PHYSICS S.S ONE(HEAT TRANSFER)

SECOND TERM

WEEK: 6

HEAT TRANSFER

Concept of transmission of heat

Different materials in the surroundings such as iron, copper, wood, paper, water, milk air respond differently when exposed to heat. This is due to the modes by which heat is transferred in our daily life; we use these modes to cook food, to prepare tea, to keep our body and house warm in harmatan  season etc.

METHOD OF HEAT TRANSFER

Generally, there are three common method of heat transfer which are:

i.                     Conduction                 ii. Convection                     iii. Radiation

There is another one named  ADVECTION.

i. CONDUCTION: Conduction of heat is the process by which heat energy is transferred through a  material, the average position of the particle of the material remaining the same.

Mechanisms of Heat Conduction

(a) Vibration of molecules     (b) Through free electrons

Factors Affecting Thermal Conductivity

1. Nature of Materials

2. The Thickness/ Diameter/ Cross Section Area of The Conductor

3. Temperature Difference

4. Length of the Conductor

ii. CONVECTION: This is heat transfer by the mass movement of a fluid in vertical (up/ down) direction. This type of heat transfer takes place in liquids and gasses by the actual movement of the heated fluid from the hotter to the colder parts. This occurs naturally in our atmosphere. Warm air is less dense than cold air, making cold air heavier than warm air.

iii. RADIATION: It allows heat to be transferred through wave energy. These waves are called electromagnetic waves. This energy is released when those waves are absorbed by an object e.g. energy travelling  from the sun to your skin, you can feel your skin getting warmer as energy is absorbed.

iv. ADVECTION: This is transfer of heat in the horizontal (north/east / south / west) direction. In meteorology, the wind transports heat by advection. This happen all the time on earth; heat is transported in many ways. For example, wind blowing over a body of water will pick up evaporated water molecules and carry them elsewhere when the air with those water molecules cools, the water is condense and release latent heat.

COMPARISON OF THE THERMAL CONDUCTIVITY OF COPPER AND WOOD

Copper is a good conductor of heat while wood is a bad conductor. Their differences can be observed when a copper foil is wound tightly round the middle of  a wooden rod and held firmly to it with a nail, some paper stick round the wood and the copper, so that it makes good contact with both.

When the rod is passed several times over a flame, it is noticed that the copper changes when it is in contact with wood but not affecting the copper. The copper being a good conductor conducts the heat away quickly from the paper, but wood conducts the heat very slowly. As a result, the temperature of the paper next to the wood rises quickly and the paper is burnt.

THERMAL CONDUCTIVITY OF LIQUIDS

Water and other liquids are poor conductors of heat. The only exception is mercury which is a metal in a liquid form. We can demonstrate that water is a poor conductor of heat by the following simple experiment.

A piece of ice is wrapped in metal gauze and put into a test tube full of water. The water is held in a tilted position and heat is applied to its top portion. It will be observed that even though the water near the top is boiling , the ice near the bottom of the tube does not melt for some time. This thus shows that water is a poor conductor of heat since little heat is conducted from the top to the bottom of the water.

GOOD AND BAD CONDUCTORS AND THEIR APPLICATION

Good conductors allow heat energy to pass through them very easily e.g copper, aluminium, silver, iron etc.

Bad conductors or insulator are materials that do not allow heat to pass easily through them. E.g water, air, wood, plastic, cloth, cotton wool, cork etc

Practical application of good and bad conductor

I. Cooking utensils      II. Use of rugs and floors      III. Home cooling in the tropics  IV. The use of cloth to keep warm.

Thermos Flask

1. The stopper rubber pad and rubber ring pad prevent heat loss through conduction since rubber is a poor conductor of heat.

2. Silvered wall prevents heat loss through radiation since the shinny surface is a poor radiator and emitter of heat.

3. Vacuum prevents heat loss through conduction and convection since the two modes of heat transfer require material medium for transfer of heat.

4. Shock absorber prevents breaking of the glass walls due to pressure from the outside.

5. Vacuum seal prevents air from reoccupying the vacuum. It is through the vacuum seal that air was sucked out.

 Thermos Flask

1. The stopper rubber pad and rubber ring pad prevent heat loss through conduction since rubber is a poor conductor of heat.

2. Silvered wall prevents heat loss through radiation since the shinny surface is a poor radiator and emitter of heat.

3. Vacuum prevents heat loss through conduction and convection since the two modes of heat transfer require material medium for transfer of heat.

4. Shock absorber prevents breaking of the glass walls due to pressure from the outside.

5. Vacuum seal prevents air from reoccupying the vacuum. It is through the vacuum seal that air was sucked out.

PRESENTATION

Step I: The teacher explains the transmission of heat

Step II: The teacher mentions methods of heat transfer

Step III: The students chorus the methods of heat transfer

Step IV: The teacher explains the comparison of the thermal conductivity of copper and wood.

Step V: The students give examples of good and bad conductors.

Step VI: The teacher states the application of heat transfer.

EVALUATION

The teacher evaluates the students by asking the following questions:

i.                     Explain transmission of heat

ii.                   Differentiates between conduction and convection

iii.                  Explain the comparison of the thermal conductivity of copper and wood.