Modes of heat transfer

The presentation is prepared to help students of Class 10 Physics to understand modes of heat transfer and study applications in daily life.

1. HEAT MODES OF HEAT TRANSFER RIFFAT SHAHEEN FFMS MALIR CAMPUS

2. Modes Conduction Convection Radiation

3. The process, in which heat energy is transferred from one system to another system without visible motion of particles, is called conduction. CONDUCTION

4. Pair discussion

5. Conduction is the mode of transfer of heat that requires a medium. It takes place in all the three states of matter. But it is much dominant in solids only. Heat energy transfers from one place to another through molecular vibrations in solids. These vibrations travel in the form of heat waves of very high frequency.

6. METALS ARE THE GOOD THERMAL CONDUCTORS ……because heat transfer in metals take place due to free electrons inside metals which move very fast and transfer heat from one place to another. On the basis of heat conduction, the materials are divided into two groups: Thermal Conductors (Good Conductors) Thermal Insulators (Poor Conductors) THERMAL CONDUCTORS The materials, which can conduct heat at an appreciably high rate, are called thermal conductors. Examples are metals like copper, iron, aluminum, steel etc. THERMAL INSULATORS The materials, which cannot conduct heat at an appreciably high rate, are called insulators or poor conductors. Examples are non-metals like wood, rubber, plastic, glass etc.

7. USES OF CONDUCTORS AND INSULATORS Cooking pots and pans are made of metals, the conductors to ensure quick and fast supply of heat. Potholders or handles are made up of wood or plastic, the non-conductors, to avoid burning of hands. Tablemats are made from insulators like wood or cloth to avoid burning of tabletops. Plastic foam and fiberglass insulators are used in the walls and ceilings of homes to keep them cooler in summer and warmer in winters

8. It was invented in 1815 by Sir Humphry Davy. It originally burned a heavy vegetable oil. It was created for use in coal mines, to reduce the danger of explosions due to the presence of methane and other flammable gases, called firedamp or mine damp .

9. WORKING OF DAVY’S LAMP A cylindrical metal guaze of high thermal conductivity surround the flame. When this lamp is taken inside the mine, the explosive gases present in the mine are not ignited because the wire gauze in the form of cylinder conducts away the heat of the flame of the lamp. The result is that the temperature outside the guaze remains below the ignition point of the gas. In absence of wire gauze the gases outside can explode.

10. CONVECTION DEFINITION The process, in which heat energy is transferred from one system to another system due to movement of particles through large distances, is called convection.

11. EXPLANATION In fluids (liquids or gases) heat transfer from one place to another inside a system takes place through convection. This mode of transfer of heat requires a material medium. The molecules take heat the heat energy and their kinetic energy increases and they carry heat through large distances.

12. VENTILATORS : Ventilators are provided in the walls of a room near the ceiling, which help to keep the room temperature moderate by continuous circulation of air. The air inside the room gets impure and heated due to our breathing. This hot air rises up and passes out through ventilators, thus allowing space for currents of fresh air from outside windows or doors.

13. As air warmed at the Equator rises towards the poles, the rotation of the Earth causes it to deflect and flow back towards the Equator. In the northern hemisphere, the winds blow from the east to the west, while they blow in the opposite direction in the southern hemisphere. These winds tend to be much stronger over open water than they are across land, which has made them ideal for sailors.

14. LAND BREEZES AND SEA BREEZES: Land is better conductor of heat than water. hence in day time, the land gets hotter than in the sea. The air above the land becomes warm and rises up being lighter and some what cold air above sea surface moves towards the sea shore. This is known as sea breeze. During night land becomes cooler than water and so warm air over the water in sea rises up. The air on the land near the sea shore begins to move towards sea side and is called land breeze

15. RADIATION DEFINITION “The process in which heat energy travels from one system to another in the form of electromagnetic waves with no need of material medium is called Radiation.” EXPLAINATION : In radiation heat travels from one system to another in the form of electromagnetic radiations (photons). Thus no medium is required for this mode of transfer of heat.

16. Heat from the sun reaches earth by radiation

17. EXAMPLES OF HEAT TRANSFER A good example would be heating a tin can of water using a Bunsen burner. Initially the flame produces radiation which heats the tin can. The tin can then transfers heat to the water through conduction. The hot water then rises to the top, in the convection process. The atmosphere would be another example. The atmosphere is heated by radiation from the Sun, the atmosphere exhibits convection as hot air near the equator rises producing winds, and finally there is conduction between air molecules, and small amounts of air-land conduction.

18. APPLICATIONS OF RADIATION

19. A thermo scope is a device for detecting and displaying temperature changes. It consists of identical glass bulbs A and B which are connected to narrow U-shaped glass tube. The tube is filled with sulfuric acid and the space above acid levels in both the arms is filled with air. Bulb A is coated with lamp black so that it may completely absorb the heat radiation falling on it. When the temperature of the two bulbs is same, there is no difference in the acid level. When bulb A is exposed to heat, it absorbs heat and air in this arm expands resulting in difference of acid level. This thermo scope is very delicate and can detect very weak radiations even DIFFERENTIAL AIR THERMOSCOPE

20. BOY’S RADIOMICROMETER

21. BOY’S RADIOMICROMETER It is a combination of moving coil galvanometer and a thermocouple. It consists of a single loop of silver or copper wire A. the lower ends of the wire are soldered to a copper disc which is coated with lamp black. When disc is exposed to heat radiations, a thermo-electric current is produced in the couple made of Bismuth and Antimony and begins to flow in the wire A. Hence we get a current in the galvanometer. The deflection shows the amount of radiation.

22. THERMO FLASK – DEWAR’S BOTTLE

23. THERMO FLASK – DEWAR’S BOTTLE A  vacuum flask  (also known as a  Dewar flask ,  Dewar bottle  or  Thermos ) is an  insulating  storage vessel that keeps its contents hotter or cooler than its surroundings. Invented by  Sir James Dewar  in 1892. USE:Vacuum flasks are used domestically to keep  beverages  hot or cold for extended periods, and for many purposes in industry.

24. The vacuum flask consists of two  flasks , placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near- vacuum  which prevents heat transfer by  conduction  or  convection . Heat transfer by  thermal radiation  may be minimized by silvering flask surfaces facing the gap. Most heat transfer occurs through the flask's neck and opening, where there is no vacuum. Vacuum flasks are commonly made of  metal ,  glass ,  foam , or  plastic , and have their opening  stopper  with  cork  or plastic How does a thermo flask work????