Environmental Issues - Radiation

The transfer of heat energy from one place to another via emission of EM energy (in a straight line with the speed of light) without heating the intervening medium is called radiation.

Procedure:

1. Light a candle and stand it upright.
2. Hold your hands on its two sides at some distance from the candle.
3. Bring your hands closer.

Observation: You feel warmth on both sides of the candle. Heat reaches your hands mainly by radiation.

How Does the Sun's Heat Reach Us?

The Sun is millions of kilometres away from us. There is no air between the Sun and the Earth — the Earth's atmosphere is only a thin layer of air close to the Earth. Yet, the heat of the sun reaches us.

Examples where heat reaches us mainly by radiation:

• Warming your hands near a candle flame
• Warming yourself near a bonfire
• Feeling warm in the morning sun in winter

The Science Behind Radiation

All objects possess thermal energy due to their temperature T (where T > 0 K). The rapidly moving molecules of a hot body emit electromagnetic (EM) waves travelling at the velocity of light. These are called thermal radiations.

These waves carry energy and transfer it to the low-speed molecules of a cold body on which they fall. This causes an increase in the molecular motion of the cold body, and its temperature rises.

• Radiation is therefore a two-step process: Conversion of thermal energy → electromagnetic waves (emission by hot body)
• Reconversion of EM waves → thermal energy (absorption by cold body)

Transfer of heat by radiation has the same properties as light (or EM waves).

Radiation of heat takes place from many objects in nature, such as

• Trees
• Mountains
• Stones
• Roads

A camera has been developed which uses these radiations to make our surroundings visible at night. It is called an infrared camera. Using this camera, it is possible to keep a watch on the movements of the enemy during the night.

Heat Absorption and Reflection

When heat rays fall on an object:

• A part of the heat is absorbed by the object.
• A part of it is reflected.

The ability of a substance to absorb heat radiation depends on its colour and also its intrinsic properties.

Apparatus: A test tube, a piece of ice, wire gauze, a burner, a candle, a test tube holder.

Procedure:

1. Take some water in a test tube.
2. Wrap a piece of ice in wire gauze and drop it into the test tube — it sinks to the bottom.
3. Using a test tube holder, hold the test tube at a slant.
4. Heat the upper part of the test tube.
5. Stop heating when the water starts boiling.
6. Observe the piece of ice at the bottom.

Observation: The water at the top boils, but the piece of ice at the bottom does not melt. The heat does not reach the bottom even though the upper part is heated.

Conclusion: The density of water decreases due to heating. Therefore, the heated water cannot sink, and the process of convection does not occur. Heat does not travel downward.

Apparatus: Two aluminium tins of the same size, two small glasses, water, thermometers, black paint.

Procedure:

1. Paint the outer surface of one tin with black paint and let it dry.
2. In both tins, place one glass each, filled with water at the same temperature.
3. Cover the tins with lids.
4. Keep them in the sun for two hours.
5. Measure the temperature of the water in the glasses in the two tins.

Observation: The temperature of water in the black-painted tin is higher than in the unpainted tin.

Conclusion: The black surface absorbs more heat radiation than the unpainted (shiny) surface. The ability to absorb heat radiation depends on the colour and intrinsic properties of the surface.

Apparatus: A steel spoon, a copper strip or rod, a divider from a compass box, a pencil, a plastic ruler, a glass beaker, and hot water (60°C–70°C).

Procedure:

1. Place all the objects in a glass beaker.
2. Pour hot water (heated up to 60°C–70°C) into the beaker.
3. Wait for a while.
4. Touch the outer end of each of the objects.
5. Record observations.

Conclusion: Some substances are good conductors of heat, while some are bad conductors of heat. Heat is easily conducted through a copper strip or pot, but not through plastic or wood.

• When water is heated from the top, its density decreases, and it stays at the top. Since hot water cannot sink, convection does not occur and the bottom remains cool.
• Radiation is the transfer of heat without a medium — through electromagnetic waves.
• Heat from the Sun reaches us through radiation across the vacuum of space.
• All objects above 0 K emit thermal radiation (electromagnetic waves).
• Radiation is a two-step process: thermal energy → EM waves → thermal energy.
• Black or dark surfaces absorb more heat radiation; absorption also depends on the intrinsic properties of the substance.
• An infrared camera uses the radiation emitted by objects to see at night — useful for military surveillance.
• Copper is an excellent conductor; plastic is a bad conductor (insulator).
• Heat readily conducts through metals (copper and steel) but not through non-metals (wood and plastic).