Lesson Notes By Weeks and Term - Senior Secondary School 1

SUBJECT: PHYSICS

CLASS:  SS 1

DATE:

TERM: 2nd TERM

REFERENCE BOOK

• New School Physics By M.W Anyakoha
• Senior Secondary School Physics By P.N Okeke

 
WEEK FIVE

TOPIC: Heat Transfer - Conduction, Convection, Radiation and Their Applications

CONTENT

• Conduction
• Convection
• Radiation
• Thermo Flask

Heat can be transferred by conduction, convection or radiation.

CONDUCTION

This is the process of transfer of heat through a material by vibration of the molecules fixed in position. N.B. Metals that allow heat to pass through them are said to be good conductors of heat. Though all metals are classified as good conductors, they differ in an ability to conduct heat. Non metal (wood / plastic) are bad conductors that does not allow heat to pass through it. Thermal conductivity is simply the ability of a metal to conduct heat.

PRACTICAL APPLICATION OF CONDUCTION OF HEAT

1. Kettle, pans and other utensils made of metals are provided with wooden or ebonite utensils so that heat from to utensils is not conducted.
2. House old source pans are made of metals because metals are good conductors of heat and electricity.
3. Thick brick walls are used in the construction of storage rooms. Bricks are bad conductors of heat, so it will not allow heat to enter the storage room.
4. A stone floor feels cold to the feet than a rug or a carpet on the same floor feels warm.

CONVECTION

Convection is the process by which heat energy is transferred in a fluid (liquid or gas) by actual movement of the heated fluid. The direction of the motion of a filter paper indicates the circulation of heated water. Circulation of a liquid is called CONVECTION CURRENT.

APPLICATION OF CONVECTION

Convection in liquid

1. Domestic water-boilers.
2. Cooling process of a motorcar vehicle.
3. Land and sea breezes.

Land and sea breezes

Convectional current set up in land and sea breezes. They occur mainly in coastal  regions.

Sea breeze

During the hot season, heat from the sun passes straight from the air with little effect. The land is heated by the sun more quickly than the sea becase:

1. It is a good absorber of heat.
2. It is a good radiator.
3. It has low specific heat capacity.

The air near the land thus warms up and rises while air from the sae moves into the land to replace the risen air. Air from the higher atmosphere moves from the sea and then a circulation current is set up. The breeze from the sea is known as the sea breeze.

Land breeze

During the night, land is not heated by the sun, so it cools very quickly. Temperature of the sea drops only slightly and since it has been heated to a very high temperature, it retains more heat as a form of energy. As a result of this, the sea is warmer than the land at night. Convectional current is therefore set in the opposite direction in the day as a result of breeze blowing from land to the sea. This breeze from the land is called land breeze.

Convection in gases

1. Chimneys in Kitchens
2. Ventilation of industrial buildings (up-cast and downcast)
3. Bonfires
4. Room radiators- when in use, the hot air rising from it can produce a shadow in the room by the sun due to convection.
5. Glider lift- lift for aircraft used in flying sport. It is done by positioning the aircraft towards the roofs of factories where air above is warmer than any other place. The rising convection current is called thermal
6. Wind- convection currents in air when hot air rises from the equator and replaced by a colder or denser air from the polar region. Wind depends on earth’s rotation, large masses of land and sea over which it passes and temperature.
7. Air conditional are placed at the top of wall in the room and not on the floor so that the hot air which are less dense rises and are cooled while the colder air which are denser sink down hence. This continuous process of rising and cooling of warm and cold air set up convectional currents which cools the room.

RADIATION

Radiation is the process by which heat is transferred or conveyed from one place / point to another without heating the interview medium. In radiation molecules are not involved, it differs from another method because it does not require a medium for heat to be transferred. Heat from the sun reaches us by this method. The energy given out as radiant heat is known as RADIANT ENERGY. This energy displace electrical and magnetic properties and it said to be electromagnetic radiation and the name is INFARED.

RADIATION CAN BE DETECTED BY

1. Radiometer
2. Thermo-pile
3. Leslie cube

Dark surface is good absorber of heat and poor emitter of radiation. Highly polished surface are good emitter of radiation and poor emitter of heat.

APPLICATION OF RADIATION

1. It is not advisable to wear a dark shirt in the tropical sun.
2. Roofs of factories are coated with aluminum paint.
3. Shinning roofs and outside walls are painted with light colour in hot climate to keep the house cool.

Cooking utensils are darkening at the bottom and polished in the upper surface. The blacking surface will allow the heat while the silver surface won’t allow the heat to go out.

EVALUATION

1. Define the following terms A. conduction B. Convection C. Radiation
2. State two application of radiation.

Thermos Flask

A thermos flask is used to keep the temperature of its content constant. The essential

Feature of a thermos flask are as follows:

1. Cork stopper – Prevent heat loss by conduction, conduction and Evaporation
2. Vacuum in the double wall – Reduce heat loss or gain by conduction and convection
3. Silvered wall surface – Minimize heat loss by radiation
4. Cork support – Prevent heat loss by conduction

Thermos flask is also known as vacuum flask.

EVALUATION

1. Define the following terms A. Conduction B. Convection C. Radiation
2. With the aid of a diagram, explain how the construction of a thermos flask minimizes heat exchange with the surrounding

Reading Assignment

New School Physics pg 46 – 53

WEEKEND ASSIGNMENT

1. The silver surface of a thermos flask minimizes heat loss by A. conduction 

1. radiation C. convection D. evaporation

1. Is it advisable to wear a dark shirt in the tropical sun? A. yes it is B. No it is not 

1. Cannot say D. All of the above

1. The following are good conductors of heat except A. steel B. aluminum C. copper D. wool
2. The thermal conductivity of copper is greater than lead A. true B. false C. cannot say D. none of the above
3. A stone floor feels warm to the feet but a rug or a carpet on the same floor feels cold A. true B. false C. cannot say D. none of the above

THEORY

• Define the following terms A. Conduction B. Convection C. Radiation.

1. With the aid of a diagram, explain how the construction of a thermos flask minimizes heat exchange with the surrounding.