10.3 – Transfer of Thermal Energy

  • Thermal energy flows from a region of higher temperature to a region of lower temperature.
  • When two objects of different temperatures are placed in thermal contact with each other, thermal energy transfer will take place between them.
  • Temperature of the initially hotter object will decrease and temperature of the initially colder object will increase until they reach the same temperature (provided there is no change in state).
  • We say they have reached thermal equilibrium.

Heat transfers between objects via three different mechanisms. Namely convection, conduction and radiation.

Conduction

Conduction is the transfer of thermal energy through a medium without any flow of the medium.

Occurs in:

Solids, liquids and gases.

Mechanism:

Thermal energy is transmitted through a medium from one particle to another through vibrations of particles (atoms or molecules) with their neighbouring particles.

Factors Affecting Rate of Heat Transfer via Conduction:

1. Presence of free electrons
Metals all contain free electrons. The process of atom-to-neighbouring-atom transfer is rather slow. Free electrons allow the energy to move much more rapidly through the solid.

2. Distance between particles
Closer packed particles allow for more rapid transfer of energy between particles. Solids (which have the closest packed particles) therefore tend to be better thermal conductors than liquids and gases.`

Applications:

Copper is used to make quality pans as it is a very good conductor. The handle of a pan is likely to be made of plastic or wood (poor conductors).

Materials with poor heat conducting abilities are used to make warm clothes or thermal insulation. Air is a poor conductor, clothes and hair/fur are good insulators as they trap air.

Convection

Convection is the transfer of thermal energy by means of convection currents in a fluid (liquid or gas) due to a difference in density.

Occurs in:

Fluids (liquids and gases)

Mechanism:

Thermal energy is transmitted from one place to another by the movement of heated particles of a gas or liquid due to density changes.

Rate of convection depends on the position of heat source and whether convection currents can be set up. [warm fluid rises, colder fluid sinks]

Factors Affecting Rate of Heat Transfer via Conduction:

1. Position of the heat source.
Not all heating of a fluid will produce convection currents. A heat source should be placed low down and the space above it should be large to allow a current of the fluid to flow.

Applications:

Sea breeze and land breezes.

Kettle
A kettle has the heating element at its base so that the water circulates around the water and the water heats evenly.

Water in contact with the heating element gets heated through conduction. The hot water expands, becomes less dense than the surrounding water and thus rises upwards forming the convection current. This moving water takes the heat away with it ensuring that the heat given to the water gets distributed all around the kettle.

Air Conditioners
An air conditioner is placed at the top of a room  so that air circulates around the whole room.

Radiation

Radiation is the transfer of thermal energy in the form of electromagnetic waves (e.g. infrared radiation) without the aid of a medium.

Occurs in:

Does not need a medium and so can occur in a vacuum. Also occurs in gases (e.g. air) and liquids.

Mechanism:

Heat sources emit electromagnetic waves (in the form of infra-red radiation).

These do not require any medium to travel.

The colour, surface texture, surface area, temperature of the emitting surface will determine the rate at which heat is emitted.

[dark/dull surfaces are good radiators or emitters of radiation].

Factors Affecting Rate of Heat Transfer via Radiation:

1. Colour of the Object
Objects that are dark in colour (black) will be the best emitters of IR and thus will cool fastest.

2. Surface Area
As IR is only emitted from the surface, the larger the surface the more heat that is able to be emitted.

3. Temperature of the Surroundings
The larger the difference between an object’s temperature and the surrounding temperature, the more heat will be transferred.

 

Applications:

Houses in hot countries and factory roofs are painted in white, light-coloured paint/ aluminium paint so as not to absorb too much heat.

The fins at the back of a fridge are painted black, and have a large surface area, This allows them to emit heat efficiently.

Greenhouse Effect

 

 

The same process occurs in the Earth.

  • Carbon dioxide gas (in the atmosphere) has similar absorption and transmission properties as glass in a greenhouse.
  • It absorbs the solar radiation and also traps the long wavelength radiation emitted from the Earth’s surface.
  • Overtime, the temperature on the Earth’s surface increases, contributing to global warming.
Example
State three ways in which heat is lost from a black cup of hot tea.

  1. The black surface will radiate heat away from the cup in the form of infra-red radiation.
  2. Convection currents will transport heat away from the heated air above the surface of the tea.
  3. Evaporation from the tea into the air causes cooling of the tea.
Example
Should the following surface be black or white? In each case explain why.

(a) The roof of the school
(b) A solar panel 
(c) The radiator of a car

(a) The roof of the school – WHITE. To reduce the amount of heat absorbed by the building, thus keeping the temperature lower.
(b) A solar panel – BLACK. To increase the amount of light absorbed by the panel.
(c) The radiator of a car – BLACK. To increase the amount of radiation from the radiator to help it lose heat faster and thus cool down the engine more effectively.
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