop hot soup
5/Wooden clothes: keep people warm on cold days
6/ Fibreglass, felt and expanded polystyrene foam: trap large amount of air, employed as insulators in the wall if the houses, iceboxes and refrigerators.

Common applications of convection

1/electric kettles: 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
2/ Household hot water systems
3/ Air conditioners
4/ Refrigerators

Common applications of radiation

1/Teapots :shiny teapots can keep tea warm for a longer time than black teapots
2/ Greenhouses
3/ Vacuum flasks

Conceptual Questions ( Try to answer by your best and I also, may be my answers are not correct) ^^! =.=
2. Place a wooden spoon and a metal spoon in the freezer. Which will cool faster? After several hours, what would they feel like? The metal spoon will cool faster as metal is the good conductor....After several hour both of the spoon will reach the same temperature which is also the same with the temperature of the freezer
4. Two identical cups of cocoa are sitting on a table. One has a metal spoon in it and one does not. After five minutes, which cup is cooler? cup with metal spoon will cooler as metal is the good conductor
3/ What happen for the car seat when the car is left in the hot day at the car park for a while?
The car seat will be very hot due to the radiation from the sun to the car and through the car glass. Tha car seat also radiates but the radiation rate is reflected ,just a part of the rate can go through the glass. The process is repeated many times. It results the car seat is much hotter than the car..