Transfer of Thermal Energy

Transfer Of Thermal Energy : Transfer Of Thermal Energy

Thermal Energy is a form of internal energy possessed by all material matter due to random motion of atoms and minute particles(kinetic molecular model of matter), and the amount depends on the temperature of the material region. : Thermal Energy is a form of internal energy possessed by all material matter due to random motion of atoms and minute particles(kinetic molecular model of matter), and the amount depends on the temperature of the material region.

What causes transfer of Thermal Energy? : What causes transfer of Thermal Energy? Thermal Energy is transferred only when there is a difference in temperature.(from a region of higher temperature to a region of lower temperature Thermal Energy always flows from a region of higher temperature to a region of lower temperature.

Slide 4 : Means by which thermal energy is transferred: Conduction Convection Radiation

Conduction : Conduction transfer of thermal energy from one place to another without any flow of the medium.

How does conduction work? : How does conduction work? Conduction works in two ways: Atomic collisions Free electron diffusion

Atomic Collisions : Atomic Collisions In a solid, particles(atoms or molecules), vibrate more vigorously or have more kinetic energy in a hotter region than in a the colder region. These particles will collide with neighbouring particles and transfer some of their kinetic energy. The neighbours collide with their neighbours. In this way, heat is conducted to colder regions and temperature increases. This is a slow process.

Free Electron Diffusion : Free Electron Diffusion In mostly metals, another much faster mechanism, free electron diffusion, takes place. Free electrons in metals gain kinetic energy and move faster. These fast moving electrons then diffuse or spread into the cooler parts of the metal. In this process, they collide with atoms in the cooler parts of the metal and transfer their kinetic energies on them. This is why metals are good conductors and are capable of transferring thermal energy much faster than insulators.

Conduction in liquids and gases : Conduction in liquids and gases The particles in liquids and gases are spaced further apart than those in solids. Therefore, collisions between molecules are less frequent in liquids and even lesser in gases. This explains why air is a poor conductor compared to water, which in turn is a poor conductor of heat compared to most of the solids.

Worked example : Worked example Why does the food keep longer if the walls are made of rigid plastic rather than of metal with double skin Solution: Bacteria becomes active in warm condition. Metals are good conductors. Metal container absorbs more heat easily from atmosphere and warms up air inside the container which leads to food decay. Plastic is a good thermal insulator and is better at keeping food fresh. With double skin, a thin layer of air will be trapped in between. Air, being a poor conductor of heat, minimizes heat gained from the atmosphere.

Convection : Convection transfer of thermal energy by means of currents in a fluid(liquid or gases).

How does convection work? : How does convection work? When a glass of liquid, such as water, is heated from the bottom, the layer of water closer to the heat source expands and hence becomes less dense compared to the water layer above it. Expanded water is less dense than the surrounding water and therefore it rises. The cooler regions of the water in the upper part of the flask, being denser, sink. This movement of liquid due to a difference in density sets up a convection current. Convection cannot occur in solids because the particles are in fixed positions.

Worked Examples : Worked Examples The diagram below shows an experiment that demonstrates convection currents in water. Why does the hot water rise? Solution Heat is transferred to the water and increases its surrounding temperature. The heated water expands. The density of the otter water is now reduced. The hot water rises and cooler water takes its place.

Radiation : Radiation transfer of thermal energy from one place to another by means of infrared radiation, without a need of an intervening medium. For example the heating of the earth by the sun. Thermal energy from infrared waved is called radiant heat.

Factors affecting the rate of infrared radiation. : Factors affecting the rate of infrared radiation. Colour and texture of the surface * Black, dull surfaces are better absorbers and emitters of radiation compared to white and shiny surfaces. Surface temperature *The hotter the object the more energy it radiates. Surface Area *The greater the area the more energy it radiates.

Worked examples. : Worked examples. A piece of ordinary kitchen aluminium foil is used to warp around food to be cooked in a barbecue fire. The foil has a shiny side and a dull side. Which of side should be on the outside, and why? Solution: Dull side should be on the outside because it absorbs radiation faster and the shiny side should be on the inside to reduce heat loss due to radiation from food.

Applications of Thermal Energy Transfer-Conduction : Applications of Thermal Energy Transfer-Conduction Uses of good conductors of heat Cooking Utensils- often made with metals where direct heating is involved Soldering iron rods- made of iron with the tip made of copper as copper is a better conductor of heat than iron Heat Exchangers- used in a large laundry facility, help save energy.

Applications of Thermal Energy Transfer -Conduction : Applications of Thermal Energy Transfer -Conduction Uses of Bad conductors of heat Handles of appliances and utensils- made of bad conductors of heat, so that our hands would not get scalded. Table mats- made of cork so that hot kitchenware can be placed on them without damaging the table top. Sawdust-used to cover the ice cubes to delay melting Wooden ladles-for stirring or scooping hot soup and also for scooping rice. Woollen Clothes –keep people warm on cold days Fibreglass, felt and expanded polystyrene-trap large amounts of air, employed as insulators in the walls of houses, iceboxes and refrigerators.

Applications of Thermal Energy Transfer-Convection : Applications of Thermal Energy Transfer-Convection Electric Kettles –heating coil of an electric kettle is always placed at the bottom of the kettle to aid transfer of thermal energy in water by convection. Household hot water systems-designed based on the convection in liquids. Air conditioners-always installed ear to the ceiling of a room to facilitate setting up on convection currents Refrigerators –convection is used to circulate cold air around the food.

Applications of Thermal Energy Transfer -Radiation : Applications of Thermal Energy Transfer -Radiation Teapots-Shiny teapots can keep tea warm for a longer time than black teapots. Greenhouses-short infrared radiation from the sun passes easily through the glass panels of a greenhouse, and is absorbed by the plants and soil inside. The plants in turn also radiate energy, but with much longer wavelength. This radiation is “trapped” by the glass panels. Thus the temperature inside the green house increases. Vacuum Flasks(next slide)

Vacuum Flask : Vacuum Flask Used to keep hot liquids Conduction and convection through the sides of the flask are prevented by the vacuum. The shiny walls of glass minimizes heat transfer by radiation by reflecting most of them. The outer container are made of poor thermal conductors. Hollow plastic stopper This silvered walls of glass Contents Vacuum Outer container

Bibliography : Bibliography thomas bond,chris hughes ,physics o-level guide Charles Chew, Chow Siew Foong,Physics Matters Cheng Chung Yu,o-LEVEL Science Physics Examination Notes Yahoo and google search engines