Evan's Space

Wonders of Physics


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Metal is a good conductor – paper over metal gets burned slower

The paper over metal or over plastic/wood will get burned faster?

Everyone knows that metal is a good conductor of thermal energy. But for experiments like this, many would have guessed it wrongly.

A paper is wrapped over metal and insulator (plastic, wood etc) and is exposed to the flame, the paper over the insulator becomes charred faster and burned faster.

The paper over the metal takes a longer time to be charred and burned. This is because metal is a good conductor of thermal energy. When the spot (paper over metal) is exposed to the flame, the metal conducts the thermal energy away from that spot to other parts of the metal. So the temperature increase at that spot is slower, hence the ignition temperature of the paper (approx. 230oC) will be reached much slower, compared to the spot where the paper is over an insulator.


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Water is a poor conductor

In the three states of matter, in general, solid is the best conductor as the particles are closely packed in an orderly manner, hence thermal energy can be passed down by the collision of the particles in the solid faster.

On the other hand, liquid and gas are relatively considered poor conductor. Gas is the worst conductor as the particles are far apart.

The following experiment demonstrates that water is indeed a poor conductor of thermal energy.

An ice cube is kept at the bottom of the boiling tube by the net. The water at the top of the boiling tube is heated and started to boil. But the ice is not fully melted.

This shows that thermal energy transferred through the water from heated water at the top to the ice is weak, hence indicating that water is a poor conductor.

How about thermal transfer through convection current? In this experiment, the heated water at the top expands, the volume of the heated water increases, becomes less dense and remains at the top. The cooler water, which is denser, remains at the bottom. Hence there is no convection current formed throughout the entire water in the tube. So thermal energy transfer to the ice cube through convection is not present here.


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Which is better to cool the food?

ice in cooling food items

Other examples in our daily lives:

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In some supermarket, the seafood are placed outside of air-conditioned place. The seafood is kept cold by putting crushed ice covering the seafood to keep the them cold and fresh.

Refer to this Sci Physics question N2008P2Q6(b)

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Solutions: 
For the solid that does not melt, when thermal energy is absorbed from the surrounding  food, its temperature starts to rise. So it is not so effective at keeping the food cool.
For ice-pack, when thermal energy is absorbed from the surrounding food, it starts to melt. During melting process, a much larger quantity of thermal energy is absorbed from the food to melt per unit mass of ice, the temperature remains constant at 1oC, and the melting process is long. Hence ice-pack is more effective at keeping the food cool.

Related posts about ice:

Density of ice – Why ice floats on water?

Will whole lake be frozen during winter?

 


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Heat Sink in PC

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Heat sink plays a very important part in cooling of the Intel chip that powers the whole computer. Removal of thermal energy from the chip is vital. If the chip overheats, the ‘brain’ will be fried and it can’t function.

The features in the heat sink are there for a purpose. The metal portion is placed on top of the chip. Metal is used as metal is a good conductor of thermal energy. It helps to conduct thermal energy quickly away from the chip.

As the metal gets hot, it will radiate the thermal energy out. The design is such that it is fin-shaped, which increases the surface area, thus increases the rate in which rate of emission of infra-red radiation to the surrounding.

Theoretically,  if this metal portion is coloured black will be better as black is a better absorber (from the chip) and emitter (emit out to surrounding) of infra-red radiation.

The fan creates a forced convection current to bring the hot air out and cool air in.

That’s the basic function of the heat sink found in most common PC.


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Double-glazed windows and doors in hotel

These photos are taken at Sentosa Resort World – Festive Hotel.

 

Most hotels have double-glazed windows and doors.

 

From thermal physics, we have learned that it has a layer of air in between 2 layers of glass. Air is a poor conductor, hence thermal energy from outside the hotel room entering the room will be reduced. In this way, the room can be kept cool, and hotel expenditure on air-conditioning can be reduced.

 

On the other hand, due to this feature, it helps to sound proof the room too. With double-glazed windows and a layer of air, external sound from outside the room will be softened due to these additional layers. Hence you are able to have a good sleep in the room!

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