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CSEC Physics: Transfer of Thermal Energy

Thermal energy, as discussed in a previous post, is the kinetic energy possessed by the particles of a substance. The transfer of thermal energy can occur in three ways:

  • Conduction

  • Convection

  • Radiation


Conduction

Conduction occurs when two objects of different temperatures are in direct contact with one another. This is a transfer of internal heat from the object/area of higher temperature to the object/area of lower temperature due to the interaction of adjacent particles.

When heat is supplied to one end of an object, the particles gain kinetic energy. The particles then begin to move more quickly and bump into the particles next to them, causing the kinetic energy to be transferred further along.

As you might already know, solids are far better conductors than liquids and gases. This is because the particles are right up against each other in close proximity in the fixed structure of the solid, so when heat is supplied to one area of a solid, the particles will bump into each other far more frequently as they are packed so closely. In gases (and liquids) however, the particles are further apart, and so they bump into one another far less frequently, causing the heat to be conducted more slowly.

Conductivity varies between materials, being greatest for metallic solids, lower for nonmetallic solids, very low for liquids, and extremely low for gases.

Air, being a gas, is a very poor conductor. Because of this, materials that contain air are also poor conductors, and therefore, good insulators.

Take for example, expanded polystyrene which is used as an insulator. Expanded polystyrene contains many air-filled pockets within it. When heat is transferred through expanded polystyrene (EPS), it has to alternate through gas pocket and solid material, so that the heat must be transferred through many interfaces causing slower conduction.



Convection

Convection is the transfer of internal energy into or out of an object by the physical movement of a surrounding fluid that transfers the internal energy along with its mass. It is the transfer of thermal energy due to the movement of the particles in fluids.

This 'fluid motion' is caused by the difference in density between hotter regions of the fluid and cooler regions. As you know, heat causes particles to move faster, and therefore causes substances to expand and become less dense. The opposite happens when the substances lose heat. So, the less dense, hotter material rises as it is heated, and the denser, cooler material from the fluid sinks to take its place. These motions are called convection currents, and they continue for as long as there is a difference in temperatures within the fluid.

This temperature difference is most obvious when there is a source of heat on one side of the fluid, such as a heater near the floor of a room. The warm air at the bottom continuously moves upward, while the cooler air moves downward to be heated and then subsequently also moves upward.

You can see convection currents and how they occur in a pot of boiling water in the following diagram:

Diagram showing convection currents in boiling water


Convection also helps to explain sea and land breezes.


In the daytime, the sun heats up both the sea and the land. However, water has a higher specific heat capacity than the land, so the land heats up faster and has a higher temperature than the sea. The land heats up the air above it, causing that air to become less dense and rise. The cooler air over the sea rushes in to fill the space made by the air that rose. This is sea breeze.






At night however, the sea becomes warmer than the land, so the process reverses. This is land breeze.











Radiation

Radiation is the emission of energy as waves or particles or rays which does not require a medium. This internal energy transfer is in the form of electromagnetic waves. For most bodies on the Earth, this radiation lies in the infrared region of the electromagnetic spectrum. Radiant energy is either reflected or absorbed by matter. Energy that is absorbed increases the kinetic energy of the particles of the object and also therefore the temperature.

The sun's rays are transferred to the Earth via radiation. This is proof that no medium is needed for radiation to occur, since outer space contains no gases or other media.


All objects are constantly emitting electromagnetic radiation , absorbing electromagnetic radiation and reflecting it over a range of frequencies (based on their temperature). At lower temperatures, most of this electromagnetic radiation is in the infrared range. At higher temperatures, objects may emit visible light.


We can tell based on the material and its temperature what electromagnetic (EM) radiation it is emitting or absorbing.

When the rate of an object's emitted radiation > absorbed radiation, it means the object is cooling.

When the rate of an object's emitted radiation = absorbed radiation, it means the object is at the same constant temperature as its surroundings.

When the rate of emitted radiation < absorbed radiation, it means the object is heating up.


Although objects constantly absorb and emit radiation, not all the radiation is absorbed because some of it is reflected. So, the nature of the surface of any materials affects the relative amounts of radiation absorbed or emitted.

Some materials are better absorbers or emitters than others based on the following factors:

  1. texture of surface (rough, smooth)

  2. nature of surface (shiny, dull)

  3. colour of surface (black, white)

  4. area of surface


Dull (matte), rough surfaces with dark colours and a large surface area are good absorbers and good emitters of infrared radiation eg rough black surfaces.

Solar panels for hot water comprise of pipes carrying water to be heated set under a black surface to efficiently absorb the infrared radiation from the Sun.

Hot water radiators should have a dark coloured, matte surface, (preferably black, but they usually don't use that colour because they don't look very attractive!)

Smooth, shiny surfaces with light colours and small surface areas are bad absorbers and bad emitters of infrared radiation, eg white gloss paint, silver surface used in vacuum flask ('thermos flask') to minimise heat transfer by infrared radiation.

Light, shiny surfaces are good reflectors of infrared radiation, this maybe to keep heat in to keep things warm or to minimise heat radiation in to keep things cool eg a vacuum flask.


Good emitters/absorbers are bad reflectors. Bad emitters/absorbers are good reflectors.


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