Transfer of Heat -Conduction, Convection & Radiation Class 7

Understanding Heat Transfer: Concepts and Applications

This section explores the fundamental principles of heat transfer, covering various methods and their practical implications.

Laboratory Thermometer vs. Clinical Thermometer

• Similarities: Both laboratory and clinical thermometers measure temperature. They rely on the principle of thermal expansion of a liquid.
• Differences: Laboratory thermometers typically have a wider range and are used for scientific experiments. Clinical thermometers are designed for measuring body temperature and have a narrower range, often with a more sensitive scale. Clinical thermometers are usually made of glass and contain mercury or alcohol, while laboratory thermometers can use various fluids.

Conductors and Insulators of Heat

• Conductors: Materials that allow heat to pass through them easily.
  • Examples: Metals (like copper, aluminum, iron) are excellent conductors.
  • Examples: Water is a good conductor of heat.
• Insulators: Materials that resist the flow of heat.
  • Examples: Wood is a good insulator.
  • Examples: Air is a relatively poor conductor, acting as an insulator.

Fill in the Blanks

(a) The hotness of an object is determined by its temperature.
(b) Temperature of boiling water cannot be measured by a maximum thermometer.
(c) Temperature is measured in degree Celsius or Fahrenheit.
(d) No medium is required for transfer of heat by the process of radiation.
(e) A cold steel spoon dipped in hot milk experiences heat transfer to its other end by the process of conduction.
(f) Clothes of dark colours absorb more heat better than clothes of light colours.

Matching

Match the following:

(i) Land breeze blows during (a) summer
(ii) Sea breeze blows during (b) winter
(iii) Dark coloured clothes are preferred during (c) day
(iv) Light coloured clothes are preferred during (d) night

• (i) - (a)
• (ii) - (b)
• (iii) - (c)
• (iv) - (d)

Why Layering Clothing Keeps You Warmer

Wearing multiple layers of clothing during winter provides better insulation than wearing a single thick garment. Each layer traps a layer of air, which acts as a barrier to heat loss. This trapped air prevents body heat from escaping, keeping you warmer.

Heat Transfer Mechanisms

(Refer to Fig. 4.13)

• Conduction: Heat transfer through direct contact.
• Convection: Heat transfer through the movement of fluids (liquids or gases).
• Radiation: Heat transfer through electromagnetic waves.

White Walls in Hot Climates

In hot climates, painting outer walls white is recommended because white surfaces reflect most of the sunlight and heat, preventing the house from absorbing excessive heat and keeping the interior cooler.

Mixing Water of Different Temperatures

When one liter of water at 30°C is mixed with one liter of water at 50°C, the final temperature will be 40°C. This is because the heat lost by the hotter water will be equal to the heat gained by the colder water, resulting in a uniform temperature.

Heat Transfer in a Metal Ball and Water

When an iron ball at 40°C is dropped into a mug of water at 40°C, there will be no net heat flow. The iron ball will reach thermal equilibrium with the water, and the temperature of both will remain at 40°C.

Cold Spoon and Ice Cream

The other end of a wooden spoon dipped in ice cream becomes cold by the process of conduction. Heat flows from the warmer part of the spoon (in contact with the ice cream) to the colder part (in contact with the air).

Copper Bottoms in Pans

Stainless steel pans often have copper bottoms because copper is a better conductor of heat than stainless steel. This ensures more efficient and even heat distribution across the pan's surface, leading to better cooking results.