9th Class Physics Unit No.9 Transfer of Heat Notes
Conduction: Heat transfer in solids by vibrating atoms and free electrons. Metals are good conductors, while insulators are poor conductors.
Thermal Conductivity: The rate of heat flow through a solid depends on its cross-sectional area, length, and temperature difference between ends. It is given by the equation Rate of flow of heat = k * A * (T2 – T1) / L, where k is the thermal conductivity of the material.
Use of Conductors and Non-Conductors: Good conductors are used when quick heat transfer is required, while insulators are used to prevent heat flow and for thermal insulation in buildings and containers.
Convection: Heat transfer in liquids and gases by the actual movement of heated molecules from hot to cold parts of the fluid. It occurs due to the expansion of fluids on heating, causing them to rise and be replaced by cooler fluid.
- Chapter No.1 Introduction to Biology
- Chapter No. 2 Solving a Biological Problem
- Chapter No.3 Biodiversity
- Chapter No.4 Cells and Tissues
- Chapter No.5 Cell Cycle
Land and Sea Breezes: They are examples of convection in nature. During the day, land gets heated faster, causing air to rise and create low-pressure areas. Cooler air from the sea flows in to replace it, causing sea breeze. At night, the land cools faster, causing high-pressure areas. Air flows from the land to the sea, causing land breeze.
Solar Radiation: Transfer of heat through electromagnetic waves. Sun emits radiant energy, including visible light and infrared rays. The Earth absorbs this energy and re-emits it in the form of infrared radiation, leading to the greenhouse effect and maintaining a moderate temperature on the planet.
Use of Solar Energy: Solar energy is harnessed using solar panels to generate electricity and for heating water.
Gliding and Birds Flight: Gliders and birds use thermals, rising columns of hot air due to convection, to stay in the air for extended periods without the need for engine power. They glide and soar on these thermal air currents.
These processes play a significant role in the transfer and distribution of heat in various natural and human-made systems.
Radiation is the mode of transfer of heat from one place to another in the form of waves called electromagnetic waves. Here are some key points about radiation and its applications:
Heat from the Sun reaches the Earth through radiation, as the space between the Sun and Earth’s atmosphere is empty.
Radiation is the process by which heat energy is transferred through electromagnetic waves.
All objects emit radiation, and the rate at which radiations are emitted depends on various factors such as color, texture, surface temperature, and surface area.
Leslie’s cube is used to compare the emission and absorption of radiation by different surfaces. Dull black surfaces are good absorbers and emitters of heat, while polished surfaces are poor absorbers and emitters.
The transfer of heat by radiation is also affected by the surface area of the body. Larger surface areas transfer more heat.
The greenhouse effect is a natural process where greenhouse gases in the Earth’s atmosphere trap some of the heat radiated by the Earth’s surface, maintaining the temperature inside the greenhouse. However, an increased concentration of greenhouse gases can lead to global warming.
Objects with black and rough surfaces absorb more heat, while white or polished surfaces reflect more heat.
Heat radiations follow the laws of reflection, and the amount of heat reflected depends on the color and nature of the surface.
Application of radiation: Cooking pots are made with black bottoms to increase heat absorption from the fire. White or light-colored clothes are worn in summer to reflect most of the heat radiation. Polishing the interior of cooking pots helps to reflect back heat radiation within them.
(a) Explain why a metal feels colder to touch than wood kept in a cold place?
A metal feels colder to touch than wood kept in a cold place because metals are good conductors of heat, while wood is a poor conductor. When you touch a metal object, it quickly conducts heat away from your skin, making it feel cold. On the other hand, wood does not conduct heat as efficiently, so it does not draw heat away from your skin as rapidly, and it feels relatively warmer to touch.
(b) Explain why land breeze blows from land towards the sea?
Land breeze blows from land towards the sea during the night because of the temperature difference between the land and the sea. At night, the land cools down more quickly than the sea due to differences in their heat-retaining capacities. As a result, the air above the land becomes cooler and denser than the air above the sea. This denser cool air over the land creates higher pressure compared to the lower pressure over the relatively warmer sea. The air flows from the high-pressure area (land) to the low-pressure area (sea), resulting in a land breeze.
(c) Explain why a double-walled glass vessel is used in a thermos flask?
A double-walled glass vessel is used in a thermos flask to reduce heat transfer through conduction, convection, and radiation. The space between the two walls of the vessel is evacuated to create a vacuum, which minimizes heat transfer through conduction and convection. Additionally, the inner and outer walls are silvered to reduce heat transfer by radiation. This way, the thermos flask can keep the contents hot or cold for an extended period by preventing heat from entering or escaping.
(d) Explain why deserts soon get hot during the day and soon get cold after sunset?
Deserts soon get hot during the day and cold after sunset due to their low moisture content and lack of vegetation. During the day, the desert ground absorbs a significant amount of solar radiation, which causes the temperature to rise rapidly. As there is little moisture or vegetation to absorb this heat, the desert can become scorching hot during the day.
After sunset, there is little to retain the absorbed heat, and the desert ground starts radiating the stored heat back into the atmosphere. Deserts have low humidity, so the heat is lost quickly by radiation to the clear night sky. This rapid loss of heat causes the desert to become cold during the night, resulting in significant temperature fluctuations between day and night in desert regions.
Why conduction of heat does not take place in gases?
Conduction of heat does not take place in gases because gases have a weak molecular structure with large intermolecular spaces. In solids, heat conduction occurs due to the vibration and movement of tightly packed molecules, which pass on kinetic energy to neighboring molecules through collisions. However, in gases, the molecules are widely spaced, and there are fewer opportunities for collisions between molecules. As a result, there is no efficient transfer of heat from one molecule to another, and conduction is not a significant mode of heat transfer in gases.
What measures do you suggest to conserve energy in houses?
To conserve energy in houses, several measures can be implemented:
Insulation: Properly insulating walls, ceilings, and floors helps to reduce heat loss during winters and heat gain during summers, making heating and cooling systems more efficient.
Energy-Efficient Appliances: Use energy-efficient appliances and lighting to reduce electricity consumption.
Programmable Thermostats: Install programmable thermostats to regulate indoor temperatures and optimize heating and cooling according to the occupants’ schedule.
Seal Leaks: Seal any air leaks around windows, doors, and vents to prevent unwanted heat exchange between the indoor and outdoor environment.
Natural Ventilation: Use natural ventilation during moderate weather conditions instead of relying solely on air conditioning or heating systems.
Renewable Energy Sources: Consider installing solar panels or other renewable energy sources to generate clean energy for household use.
Energy-Efficient Windows: Install energy-efficient windows that help to maintain indoor temperatures and reduce the need for heating or cooling.
Why transfer of heat in fluids takes place by convection?
Transfer of heat in fluids takes place by convection due to the movement of fluids (liquids and gases). When a region of the fluid is heated, it becomes less dense and rises, while the cooler, denser fluid moves in to take its place. This creates a continuous flow of fluid, which results in the transfer of heat from one part of the fluid to another. This process is called convection, and it is an efficient mode of heat transfer in fluids because it involves the actual movement of the heated material.
What is meant by convection current?
A convection current refers to the circular flow of fluid (liquid or gas) that occurs when there is a temperature difference within the fluid. Convection currents are responsible for transferring heat from one part of the fluid to another. In a convection current, the heated fluid rises, while the cooler fluid sinks, creating a continuous circulation or flow pattern.