CHAPTER 7: HEAT

7.1     HEAT AS A FORM OF ENERGY

The Sun Gives Out Heat

1.       Heat is a form of energy.

2.       The Sun is the primary source of heat energy.

Other  Sources  Of   Heat  Energy

      1. Apart from the sun, we can get heat energy from :                          

a.     Fossil fuel and electricity

b.     Radioactive metals

c.      The mantle of the Earth.

2.       Heat is produced in our daily life wherever there is friction.

3.       Friction occurs when two objects rub against each other.

The  Uses Of  Heat  In Our Daily Life.

1.  Heat is a useful form of energy.

2.  The uses of heat in our daily life include : 
-Cooking, food drying, boiling water, drying cloth and providing warmth.

The Differences  Between Heat And Temperature

Heat	Temperature
Is a form of energy	Is a degree of hotness a substances
Is measured in the joule ( J )	Is a measured in the Kelvin ( K )
Is the total amount of kinetic energy of a particles	Tells us how fast the particles are moving.

7.2     HEAT  FLOW  AND  ITS  EFFECTS

1.       When a substance is heated, its particles receive more energy and move 
           faster.

2.       As a particles move more vigorously, they take up more space.

3.       This causes the substance to expand ( get bigger ).

4.       When a hot substance cools down, it contracts ( get smaller ) and goes back to its original size.

7.3     HEAT  FLOW.

1.       Heat can flow from one place to another.

2.       It can travel in three different ways :

                                   i.      By conduction through solids

                 ii.  By convection through liquids and gases

                 iii. By radiation through empty space
                               

a. CONDUCTION

1.       When one end of a metal rod is heated by a Bunsen flame, the particles of the metal rod near the flame get extra heat energy from the flame.

2.       The particles vibrate more vigorously, knocking into neighbours particles and transferring energy to them.

3.       The neighbouring particles, in turn, knock their neighbours and pass

          energy  to them.

4.      Thus, heat is carried along the rod from the hot end to the cold end.

b. CONVECTION

1.  When air is heated, it expand.

2.  The hot air becomes less dense than the cooler air around it.

3.  Thus, the hot air rises.

4.  As the hot air rises, cooler air rushes in to take its place.

5.  Soon, an air current is formed.

6.  When the hot air rises, heat energy is carried from one place to another.

7.  Convection is the transfer of heat energy from one place to another by the movement of the material itself.

c. RADIATION

1.  Heat travels through empty space (vacuum) as radiation, or heat rays.

2.  Heat rays are electromagnetic waves.

3.  They travel very fast through space.

4.  Eg :  Heat from the Sun reaches the Earth.

The Difference Between Conduction, Convention and Radiation

Differences in terms of	Ways of heat transfer
	Conduction	Convection	Radiation
Medium	Solids	Liquids or gases	Does not need a medium
How is heat transferred?	Heat flows from particle to particle which remain in their fixed position	Heat is carried by particles that move, following the convection current	Heat is transferred in the form of electromagnetic waves
Can the process take place in vacuum?	No	No	Yes
Is the rate of heat transfer fast or slow	Slowest	Fast	Fast

HEAT  FLOW  IN  NATURAL  PHENOMENA

A.   LAND  BREEZE

1.  Land breeze blows during the night from land to sea.

2.  During night time, the land becomes cool faster than the sea.

3.  The air above the sea becomes less dense and rises.

4.  Cool air from the land moves in to take its place.

5.  The convection current that forms is known as land breeze.

B.  SEA BREEZE

1.  Sea breezes blow during the day.

2.  During the day, the land heats up faster than the sea.

3. As a result, the air on land becomes less dense and rises.

4. The cooler air over the sea which is denser (cooler) flows in to take the place of the warm air, causing what is known as a sea breeze.

C. THE  WARMING  OF  THE  EARTH  BY  THE  SUN

1. The heat from the sun reaches the earth by radiation through a vacuum in

      space (no medium)

2.   The radiant heat which is absorbed heats up the earth.

EXPANSION AND CONTRACTION

The application of the principle of expansion and contraction of matter in daily life.

Mercury in glass thermometer

1. A mercury in glass thermometer works on the expansion and contraction of

     mercury in the bulb of the thermometer.

2. When the thermometer is used to measure the temperature of a hot object, the

     mercury in the bulb expands and forces the mercury in the glass tube to go up.

3.  When the thermometer is used to measure the temperature of a cold object,

      the mercury in the bulb contracts and the mercury in the glass tube goes

     down.

Bimetallic Strip in Fire Alarm      PMR 08

   1. A bimetallic strip is made of two different metal strip, such us copper and

        iron. Copper expands faster than iron when heated.

2.     The forces that arise due to en equal expansion of the two metals 

      cause  the bimetallic strip to bend.

3.     Bimetallic strips are used as automatic switches in fire alarms and as

     thermostats in electrical appliances such as iron, air-conditioners, rice 

      cookers and refrigerators to keep the temperature in these appliances

      constant.

Gaps  In  Railway  Tracks

1. Gaps are left in between sections of rails to allow for the expansion of the rails

     during very hot weather.

2. Concrete roads are built with expansion gaps to allow for the expansion of

     the concrete during hot weather

Rollers in concrete and steel bridges

1.   Concrete bridges and steel bridges have rollers on one end to allow for

      expansion on hot days and cold days.

The use of the principle of expansion and contraction of matter in solving simple problems

a. Opening a tight cap PMR 08

1. The cap can be removed easily by dipping it into hot water for a few minutes. This is because the cap expands when it is heated by the hot water and became loose.

DARK,  DULL  OBJECTS  ABSORB   AND  GIVE  OUT  HEAT  BETTER

1. Dark and dull objects :

a.  absorb heat better than white, shiny objects.

b.   give out (transmit) heat better than white shiny objects.

2. A white, shiny surface is a poor absorber and poor radiator of heat.

3. The ability of an object to absorb and give out heat depends on :

a. the surrounding temperature

b. the surface properly of an object.

4. White and shiny objects reflects heat better than dark and dull objects.