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Experiment to investigate the....

Absorption of radiation by a black(dull) and silvery surface

Appratus needed: 1. 2 temperature sensors
2. data logger
3.100w bulb
4.aluminium foil

Procedures: 1. place the 2 sensors at equal distance away from the 1002 bulb.
2. connect the 2 sensors to the data logger where a graph of temperature
against time will be plotted.
3.one sensor is blackened and the other wrapped with aluminium foil.
4.switch on the sensors and the light bulb.
5 .after some time, switch off the light bulb.


conclusion:1. the blackened sensor shows a higher temperature recorded.
2.therefore, black(dull) surfaces are better absorbers of radiation.



Emission of radiationby a black(dull) and slivery surface

Appratus needed: 1. temperature sensors
2.data logger
3. 2 identical tins(one painted black and one with silvery surface)
4. boiling water maintained at 100 c.


Procedures: 1.place the temperature sensors into the tins filled with boiling water.
2.one tin is blackened on the surface and the other with silvery surface.
3. start the data logger
4.stop the data logger after some time
5.the graph is plotted with temperature against time.


conclusion: 1. the graph shows that the temperature of water in the blackened tin drops
faster compared to the water in the silvery tin.
2.the blackened surface is a better emitter of radiation.

common applications of radiation

1.teapots

Since shiny surfaces are bad emitters of radiation, shiny teapots can keep tea warmer than a black teapot for a longer time. In addition, since shiny surfaces are bad absorbers of radiation, shiny containers can keep a cold liquid cooler for a longer time than black containers.

2.Greenhouse

The greenhouse traps heat to help plants grow better. On a warm day, infra-red radiation from the Sun passes through the glass of the greenhouse and is absorbed by the soil and plants inside. The soil and the plants therefore become warmer and they emit infra-red radiation of longer wavelengths. These longer wavelengths of radiation are unable to pass through the glass and so heat is trapped inside the greenhouse. This causes the temperature inside the greenhouse to rise and this is good for plant growth.

3.Vacuum flasks

It is designed to keep liquids hot by minimising heat losses in 4 possible ways-Conduction, Convection, Radiation and Evaporation.
Conduction and covection through the sides of the flask are prevented by the vacuum between the double-glass walls of the flask. Conduction through the trapped air above the liquid is mionimal since air is a very poor conductor of heat. The stopper is made of plastic which is also a poor conductor of heat.
Convection and evaporation can only occur when the plastic stopper is removed during use. Heat loss by radiation is harder to stop as radiant heat can pass through vacuum.
The minimise heat loss through radiation, the walls of the glass are silvered to reflect the radiant heat back into the hot liquid.

common applications of convection

1. Electric kettles
the 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. when the power is switched on ,the water near the heating coil is heated up, expands and becomes less dense. The heated water therefore rises while the cooler regions in the upper part of the body of water descend to replace the heated water.

2.household hot water systems

1Water is heated in the boiler by gas burners. The hot water expands and becomes less dense. Hence, it rises and flows into the upper half of the cylinder.

2To replace the hot water, cold water from the cistern falls into the lower half of the cylinder and then into the boiler due to the pressure difference.

3The overflow pipe is attached to the cylinder just in case the temperature of the water becomes too high and causes a large expansion of the hot water.

4 The hot water tap is led from the overflow pipe and must be lower than the cistern so that the pressure difference between the cistern and the tap causes the water to flow out of the tap.

3.Air conditions

The rotary fan inside an air conditioner forces cool dry air out into the room. The cool air, being denser, sinks while the warm air below, being less dense, rises and is drawn into the air conditioner where it is cooled.

4.Refrigerators

Refrigerators work in very much the same way as air conditioners. The freezing unit is placed at the top to cool the air so that the cold air, being denser, sinks while the warm air at the bottom rises. This sets up convection currents inside the cabinet which help to cool the contents inside.

common applications of conduction

use of good conductors of heat

1. cooking utensils (e.g.kettles) usually made of aluminium or stainless steel where direct heating is involved.
2. soldering iron rods are made of iron with the tip made of copper .
3. heat exchangers

use of bad conductors of heat (insulators)

--insulators is to minimise loss of thermal energy , or prevent thermal energy from being transferred quickly.

1. handles of appliances and utensils -----made of poor conductors (e.g.wood,plastic)
2. table mats----made of cork so that hot kitchenware can be palced on them without damaging the table-top.
3. sawdust----used to cover ice blocks because of its good insulating property.
4. wooden ladles----stirring or scooping hot soup and scooping rice that has just been cooked.
5. wollen clothes----keep people warm on cold days.
6. fibreglass,felt and expanded polystyrene foam-----trap large amouts of air are employed as insulators in the walls of houses,ice boxes and refrigerators. Double-glazed windows have air trapped between two panes of glass,which reduce thermal energy transfer through the windows.

factors affecting the rate of infrared radiation

1. colour and texture of the surface
--dull and black surfaces are better absorbers of infrared radiation than shiny ,white surfaces.
--dull and blck surfaces are better emitters of infrared radiation.

2.surface temperature
--the higher the temperature of the surface of the object relative to the surrsounding temperature, the higher the rate os infrared radiation.

3.surface area
--two object of the same mass and material, but with different surface areas, the object with the larger surface are will emit infrared radiation at a higher rate.

Absorption and Emission of Radiation