Friday, June 19, 2009

Applications of Thermal Energy Transfer

Common Applications of Conductions
Uses of good conductors of heat:
If thermal energy has to be transferred quickly through a substance, good conductors of heat such as metals are used.
1) Cooking utensils - Kettles and pots are made of aluminium or stainless steel where direct heating is involved.

2)Soldering iron rods - Made of iron with a copper tip as copper is a better conductor of heat compared to iron.

3)Heat Exchangers - made of copper so that the cold water can cool the heated water and the heated water can warm the cold water up.

Uses of bad conductors of heat (insulators)
Insulators are useful if we want to minimise loss of thermal energy, or prevent thermal energy from being transferred quickly.

Common uses of insulators:
1) Handles of appliances and utensils: utensils such as kettle and pans have handles that are insulators of heat. This way, the hot utensil can be picked up without scalding our hands.

2) Table mats: Usually made of cork so that hot kitchenware can be placed on top of them so as not damage the table. (for example, melting the plastic covers with hot pots)

3) Sawdust: Used to cover ice blocks to keep ice frozen during the summer.

4) Wooden ladles: Useful for stirring/scooping hot soup and for scooping rice that has just been cooked.

5) Woollen clothes: Used to keep people warm on cold days

6) Fibreglass, felt and expanded polystrene foam: Used to trap air to reduce thermal energy transfer between windows and walls.

Common Applications of Convection
1)Electric Kettles - The heating coil will be placed at the bottom of the kettle to aid transfer of thermal energy in water by convection. When the kettle is being heated up, the water around the heating coil will be heated up and it expands, causing it to be less dense . The heated water would then rise while the cooler regions in the upper part of the kettle would then descend to replace the heated water.

2) Heater - Household hot water system is designed based on the convection in liquids.
a) Water is heated in the boiler. The hot water wexpands and become less dense. It then rises and flows to the upper half of the cylinder.
b)To replace the hot water, cold water from the cisterm falls into the lower half of the cylinder and then it will eventually flow into the boiler due to the pressure difference.
c) The overflow pipe is attached to the top half of the cylinder just in case the temperature of the water gets too high and causes a large expansion of water. If the water expands a lot, it go into the overflow pipe which leads back to the cistern.
d) The hot water tap which is led from the overflow pipe must be lower than the cistern so that the pressure difference between the cistern and the tap causes the water to flow out of the tap

3) Air conditioner - It is always installed near to the celing of a room to facilitate setting up of convection currents. The rotary fan inside an air conditioner releases cool dry air into the room. As cool air is denser, it sinks. The warm air below would then rise and is drawn into the air conditioner where it is cooled. This way, the air is recirculated and the temperature of the air would eventually fall to the desired value.

4)Refrigerators - It works the same way as the air conditioner. The freezing unit is usually placed at the top to cool the air and facilitate setting up of convection currents. The convection currents inside the refrigeration cabinet help cool the contents inside the refridgerator.

Common Applications of Radiation
1) Teapots - Teapots have shiny surfaces which are bad emitters of radiation. It enables them to keep tea warm for a longer time than black teapots. The shiny surface are also bad absorbers of radiation, thus, enabling them to keep cold liquids cool for a longer time than black containers.

2) Greenhouses - During the day, infrared radiation from the Sun passes through the glass roof of the greenhouse. This warms up the soil and plants in the greenhouse. As the contents in the greenhouse get warm, they emit infrared radiation also.
The infrared radiation that the contents in the greenhouse emit is slightly different from the infrared radiation emitted by the Sun. The infrared radiation from the contents in the greenhouse are not able to pass through the glass roof. Therefore, the infrared radiation by both the Sun and the contents in the greenhouse gets trapped. The amount of infrared radiation would then slowly increase over time, causing the temperature in the greenhouse to increase.

3) Vacuum flasks - It is designed to keep liquids hot by minimising heat loss in four possible ways, namely conduction, convection, radiation and evaporation.

-The cork is usually made of an insulator of heat.

-The trapped air above the liquid is minimal as air is a very poor conductor of heat.

- Conduction and convection through the sides of the flask are prevented by the vacuum between the double-glass walls of the flask.

- To minimise heat loss through radiation, the walls of the glass are silvered so as to reflect heat back into the hot liquid. Convection and evaporation can only occur when the cork is removed during use. Heat loss by radiation is harder to stop as radiant heat can pass through a vacuum.

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