chp10 Transfer of thermal energy

• Heat is the transfer of thermal energy from a hotter to a colder region.
• Thermal energy always flows from a region of higher temperature to a region of lower temperature. This means that thermal energy is transferred only when there is a difference in temerature.
• Thermal energy is transferred by any of these three processes: Conduction, Convection and Radiation.

Conduction

• It is the transfer of thermal energy through a medium, without the medium moving , is called conduction.
• It always takes place from a region of higher temperature to a region of lower temperature, and acts to equalize temperature differences.
• Conduction takes place in all forms of matter, viz. solids, liquids, gases and plasmas.
• In solids, it is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons.
• In gases and liquids, conduction is due to the collisions and diffusion of the molecules during their random motion. (e.g. stove)
• Different materials conduct heat at different rates. Another term for poor conductors of heat is insulator.

Substance Thermal Conductivity k [J/(s-m-C)]
1.Syrofoam 0.010
2.Air 0.026
3.Wool 0.040
4.Wood 0.15
5.Body fat 0.20
6.Water 0.60
7.Glass 0.80
8.Concrete 1.1
9.Iron 79
10.Aluminum 240
11.Silver 420
12.Diamond 2450



conduction in solids

All solids (metals and non-metals) are made up of tiny particles called atoms and molecules. The difference between metals and non-metals is that metals contain many free electrons which move randomly between the atoms or molecules, while non-metals do not have such free electrons.

example:

• The above two pictures shows the heat transfer in copper and wood.
• When thermal energy is supplied to one end of the rod, the particles at the hot end vibrate vigorously.
• These particles will collide with neighbouring particles, making them vibrate as well.
• Thus the kinetic energy of the vibrating particles at the hot end is transferred to the neighbouring particles.
• This process of transferring thermal energy from the hot end to the cold end by atomic or molecular viration takes place in both metal and non-metal. In this process, there is no transfer of particles.
• In metals , another much faster mechnism of thermal energy transfer takes place at the same time: free electron diffusion.
• when copper rod is heated , the free elctrons in the copper gain kinetic energy and move faster as a result. The fast moving electrons then diffuse or spread into the cooler parts of the metal.
• In this process, they collide with the atom in the cooler parts of the metal and transfer their kinetic energies to them.
• This is why good conductors like metals are capable of transferring thermal energy much faster than insulators(non-metals).

Conduction in liquids and gases

• In liquids and gases, thermal energy can also be conducted from a hotter region to a cooler region.
• The process is inefficient.

REASON

• The particles in liquids and gases are spaced further apart than those in solids.
• Collisions between molecules are less frequent in liquids and even lesser in gases.
• The transfer of kinetic energy from fast-moving molecules to neighbouring molecules is slower.
• This is why air is a poor conductor of heat as compared to water.

convection

• Convection is the transfer of thermal energy by means of currents in a fluid. (liquids or gases)

liquid:

• When the liquid at the bottom of the container is heated, it expands. The expanded liquid is less dense than the surrounding liquid and therefore starts to rise.
• The cooler regions of the liquid in the upper part of the container, being denser, sink.

Gases:

• The gas above the candle gets heated and expands. As the air is now dense than the surrounding air, it raises out of its container.
• The cooler surrounding air, being denser, sinks through the other container to replace the less dense air

Convection currents occur only in fluids such as liquids and gases but not in solids.

Radiation

• Radiation is the continual emission of infrared waves from the surface of all bodies, transmitted without the aid of a medium.
• Radiation does not require a medium for energy transfer. It can take place in a vacuum.
• . Heat transfer by radiation occur between solid surfaces, although radiation from gases is also possible. Solids radiate over a wide range of wavelengths, while some gases emit and absorb radiation on certain wavelengths only. When thermal radiation strikes a body, it can be absorbed by the body, reflected from the body, or transmitted through the body.

Absorption of infrared radiation:

• It's radiation is absorbed by all objects and surfaces. The absorption of radiant heat causes a temperature rise.

Emission of infrared radiation:

• Infrared radiation is emitted by all objects and surfaces.
• This emission causes the temperature of the objects themselves to fall.