SSS 1: Heat energy (III)

MODE OF HEAT TRANSFER

Heat flows from a body at a higher temperature to another at a lower temperature.

The modes of heat transfer are:

a.       Conduction

b.       Convection

c.       Radiation

A.      CONDUCTION

Conduction of heat is the process by which heat energy is transferred through a material with the average particles of the materials remaining the same.

Thermal Conductivity

Thermal conductivity is the ability of a metal to conduct heat.

where;

K        =        Thermal conductivity

Q       =        Quantity of heat transferred

d        =        Distance between the two isothermal planes

A        =        Area of the surface

∆T     = temperature difference.

The S.I. unit of thermal conductivity is 

watt per meter – kelvin (wm^-1k^-1)

The dimension is MLT^-3ϴ^-1

Good Conductors of Heat

Good conductors are materials that allow heat energy to pass through them easily e.g. copper, aluminum, iron, silver, etc. Generally, all metals are good conductors of heat

Bad Conductors of Heat

Bad conductors are materials that do not allow heat to pass through them easily e.g. plastic, rubber, air, wood, cloth, glass, cork, water, asbestos, non-metals, etc.

Application of Good Conductors

i.        They are used in cooking utensils

ii.       They are used in Humphry Davy’s Lamp

Application of Poor Conductors

i.        Use of rugs on the floor

ii.       Use of cloth to keep us warm

iii.      Thatched-roofed houses

iv.      Use of asbestos ceiling in the house

B.      CONVECTION

Convection is the process by which heat energy is transferred in a fluid by the movement of the heated particles.

Application of convection

i.        Ventilation in houses

ii.       Land and sea breeze

iii.      Cooling of motor engine

iv.      Use of smoke in the chimney

v.       Domestic hot water system

Sea Breeze

 

During the day, the land is hotter than the sea. Warm air above land rises and its place is taken by cooler air from the sea. The specific heat capacity of land is less than that of the sea.

Land Breeze

At night, the land cools faster than the sea, and warm air from above the sea rises upward. Its place is taken by cooler air from above the land.

C.      RADIATION

Radiation is the process by which heat is transferred from a hotter to a cooler place without heating the intervening medium

Detection of Radiation

The instruments used in detecting radiant heat are:

i.        Radiometer

ii.       Thermopile

Emission and Absorption of Radiation

Different types of surfaces at the same temperature emit or radiate heat at different rates which is dependent on the nature of the surface.

i.        An object that is good at absorbing heat is a good emitter

ii.       A dull-black surface is a good absorber and good emitter of radiation

iii. highly polished or shiny surfaces are poor absorbers and poor emitters of radiation.         

 

                  White Body

The white body reflects almost all radiations incident upon it and does not absorb or transmit any part of it.

 

Thermos Flask

i.        The vacuum reduces heat loss by conduction and convection.

ii.       Silvered double walls reduce heat loss by radiation

iii.      The cork reduces heat loss by conduction

iv.      The cork stopper reduces heat loss by evaporation and convection

Newton’s Law of Cooling

Newton’s law of cooling states that the time rate of heat loss of a body is directly proportional to the difference in the temperature between the body and its environment.

The temperature of the environment of a body is called its ambient temperature. Ambient temperature is used to determine a victim’s time of death.

Assignment

Discuss the experiment to show that water is a poor conductor of heat