# Transfer of Thermal Energy

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Transfer of Thermal Energy
Name: ________________ Class: _________________ Index: ________________

Learning Objectives show understanding that thermal energy is transferred from a region of higher temperature to a region of lower temperature. describe, in molecular terms, how energy transfer occurs in solids. describe, in terms of density change, convection in fluids. explain that energy transfer of a body by radiation does not require a material medium and the rate of energy transfer is affected by: (i) colour and texture of the surface. (ii) surface temperature. (iii) surface area. apply the concept of thermal energy transfer to everyday applications.

In general, heat travels from higher temperature region to lower temperature region.
There are three processes by which heat may transmitted: Conduction, Convection, and Radiation

Conduction of Heat

Conduction Conduction is the process by which heat is transmitted through a medium from one particle to another.

Conduction Heat travels through some material better than others.
Pins are hold by wax Material Time taken for pin to drop(s) wood did not drop copper steel aluminium

A good conductor has a high conductivity.
Metals are good conductors of heat. Silver and copper have highest conductivity. Non-metals, such as plastic and air, are poor Conductor (insulators). Liquids and gases are usually poor conductors The poorest conductor is vacuum.

Test Yourselves the water at the top starts boiling, the ice cube
Heat a test tube of water near the top with a ‘weighted’ ice cube near the bottom. Even when the water at the top starts boiling, the ice cube does not melt. Why ? Ans.: Water is a poor conductor of heat.

Insulators Insulators are used to lag pipes,
lofts, hot water tanks and many other objects. Insulators are very poor conductors. Wrapped round hot objects, they act as barrier, reducing the heat loss. Fibre glass insulation is a popular choice for insulating the loft of homes.

Insulation Lagging in a cavity wall provides extra insulation

Convection of Heat

Convection Convection is the process by which heat is
transmitted from one place to another by the movement of heated particles of a gas or liquid due to difference in density.

Convection in Liquids To demonstrate convection in water, drop a
few tiny crystals of potassium permanganate into a flask filled with water. Gently heat the flask, purple streaks of water will rise upwards and then fan outwards. The water becomes uniformly purplish after some time. The circulation of a liquid in this matter is called a convection current. Thinking : Why hot water rises and cold water sinks ?

Convection in Liquids

Convection in Gases The figure below shows a simple demonstration of convection of gas. The hot gases from the burning candle go straight up the chimney above the candle. Cold air is drawn down the other chimney to replace the air leaving the room.

Concept on ventilation of a room with fire place

Sea Breeze Land Breeze (continue on next slide)

Sea Breeze During the day the sun heats the land must faster than the sea. (Why ?) The air above the land is heated, expands and rises. Cold air from the sea moves inland to take its place. Hence, sea breeze is obtained. Discussion : How land breeze is produced ?

Test Yourselves 1. Usually, it is better to install air conditioner
in the higher portion of the wall. Please explain this statement with reasons. 2. How are winds forms ? 3. Why is the heating coil of an electric kettle placed near the bottom of the vessel ?

Radiation Radiation is a method of heat transfer that does not require any medium. It can take place in a vacuum. In radiation, heat transmits energy in the form of waves.

Heating an object up make it radiate more energy. A dull dark surfaces is a better emitter or radiator than a shiny one.

What type of surface is the best absorber of heat
Fig. below shows one way to test different surfaces. Results from this type of test show that, a dull black surface is the best absorber of radiation, a shiny silvery surface is the worst absorber of radiation.

What type of surface is the best absorber of heat

A black body is defined as a body that absorbs all radiation that falls on its surface. Actual black bodies don't exist in nature - though its characteristics are approximated by a hole in a box filled with highly absorptive material. A black body is a hypothetic body that completely absorbs all wavelengths of thermal radiation incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self-luminous. All blackbodies heated to a given temperature emit thermal radiation.

The radiation energy per unit time from a blackbody is proportional to the fourth power of the absolute temperature and can be expressed with Stefan-Boltzmann Law as q = σ T4 A         (Not Tested) where q = heat transfer per unit time (W) σ = (W/m2K4) - The Stefan-Boltzmann Constant T = absolute temperature Kelvin (K) A = area of the emitting body (m2) Therefore, the rate of energy transfer increases with temperature and surface area.

Worked Example

The space between the two walls is a vacuum. This can stop energy transfer out by conduction and convection. The shiny bright silvering coated glass surfaces can reduced the heat lost by radiation. (Why ?)

Test Yourselves 1. Brightly polished kettle do not lose much energy
by radiation. Why ? 2. The cooling fins on the back of a refrigerator, in a car radiator should be dull black. Why ?

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