Heat transfer: Conduction, Convection & Radiation

 

What is heat transfer?

• Heat transfer is the exchange of thermal energy between physical objects.

• Heat will naturally flow from a hotter to a colder object (2nd Law of Thermodynamics)

• Thermal equilibrium happens when all involved objects and their environment reach the same temperature.

• There are essentially three ways that heat can be transferred: Conduction; Convection; & Radiation

 

Conduction

• Conduction is the main method of heat transfer within solid objects or between solid objects in contact with each other.

• Molecules in the hotter part of the object vibrate faster than the molecules in the cooler parts. The faster moving molecules transfer part of the energy to their slower moving neighbours – so transferring heat through the object

• A ‘steady state’ is achieved when the heat entering the object at one side is balanced by that being emitted from the other side. Throughout the period of time, the object’s heat remains constant.

• Metals are highly conductive whereas gases are not. 

 


 

Convection

• Liquids and gases are fluids.


Typical example of convection used to warm space 


• Convection is the movement of heat via currents as heated molecules move from hot places to cooler places.

- When molecules are heated in a fluid, they vibrate faster.

- Higher temperature fluid molecules expand the space they occupy.

- The resulting heated space is less dense compared with the cooler spaces

- The heated, less dense fluid rises to replace the denser, cooler fluid, which in turn sinks into the warmer areas.

- The repeated rising and falling of warming and cooling liquid or gas sets up the convection current.

- Through warming up one side of a solid container of fluid, the convection currents can transfer heat to the other sides; or heat between two solid objects sandwiching a fluid.

 

 

 

Radiation



Electromagnetic Radiation & Electromagnetic Spectrum (courtesy Astronomy News)

• All objects absorb and emit thermal radiation (aka infrared electromagnetic radiation).

• Heat energy is transferred via waves, not particles.

• The Earth is heated through thermal radiation from the Sun.


Solar radiation heating concrete floor

• Thermal radiation is emitted as a result of the random movements of atoms and molecules in matter. Since these atoms and molecules are composed of charged particles (protons and electrons) their movement causes the emission of electromagnetic radiation, which carries energy away from the surface of the body.

• Thermal radiation is absorbed well by :

- Water, water vapour, glass, wood, brick, stone, concrete, asphalt, copper,

• Thermal radiation is reflected well by :

- Aluminium foil

 

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