Heating and CoolingMS ANNEAR

Unit Description

Heat Transfer: Conduction Process by which thermal energy is

transferred from one place to another Most significant in solids, important in

liquids and of lesser importance in gases Materials that conduct readily are referred

to as good conductors and materials that conduct heat poorly are referred to as insulators

How? Heat Transfer by molecular collisions Kinetic Molecular Theory of matter (Particles in a solid are

constantly vibrating within the crystal structure and interact with neighbouring particles)

If one part of a material is heated then the particles in that region will vibrate more rapidly and transfer energy to neighbouring particles.

Process is slow: mass of each particle is large and their vibrational velocities are relatively low, therefore are slow to pass on their vibrational energies.

Substance that purely rely on this method of conduction are likely to be poor conductors: plastic, glass, wood, paper, ceramic and concrete

How?Heat transfer by free electrons

Metals and some other substances contain electrons that are not involved in a particular chemical bond and are free to move through the solid. These are referred to as delocalised.

These electrons have a very small mass compared to atoms and even a small energy gain from heating will result in a very large fain in velocity.

These delocalised electrons transfer heat energy throughout the whole material very quickly.

Therefore good electrical conductors are also good heat conductors.

Factors affecting thermal conduction

The rate at which heat energy is transferred through any object will depend upon the: Temperature difference across material [T] Thickness of material [L] Surface area [A] Nature of material (thermal conductivity) [k]

Thus heat transfer (energy per time) through a material is given by

Convection

Convection is the transfer of heat energy within a fluid (liquid or gas).

If one part of the fluid is heated the material there will expand and become less dense. The hotter material, being less dense, will rise and the colder denser materials will tend to sink.

Even though liquids and gases are not good conductors they can transfer heat quite rapidly through convection.

This circulation of a material due to convection, is called a convection current.

Convection

What happens to the smoke once the candle is places underneath the chimney/exit tube.

Explain by observation and by description of particles

Convection: Sea Breeze vs Land Breezehttps://www.youtube.com/watch?v=gM0d3fGew-0

Radiation Common Knowledge that the earth is heated by the sun through

the suns radiation. Heat energy usually transferred by action of particles.

Radiation represents a case where heat is transferred without the movement of particles

Particles?

Radiation Radiation represents a case where heat is transferred

without the movement of particles Radiation is another expression for electromagnetic waves (which

include visible, ultraviolet and infrared light) Electromagnetic waves travel at the speed of light (c) and can be

focused, refracted and reflected Radiation follows other wave-like phenomenon as when in

contact with an object radiation is partly reflected, partly transmitted and partly absorbed.

The absorbed part transfers heat energy to the absorbing object causing it to rise in temp.

Radiation is emitted from all objects who temp is above absolute zero (-2730

) Thermograph / heat signature

Radiation: Emission and Absorption All objects emit some radiation, but all do not emit or absorb at

same rates. Eg. Matt black surfaces emit radiant energy at a greater rate than shiny, light coloured surfaces.

Black matt surface heat up faster than shiny light surfaces. Black matt surfaces also cool down faster. Eg. Car radiator painted black to increase emission of energy collected from car engine by conduction and convection.

Other factors that affect rate of both emission and absorption of radiation: Surface area Temperature Wavelength of incident radiation Surface colour and texture (emissivity e, varies in between 0 and 1)

1:better emission and absorption (black rough) 0: poor emission and absorption (white shiny)

6.8 and Set 11 Review