Thursday, June 11, 2009

Transfer Of Thermal Energy

What causes the transfer of thermal energy?
The answer is heat.
NOTE: Thermal Energy is transferred only when there is a difference in temperature

For example,
if you put your left hand into a pail of cold water and you put your right hand into a pail of hot water, your left hand would feel cold and your right hand will feel hot.
However, when you put your hands into a basin of water which as the same temperature as your hands, there is thermal equilibrium, which means your hands will feel neither cold nor hot.

How is thermal energy transferred?
- Conduction
- Convection
- Radiation

First watch the videos below

Part 1 (skip to 1:10)

Part 2

Definition: Conduction is the process of thermal energy transfer without any flow of the material medium.
If you had watched the first part video,
he had explained that when he put one iron rod and copper rod into hot oil, one side will be hotter then the other.

2 important ideas can be drawn from this:

1) Different materials conduct heat at different rates.
The faster and hotter the rod becomes, the better the conductor of heat it is.
Poor conductors of heat are also known as insulators (eg. wood)

2)Thermal energy flows through the material of the rods without any flow of the material itself. Such transfer of energy is called conduction.

Generally, metals are good conductors of heat and non-metals are poor conductors of heat.
Why is this so?
ALL solids are made up of tiny particles called atoms and molecules.
However, there is a difference between metals and non-metals that determines their conductivity.
Difference: Metals contain many free electrons which move randomly between atoms or molecules, while non-metal do not have such free electrons.

How does conduction work?
-When thermal energy is supplied at one end of a rod, the particles at that end would vibrate vigorously.
-It would then collide into its neighbouring particles, causing them to vibrate as well.
-Kinetic energy would also be transfered from the vibrating particles to its neighbouring particles.

In simple words, the kinetic energy is to make the particles collide so they can PASS the heat from one end to the other!

It is quite slow to pass thermal energy this way.
However, in metals, they have another much faster mechanism that takes place at the same time: FREE ELECTRON DIFFUSION

When a metal is heated, the free electrons in the metal gain kinetic energy and move faster a result. These fast-moving electrons then spread into the cooler parts of the metal and transfer kinetic energy to them, causing them to vibrate and pass the heat.

To put it simply, these free electrons do exactly what a particle does.
NOTE: ''at the same time'' means that besides the normal way of transferring thermal energy by the vibration of particles, THERE IS AN EXTRA MECHANISM which is the work of the free electrons.

This explains why good conductors like metals are capable of transferring thermal energy much faster than insulators (non-metals).

Conduction in liquids and gases
Thermal energy can also be conducted in liquids and gases from a hotter region to a cooler region. However, it is inefficient.
Reason: Particles in liquid and gases are spaced further apart than those in solids.
Thus collisions between molecules are less in liquids (and are even lesser in gases due to even more spaced apart particles as compared to liquid and solid) which results in slow transfer of kinetic energy from fast moving molecules to neighbouring molecules .
This is why air is a poor conductor of heat as compared to water and why water is a poor conductor of heat compared to most solids.


Definition: Convection is the transfer of thermal energy by means of currents in a fluid (liquids or gases).
Please watch the video below

The circulation of the purple streams of water represents convection current in water.

Explanation: When the water at the bottom of the flask is heated, it expands. The expanded water is less dense than the surrounding water and therefore it starts to rise. The cooler regions of the water in the upper part of the flask, being denser, sink.
NOTE: The reason to why heated water becomes less dense is due to the formula (Density = Mass/Volume). When the water is heated, the volume increases, causing the density to be lower.

Look at the convection corner.
The air above the fire gets heated and expands. Now the heated air above the fire is less dense than the surrounding air, thus causing it to rise to warm the hands. The surrounding air, which is less dense, would now sink to replace the denser air which has risen.

NOTE: Convection currents occur only in fluids such as liquids and gases but not in solids.
Reason: Convection requires bulk movement of the fluids which carry thermal energy. However, for solids, thermal energy is transferred from one particle to another by the vibrations of the particles, which means there is no bulk movement of the particles itself.
Definition: 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 (unlike conduction and convection)
This means that radiation can take place in vacuum, which convection and conduction cannot take place in.
One example of radiation is thermal energy reaching the Earth from the Sun.

The Sun emits electromagnetic waves. Part of this family of electromagnetic waves makes us feel warm. The group of electromagnetic waves is called infrared waves. The thermal energy from the infrared waves is called radiant heat.
NOTE: The thermal energy FROM infrared waves are called radiant heat. Infrared waves are NOT radiant heat.
All objects emit some radiant heat.
The hotter the object, the greater the amount of radant heat emitted.

Absorption of infrared radiation
Infrared radiation is absorbed by all objects and surfaces. The absorption of radiant heat causes a temperature rise.

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

Let's take the sun for example

As you can see, the radiant heat from the Sun is absorbed by the surface but there is radiant heat also being emitted from the surface itself.

In general, a good emitter of radiant heat is also a good absorber of radiant heat. Conversely, a poor emitter of radiant heat is also a poor absorber of radiant heat.
Reason: A good absorber reflects LITTLE radiant heat and since lesser heat is reflected, the object will become hotter. When it's hotter, it tends to lose heat faster to the surroundings Thus, it's a good emitter

Factors affecting rate of infrared radiation:
1)Colour and texture of the surface
2)Surface temperature
3)Surface area

Colour and texture of the surface
As you all have learnt in primary school, dull and black surfaces are better absorbers of infrared radiation compared to shiny and white surfaces. Similarly, this shows that dull and black surafces are better emitters of infrared radiation.

Surface Temperature
The higher the temperature if the surface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.

Surface Area
The object with a larger surface area will emit infrared radiation at a higher rate.

Compare square A and square B.
Which box do you think emits/absorbs more radiant heat?

Ans: Box A
Explanation: Firstly, Box A is dull and black compared to the shiny white box beside it
Secondly, Box A has a larger surface area compared to Box B

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