Heat Transfer

Heat Energy is also known as Thermal Energy. Transfer of thermal energy is also known as HEAT TRANSFER.

Thermal energy ALWAYS flow from a region of HIGH temperature to a region of LOW temperature.

Only when thermal equilibrium is reached between two bodies (i.e both bodies are at the same temperature), there will be NO net flow of thermal energy between them. This means that there will not be HEAT TRANSFER.

Below is a video that talks about one processes of transfer energy transfer - CONDUCTION



I've found this video online from youtube. It is useful for the people who are slower in understanding as it helps us to understand the concept of conduction easier and better.

What is conduction then?
Conduction is the the process of thermal energy transfer without any flow of the material medium.

In heat transfer, conduction (or heat conduction) is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. 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. 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.

I've found a formula online that can calculate the conductivity gradient for a given system:
q = - kA (Δ T/Δ n)
Where Δ T/Δ n is the temperature gradient in the direction of area A, and k is the thermal conductivity constant obtained by experimentation in W/m.K.

Have you ever wondered how come thermal energy is transferred much faster in iron than glass?
This is becuase both conductors (iron, brass, steel) and insulators (glass, wood, rubber, plastic) have different mechanisms to transfer thermal energy.

The difference between conductors of heat (metals) and insulators of heat (non-metal) is that the conductors have MANY electrons which moves about randomly between atoms or molecules whereas insulators does not.

All matter conducts heat by atomic and molecular vibration

Thermal Energy is passed by the vibration of molecules in ALL matter.

When the matter is being heated, the particles will vibrate vigorously, colliding with the neighbouring particles, making them vibrate as well. Thus, transferring heat to the neighbouring particles.

This picture below illustrate how conduction takes place in solids.

However, this way of transferring heat is VERY SLOW. That's why, there's a second method of transferring thermal energy - Free Electron Diffusion

PLEASE TAKE NOTE:
1. Free Electron Diffusion occurs in BOTH METALs and NON-METALs.
2. Non-Metals STILL have Electrons, just that, that have lesser electrons than metal, that's why, metal conduct heat FASTER

Metals, like I've said before, contains MANY electrons that moves around. When a metal rod is heated, the free electrons will then diffuse and spread into the colder part of the metal, and, while they move, they'll collide with the atoms in the cooler parts of the metal and transfer their kinetic entergies to them. This explains why good conductors like metals are capable of transferring thermal energy FASTER than insulators.

Of course conduction TAKES place in liquids and gases too...
But, at a SLOWER process because the particles are spaced much further apart and it WILL be much more difficult to COLLIDE with neighbouring particles..

Imagine ...
you are in a crowd of people at orchard road...
Will it be easier for you to knock into someone that is beside you?
Of course it would be!
That's like SOLIDS actually.. very closely packed :)

NOW. IMAGINE AGAIN
You are in school and there's a break in between 2 lessons. and you want to go to the washroom.
There are SOME people walking along the corridor to the toilet
Will you have the tendancy to bang into someone?
of course you will, but there'll be LESS tendency right?
THAT is an example of conduction in liquids

FINALLY, IMAGINE THIS
You came back to school during the school holidays JUST TO RETURN YOUR REPORT BOOK.
Some people will DEFINATELY BE in school for CCAs, Trainings etc...
But, WILL YOU HAVE a HIGH Possibility of colliding into them?
it's like NEAR ZERO!!!
This is an example of conduction in GAS.

Well, now, I hope you'll know that Conduction takes place
-Fastest in solids
-Followed by liquids
-Slowest in GAS

We'll go on to the Applications of conduction
1. Cooking utensils
- handles: plastic, rubber [poor conductor]
- Pans: metals like steel

2. Table Mat
- Prevent the table from damaging by hot kitchenware

3. Gloves/Mitten
- Keep us warm during winter :)

CONVECTION
Convection is the flow of heat through currents within a fluid (liquid or gas). It is another form of thermal energy transfer.

Convection takes place in ONLY in both gases and liquids because, Convection INVOLVES the movement of the fluids which carry thermal energy with them. [They practically MOVEEES]
But, Solids don't. They stay about their fixed position and VIBRATE.

Have you ever notice a kettle?
Did you ever wonder WHY the heating coil is placed at the bottom of the kettle?

THAT's BECAUSE it makes use of the convection concept.

When water is heated in a kettle, it will expand and rise as it is less dense than the surronding water...
As it rise, the cooler water at the top would sink as it is denser..
The movement of the water is known as CONVECTION CURRENT.

**Note: Density= Mass / Volume. When it expands, it'll have more volume, hence, it becomes LIGHTER*

The diagram ABOVE describes how convection in air works.
*By the way, this is not found in the textbook.*
When you place the flame in tube 1, the flame will extinguish very quickly because the oxygen in the tube will soon be used up.
However if a glass rod is placed FULLY inside a tube, like tube 2, the flame will continue to burn and will not extinguish

why?
The air above the candle gets heated and expands. Since the air expands, it becomes less dense than the surrounding air. Hence, it rises out of the tube from one side of the glass. IN doing so, the surrounding air will then enter the tube from the other side of the glass, replenishing the oxygen used in heating.

One last example that I'll like to use is the illustration of sea breeze and land breeze at the beach.

Did you realise, the beach tends to be very windy?
If you sit facing the sea...

During the day, the air above land gets hotter than the sea. Hence, the air above land will rise and cool air above the sea will sink to replace the air above the land. Hence, the wind seems to be coming FROM THE SEA

During the night, the air above the land is cooler than the sea. Hence, the air above the sea will rise and the air above the land will sink to replace the air above the sea. This time, the wind seems to be coming FROM the LAND.

Now that we've come to the end of introducing convection, I'd like you to think
WHY IS IT THAT PEOPLE DONT BOIL AT THE SIDE OF THE PAN BUT AT THE CENTRAL BOTTOM?
Do you think it is because of the way the covection current moves?

APPLICATIONS OF CONVECTION

1. Kettle
- The coil of the kettle is placed at the bottom

2.Air-Conditioner
-It is ALWAYS placed near the ceiling of

3. Refridgerator

-The Freezer unit is at the top so that the cool air will sink and the hot air will rise. This saves electricity.

4. Oven

- It is similar to the kettle as the heating coil is at the bottom.

Here's a short videoclip from Youtube that summarises the whole chapter.