Evan's Space

Wonders of Physics


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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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Gas expands and contracts the most

The 3 states of matter –  solid, liquid and gas.

In general, when a body is heated, it expands and volume increases. The mass remains the same. Since density = mass/volume, its density decreases (less dense). For instance, warm air rises as it is less dense. In terms of kinetic theory, the particles will increase in kinetic energy. The average spacing between the particles increases (assuming not in a closed container).

Likewise, when a body is cooled, the opposite occurs. The body contracts and volume decreases. It becomes denser.

Due to the differences in particles arrangement of solid, liquid and gas, each expands by different amount when heated and vice versa. Which expands the most when heated and contracts the most when cooled?

The following demonstration of the ‘Pee Boy’ is a good video to show the concepts.


<p><a href=”https://vimeo.com/25123645″>Pee Boy</a> from <a href=”https://vimeo.com/user7367248″>evantoh</a&gt; on <a href=”https://vimeo.com”>Vimeo</a&gt;.</p>

Explanation:

The tiny hole at the penis is too small for any water to enter on its own. So using thermal transfer in the different states, the following steps are taken:

  1. Put the hollow empty boy into the hot water. [air inside the boy expands more than the solid ceramic, hence bubbles are seen coming out of the hole]
  2. Put the hollow empty boy now into the cold water. [The air inside contracts and volume decreases. This creates a low pressure and water is then sucked into the boy through the tiny hole]
  3. Place the boy on a platform. [The boy is only partially filled with water. The head portion is filled air while the bottom portion is filled with water]
  4. Pour hot water over the head. [As the whole boy is heated by the running hot water, the air in the head portion expands much more than the water at the bottom and the solid ceramic of the boy. Hence the air pressure increases and it pushes the water out of the boy]
  5. And he pees!!! Quite powerful indeed!


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Boiling water at 60 °C in lab!

Water boils at 100°C. In the view below, the water is initially at around 60°C.

How is this possible?

Explanation:

At sea level, where the pressure is atmospheric pressure (approx 105 000 Pa), the boiling point of water is 100°C.

In the video as air is sucked from the sealed container, the pressure in the container decreases. This lowers the boiling point of the water to around 60°C, hence water will boil and bubbles are formed.

It is similar to boiling water at high altitude, like on top of Mount Everest (about 8 km high). Water will boil around 70°C to to lower atmospheric pressure at high altitude.


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Spraying of mist in air – what’s the purpose?

This structure is located at Marina Bay, beside the Marina Bay Sands. It sprays out mist into the air. What is the main purpose of this in terms of thermal physics?

<p>Spray out mist. What’s the function? from evantoh on Vimeo.</p>

You might have come across similar features like this in other places like coffee shop, al fresco Starbucks etc.

Explanation:

Mist contains tiny droplets of water. When it is sprayed out to the surrounding, it will evaporate very fast.

The higher rate of evaporation is due to the tiny droplets of water having a relatively higher expose surface area compared to a body of water of the same mass.

As the droplets of water evaporate, thermal energy is taken from the surrounding air to change the water to a gaseous state. Hence temperature of the surrounding air drops, giving the cooler and conducive environment in a hot tropical weather in Singapore.