Why evaporative cooler does not work well in Singapore?

Recall the factors the affect the rate of evaporation

Recall: evaporation causes cooling

Convection Toy – Angels Carousel

This simple toy is made possible using convection current in the air.

The ‘fan’ of the carousel is similar to the windmill we are familiar with.  Just that the kinetic energy of the wind is created by the stream of hot air rising up from the bottom. It is the opposite of an electric fan.

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 71^oC. 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!

 

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

Which is better to cool the food?

Other examples in our daily lives:

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)

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?

 

Thermal Properties of Matter

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.

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!

Heating and Cooling Curve

 

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.

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.

N2007P1Q17 – Thermal Properties

Four bars, all exactly the same size, are each placed with one end in boiling water.The times taken for the temperature of the other end to increase by 2 ^oC are measured.

Material of bar	Time for 2oC rise / s
Aluminium	10
Copper	5
Cork	800
Styrofoam	1200

To make a large metal tank with the least heat loss, which materials should be used for the tank and its insulation?

	Tank	Insulation
A	Aluminium	Cork
B	Aluminium	Styrofoam
C	Copper	Cork
D	Copper	Styrofoam

Solutions: Option B

This question can be a bit tricky. It depends on which approach you view the question. Simply using conduction will be a much easier way to get the answer. The hint to use conduction is from the first paragraph. Time taken for temp of the other end to increase by 2 ^oC is about conduction.

To build such a tank, metal (aluminium or copper) has to be used and insulation on the external wall (cork or styrofoam).

To contain boiling water with least heat loss, both tank and insulation have to be good insulator (poor conductor) to reduce heat lost to surrounding. Hence tank should be aluminium and insulation should be styrofoam (option B)

On the other hand, if you approach the question in term of heat capacity, it will be a bit tedious and you do not have the values.Firstly, if aluminium takes a longer time to rise by 2 ^oC, it has a higher specific heat capacity. But you cannot merely look at specific heat capacity. You have to look at heat capacity as the mass of the container is important.

Copper: Density = 8940 kg/m³ and specific heat capacity = 400 J/kg.K

Aluminium: Density = 2700 kg/m³ specific heat capacity = 900J/kg.K

Assumptions: Volume of copper and aluminium are the same (same shape of container) say 0.02 m³, Initial temperature of metal is 30^oC

Considering the amount of thermal energy gained from water as metal temperature reaches 80^oC

Heat lost by water = heat gain by metal

Copper

Mass of copper = 8940 x 0.02 = 178.8 kg

Heat lost be water = heat gained by copper = mcθ = 178.8 x 400 x (80-30) = 3576000 J

Aluminium

Mass of Aluminium = 2700 x 0.02 = 54 kg

Heat lost be water = heat gained by aluminium = mcθ = 54 x 900 x (80-30) = 2430000J

From the calculation, it is obvious that the aluminium gains lesser thermal energy from the water, hence water will remain warmer compared to when using copper. Hence using this approach, it is still Option B. But important to know that you have to consider heat capacity [C], not specific heat capacity (c). [ C = mc]