REG.10
EKWEI LING,JOLENE
3B1
THERMAL PHYSICS.
Chapters
SEVEN ,EIGHT, NINE




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CHAPTER 8;
Transfer of Thermal Energy.

1. Thermal energy is the energy portion of a system that increases with its temperature.
   
2. It is also a term used to describe the energy content of a system related to heating effects.


3. It can only be transferred when temperatures are in difference.


4. & in conclusion, Thermal Energy flows from a region of HIGHER temperature to a region of LOWER temperature.

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Thermal Energy is being transferred by 3 processes:

1. CONDUCTION.
   
2. CONVECTION.
   
3. RADIATION.

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Conduction:
It is the process of thermal energy being transferred without any flow of material medium.

An example of HEAT TRANSFER OF CONDUCTION:
Conduction occurs through collisions between atoms and molecules in the substance and the subsequent transfer of kinetic energy.

Study the following diagram:

1. When the barrier is removed, the fast ('hot') atoms collide with the slow ('cold') ones.


2. In such collisions, the faster atoms lose some speed & the slower ones gain speed; thus, the fast ones transfer some of their kinetic energy to the slow ones.


3. This transfer of kinetic energy from the hot to the cold side is called a flow of heat through CONDUCTION.

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An example of THERMAL CONDUCTIVITY:
Different materials transfer heat by conduction at different rates;

Study the following diagram:

Thus,

1. Materials with a HIGH thermal conductivities, will transfer large amounts of heat over time - e.g: Copper, that are good thermal conductors.


2. Similarly, materials with LOW thermal conductivities, will transfer small amounts of heat over time - e.g: Concrete, that are poor thermal conductors.

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Poor thermal conductor keeps as much heat in, as possible. Insulation is therefore also known in terms of its thermal resistance.

& in conclusion,

Materials that have a HIGH thermal conductivity, have a LOW thermal resistance -
Similarly, materials that have a LOW thermal conductivity, have a HIGH thermal resistance -
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Convection:
It is a process of thermal energy being transferred by means of currents in liquid or gases.

An example of HEAT BY CONVECTION:

1. As air heats, the molecules spread out; causing this region to become less dense than the unheated air.


2. Being less dense than the unheated air, the hot air will subsequently rise due to buoyant forces.


3. This movement of hot air into a cooler region is then said to transfer heat by convection.

Study the following diagram:

(arrows in the diagram are the CONVECTION CURRENTS)

1. Heating a pot of water on a stove is an example of transfer heat by convection.


2. When the stove is first turned on, heat is being transferred first, by conduction between the element through the bottom of the pot to the water.


3. However, when the water starts bubbling - these bubbles are actually regions of hot water rising to the surface, thereby transferring heat from the hot water, that is at the bottom; to the cooler water at the top, by CONVECTION.


4. At the same time, the cooler & denser water at the top, will sink to the bottom, where it is subsequently heated.
   

This is the process of CONVECTION, by heat transfer to form convection currents.

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An example: Flow of material through pressure difference

Study the following diagram:

1. When the barrier is being removed, material in the high pressure (high density) area will flow to the low pressure (low density) area.


2. This movement of material is said to be of heat flow by convection.

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Radiation:
It is a process of continual emission of infrared waves from the surface of all bodies, that transmit without a medium. (it can take place in vacuum.)

1. The energy of heat can go in making light, such as coming from a hot campfire.


2. This light, being a wave, carries energy. It can move from one place to another without requiring a medium.


3. When this light reaches us, part of the energy gets converted back into heat, which is why we are able to feel warm sitting beside a campfire.


4. Some of the light can be in the form of visible light, but a great deal of the light emitted, is infrared light.


5. The hotter the object is, the less infrared light is emitted, and the more visible light as well.

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Factors that affects the rate of infrared radiation:

1. Colour & texture of surface. Black surfaces are much better absorbers & emitters of infrared radiation, than white or shiny surfaces.


2. Surface temperature. The HIGHER the temperature of the surface of the object, the HIGHER the rate of infrared radiation.


3. Surface area. The LARGER the surface area is, the HIGHER the rate of emitting infrared radiation.

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Application of Thermal Energy Transfer (conduction):

Metals that are good conductors of heat are:

1. Cooking Utensils. e.g: Kettles, saucepans & boilers.
   
2. Soldering iron rods. (made out of Copper)


3. Heat exchangers. (made out of Copper too)

Non metals that are good insulators of heat are:

1. Handles of appliances & utensils. e.g: (handles of the above stated no.1 & no.2 )


2. Table mats. (made out of cork)


3. Sawdust.


4. Wooden ladles.


5. Woollen clothes.


6. Fibreglass, felt & expanded polystyrene foam.
   

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Application of Thermal Energy Transfer (convection):

1. Electric Kettles.


2. Household hot water systems.


3. Air conditioners.


4. Refrigerators.
   

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Application of Thermal Energy Transfer (radiation):

1. Teapots.


2. The Greenhouse. (used to help plants grow better by trapping heat)


3. Vacuum Flasks. (to keep liquids hot by minimising heat loss in CONDUCTION, CONVECTION, RADIATION & EVAPORATION)

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VIDEOS!

Thermal Conduction:

Thermal Convection:

With this, marks the end of Chapter 8, Transfer of Thermal Energy. :D
(officially started at 5:04pm and ended at 7:49pm) 1st half.
(officially continued at 10:06pm and ended at 11:09PM) 2nd half.

with truckloads of love ♥