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Physics Chapter 8 Short Questions Notes
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Class 9 Physics Chapter 9 Transfer of Heat Short Questions Notes
- Heat flows from a body at a higher temperature to a body at a lower, temperature.
- The mode of transfer of heat by vibrating atoms and free electrons in solids from hotter to colder parts of a body is called conduction of heat.
- The amount of heat that flows in unit time is called the rate of flow of heat.
- The rate at which heat flows through solids depends on the cross-sectional area of the solid, the length between hot and cold ends, the temperature difference between hot and cold ends, and the nature of the material.
- The rate of flow of heat across the opposite faces of a meter cube maintained at a difference of 1 K is called the thermal conductivity of the material of the cube.
- Good conductors are used for quick transfer of heat. Thus cookers, cooking plates, boilers, radiators condensers refrigerators, etc. are made of metals.
- Water is a poor conductor of heat.
- Materials that trap air are also bad conductors such as wool, felt, fur, feathers, polystyrenes, and fiberglass.
- Transfer of heat by the actual movement of molecules from a hot place to a cold place is known as convection.
- Land and sea breezes are also examples of convection.
- Gliders use the upward movement of hot air currents due to the convection of heat. Air currents help them to stay in the air for a long period.
- Birds are able to fly for hours without flapping their wings due to the upward movement of air currents.
- The term radiation means the continual emission of energy from the surface of a body in the form of electromagnetic waves.
- Radiations are emitted by all bodies. The rate at which radiations are emitted depends on various factors such as color and texture of the surface, temperature, and surface area.
- A dull black surface is a good absorber of heat as its temperature rises rapidly.
- A polished surface is a poor absorber of heat as its temperature rises very slowly.
- Radiations from the Sun pass easily through glass/polythene and warm up the materials inside a greenhouse. The radiations given out by them are of much longer wavelengths. Glass/polythene does not allow them to escape and thus maintains the inside temperature of the greenhouse.
- Earth’s atmosphere contains carbon dioxide and water vapors. It causes the greenhouse effect and thus retains the temperature of the Earth.
- The bottoms of cooking pots are made black to increase the absorption of heat from fire.
- White surfaces reflect more heat than colored or black surfaces. Similarly, polished surfaces are better reflectors than rough surfaces, and reflection of greater from polished surfaces. Therefore, We wear white or light-colored clothes in summer.
- We polish the interior of the cooking pots to reflect back most of the heat radiation inside the hot pots.
- A thermos flask consists of a double-walled glass vessel. It reduces the transfer of heat by conduction, convection, and radiation.
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