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

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

ConductionConduction Heat travels through some material better than others.

Material Time taken for pin to drop(s)

wood did not drop copper 14 steel 73 aluminium 16

Pins are hold by wax

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 poorConductor (insulators).

Liquids and gases are usually poor conductorsThe poorest conductor is vacuum.

Test YourselvesTest Yourselves • 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 wallprovides extra insulation

Convection of Heat

ConvectionConvectionConvection is the process by which heat istransmitted from one place to another bythe movement of heated particles of a gasor liquid due to difference in density.

Convection in LiquidsConvection 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 LiquidsConvection in Liquids

Convection in GasesConvection 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 BreezeSea 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 of Heat

RadiationRadiation 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 ofwaves.

Heat Radiation

• All objects emit heat radiation.• 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 = 5.6703 10-8 (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 flask is a double-walled glass bottle.

• 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 ?)

Vacuum Flask

Test Yourselves1. 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 ?

References:

http://blogs.saschina.org/sophie01pd2016/files/2009/10/heat-transmittance-me

http://www.engineeringtoolbox.com/radiation-heat-transfer-d_431.html

http://www.free-online-private-pilot-ground-school.com/images/sea-land-breeze.gif

http://www.auburn.edu/projects/sustainability/website/images/insulation.jpg

http://www.physics.arizona.edu/~thews/reu/the_science_behind_it_all.html