Transfer of Heat - Radiation

Radiation is the transfer of heat through invisible waves. Many natural objects like trees, mountains, and roads release heat through radiation. Scientists have created a special camera called an infrared camera that uses these heat radiations to see things at night. It helps monitor enemy movements during the night when regular cameras can’t work.

When heat rays hit an object:

• Some heat is absorbed, and
• Some heat is reflected.

How much heat an object absorbs depends on its colour and other properties. For example, dark colours absorb more heat, while light colours reflect more heat.

1. Aim: To observe how heat transfer works through conduction and radiation in different conditions.

2. Requirements: test tube, piece of ice, wire gauze, burner, candle, and test tube holder.

3. Procedure

A. Conduction experiment

• Take some water in a test tube.
• Wrap a piece of ice in wire gauze and place it at the bottom of the test tube.
• Hold the test tube at a slant using the holder.
• Heat the upper part of the water with a burner until it starts boiling.

Observation: Even though the water at the top boils, the ice at the bottom doesn’t melt.
Reason: Heat transfer by convection doesn’t happen because hot water stays at the top (less dense) and can’t sink.

The relation between density and convection

B. Radiation experiment

• Light a candle and hold your hands on either side, close but not touching.
• Slowly bring your hands closer to the candle.

Observation: You can feel the heat from the candle even though your hands don’t touch it.
Example: Similar radiation heat is felt from the sun or a bonfire, even though no solid medium exists between them and us.

Radiation

4. Conclusion

• Conduction: Heat does not always reach every part of a material if there is no movement (like in the ice experiment).
• Radiation: Heat can travel without any medium, as shown by the warmth felt from the candle or sun.

1. Aim: To observe which materials conduct heat well and which do not by testing various objects in hot water.

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

3. Procedure

• Place the steel spoon, copper strip, divider, pencil, and plastic ruler inside a glass beaker.
• Pour hot water (heated to 60°C–70°C) into the beaker.
• Wait for a few minutes to allow heat to transfer through the objects.
• Touch the outer ends of each object and record how hot they feel (very hot, hot, warm, or cool).

4. Observation Table