Evan's Space

Wonders of Physics


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Fiery Re-entry into Earth’s Atmosphere

SPACE CAPSULE simple

SPACE CAPSULE

SpaceX’s Crew Dragon heat shield shown off after first orbital-velocity reentry

How do spacecraft re-enter the Earth? | HowStuffWorks

Why Is It So Difficult For A Returning Spacecraft To Re-Enter Our Atmosphere?

Returning from Space: Re-entry – PDF format

SpaceX In-Flight Abort Test

SpaceX Falcon Heavy- Elon Musk’s Engineering Masterpiece

Shuttle Atlantis STS-132 – Amazing Shuttle Launch Experience

How to Land the Space Shuttle… from Space


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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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Boiling water at reduced pressure – Water boils at 75 oC

At sea level (where most of us are), the standard atmospheric pressure is about 101325 Pa. The boiling point of water is at 100 oC which we are familiar with.

But as you climbed up e.g. Mount Everest at 8,848 m, the pressure is low and the boiling point of the water is about 71oC. So that’s the hottest cofe you can have on top of the cold mountain!

Hence as the pressure decreases, the boiling point of the water decreases. As with lower pressure, the water molecules requires lesser energy to break the intermolecular forces to escape into the atmosphere, hence boiling point is lower.

This video shows the same effect. Using the syringe, the air is pumped out of the container to reduce the pressure. The water at 75 oC , (below the usual boiling point of 100 oC) will start to boil and you can observe the bubbles forming!

 


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Finding specific latent heat of vaporisation (lv) (pure physics)

Take note that the question is looking for specific latent heat of vaporisation during boiling. Hence information of mass m1 before heating is not relevant. There is no change of state of the water, it is merely the heating of the water (which involves specific heat capacity of water).

Refer to the worked solution below.

Solutions: Option D


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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?