Unit 11: Transfer of Thermal Energy — Short Questions

The three primary types of heat transfer are conduction, convection, and radiation. Conduction is the transfer of heat through a material by direct contact between particles in solids. Convection is the transfer of heat through the actual movement of molecules (liquids and gases). Radiation occurs through electromagnetic waves without requiring a medium.

Heat travel faster in metals compared to non-metals.
Reason
Metals have free electrons that move freely and quickly transfer energy from hot to cold areas. Non-metals lack these free electrons and rely on slower lattice vibrations for heat transfer. This is why heat travels more rapidly through metals than non-metals.

When liquid or gas is heated, it becomes less dense and rises above the heat source. Cooler, denser fluid from the surrounding area moves downward to take its place. This cooler fluid then gets heated and rises again. This continuous circular movement of heated molecules from hot regions to cooler regions is called convection.

Land and sea breezes are caused by convection currents. During the day, land heats up faster than the sea because land has a lower specific heat capacity. The warm air above the land rises, and cooler air from the sea moves in to replace it, creating a sea breeze. At night, the land cools down more quickly than the sea, so the air above the sea becomes warmer and rises, while cooler air from the land moves towards the sea, creating a land breeze.

Birds and gliders use rising warm air currents called thermals, which are created by convection. They stretch their wings and circle in these thermals to gain altitude. In this way they glide over long distances without flapping their wings, saving energy.

Radiation is the transfer of heat through electromagnetic waves without needing a medium. Heat transfer occurs through radiation when energy travels across space in the form of electromagnetic waves. Example: Heat from the Sun reaches Earth by radiation through space.

A dull black surface emits and absorbs more radiation than a shiny silver surface.
Reason
A dull black surface is rough and dark, making it a good absorber and emitter of infrared radiation. A shiny silver surface is smooth and reflective, so it reflects most radiation and absorbs/emits very little. This is why a dull black surface heats up quickly and cools down faster, while shiny surfaces like polished metals stay cooler under the same conditions.

The greenhouse effect traps some of the Sun's heat in the atmosphere. Greenhouse gases such as carbon dioxide and water vapors trap the long-wavelength thermal radiations (infrared) emitted from the Earth's surface, reflecting them back into the atmosphere. This prevents excessive heat loss and maintains a warm, stable temperature on Earth.

In non-metal solids, atoms and molecules are tightly packed and vibrate around fixed positions. When one end is heated, the particles there vibrate more vigorously and collide with neighboring particles. These collisions transfer kinetic energy to the neighbours, causing them to vibrate faster and pass energy further along, slowly spreading heat through the solid.

A vacuum flask uses a double-walled structure with a vacuum in between to stop heat transfer by conduction and convection, as there is no medium for heat to travel. The inner walls are silvered to reflect heat radiation back into the liquid, while the plastic cap and outer casing act as insulators to prevent heat loss from the top and outside.

Double-glazed windows consist of two sheets of glass with an air gap or vacuum between them. Since air is a poor conductor and a vacuum prevents conduction and convection, this design stops warm air from escaping in winter and prevents outside heat from entering in summer, reducing the need for artificial heating or cooling.

When a fluid is heated, the molecules in the hot region expand and become less dense (lighter). These lighter molecules rise upward, while cooler, denser fluid from the surroundings sinks to take their place. This cooler fluid then gets heated and rises, creating a continuous circulation pattern known as a convection current that transfers heat.

During the day, the land heats up faster than the sea because land has a lower specific heat capacity. The air above the land becomes hot, expands, and rises. Cooler, denser air from over the sea then flows inland to replace the rising warm air, creating a breeze that blows from the sea toward the land.

At night, the land cools down much faster than the sea. As a result, the air above the relatively warmer sea remains warm and rises. The cooler, denser air from the land then flows out toward the sea to replace the rising air, creating a breeze that blows from the land to the sea.

Sunlight heats the ocean surface, causing warm water to stay at the top while cold water sinks. This temperature difference drives convection currents that continuously mix the ocean water. This mixing process moves oxygen from the surface to deeper layers and brings essential nutrients from the ocean floor up to the surface, sustaining marine ecosystems.

When a heater is turned on, it warms the air immediately adjacent to it. This warm air expands, becomes less dense, and rises toward the ceiling. Cooler, heavier air from other parts of the room sinks down to the floor to replace it near the heater. This cycle repeats, circulating warm air until the whole room is heated.

In a storage tank system, cold water enters the bottom and is heated by a gas burner or electric element. As the water heats, it becomes less dense and naturally rises to the top of the tank. When a tap is turned on, hot water is drawn from the top, while fresh cold water enters at the bottom to continue the cycle.

All objects constantly emit and absorb radiation. If an object is hotter than its surroundings, it emits more heat than it absorbs, causing it to cool down. Conversely, if it is cooler, it absorbs more than it emits, warming up. This process continues until the object and its surroundings reach the same temperature (thermal equilibrium).

The nature of a surface determines its ability to handle radiation. Dull, black, and rough surfaces are excellent absorbers of infrared radiation and heat up quickly. In contrast, shiny, white, and smooth surfaces are poor absorbers because they reflect most of the incident radiation, staying cooler for longer.

A Leslie's cube is a metal box with different faces (dull black, shiny silver, white, etc.) filled with hot water. A radiation detector placed at equal distances from each face measures the heat emitted. The experiment shows that the dull black surface emits the most radiation, while the shiny silver surface emits the least, proving surface nature affects emission.

The factors are (i) the colour of the surface,
(ii) the texture of the surface,
(iii) the surface temperature,
(iv) the surface area
Dull, dark, and rough surfaces are better emitters and absorbers than shiny, light, and smooth ones.

The rate of radiation is directly linked to these two factors. First, hotter objects emit significantly more energy than cooler ones. Second, a larger surface area allows more radiation to be emitted or absorbed simultaneously. For example, radiators use fins to maximize surface area for efficient heat transfer.

The glass or plastic walls of a greenhouse are transparent to short-wavelength solar radiation (light), allowing it to enter and warm the interior. However, they are opaque to the long-wavelength infrared radiation (heat) emitted by the warmed objects inside. This traps the heat within the structure, keeping it warm even in cold weather.

The Earth's atmosphere acts like the glass of a greenhouse. Gases such as carbon dioxide and water vapor allow solar energy to reach the surface but absorb and trap the heat radiated back from the Earth. This natural process maintains the planet's temperature at a level suitable for life, preventing it from freezing.

Human activities have significantly increased the concentration of carbon dioxide in the atmosphere. Since CO2 is a greenhouse gas, higher concentrations trap more terrestrial radiation than normal. This "enhanced" greenhouse effect leads to a rise in the Earth's average temperature, a phenomenon known as global warming.

Shiny and light-coloured surfaces are excellent reflectors of infrared radiation. When exposed to the Sun, they reflect a large portion of the incoming radiant heat rather than absorbing it. Dark surfaces, however, absorb most of the radiation, converting it to heat, which causes them to reach much higher temperature.

The dull black surface radiates more heat.
Reason: The emission of radiation depends on surface colour and texture. Dull and dark-coloured surfaces are naturally better emitters of thermal radiation. The shiny silver surface, being smooth and reflective, is a poor emitter of radiation and tends to reflect much of the heat back rather than emitting it. Therefore, when the Leslie's cube is filled with hot water at the same temperature, the dull black surface releases more thermal energy through radiation compared to the shiny silver surface.

A larger surface area allows an object to emit more heat at a faster rate through radiation.
Explanation: If an object has a larger surface area, it can emit or absorb more heat simultaneously because more of the surface is exposed to radiate energy. Radiators used for heating rooms are designed with lots of thin slots or fins to increase the surface area. The large number of fins effectively increases the total surface area which allows them to heat the room efficiently without occupying too much space.

Convection requires the actual movement of molecules. In solids, atoms and molecules are tightly packed and vibrate only around their mean positions; they cannot flow through the material. Therefore, Convection is not possible in solids.
Convection currents in liquids and gases: Liquids and gases are poor heat conductors because their particles are loosely packed and not in direct contact as in solids. To compensate for this poor conductivity, convection currents form in liquids and gases. When heated, the fluid becomes less dense and rises, while cooler fluid sinks to replace it, creating a circular motion that distributes heat throughout the fluid much more effectively than conduction alone could achieve.

Birds like eagles and gliders stay aloft for hours without flapping their wings by riding on thermal currents. The scientific principle involved is convection.
Explanation: Birds and gliders use rising warm air currents called thermals, which are created by convection. They stretch their wings and circle in these thermals to gain altitude. In this way they glide over long distances without flapping their wings, saving energy.

A hot metal object cools down because it radiates more heat than it absorbs from the cooler room.
Reason: All objects continuously emit and absorb thermal radiation. When hotter than surroundings, the object radiates more heat energy than it absorbs, losing thermal energy through radiation. This continues until its temperature equals the room temperature, reaching thermal equilibrium where emission equals absorption.