Evan's Space

Wonders of Physics


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Will the whole lake be frozen during winter? At what temperature is water at its max density?

water heaviest at 4oC smallDuring summer, though the surrounding is hot, the water in the lake will not reach very high temperature mainly due to its high specific heat capacity. In general, cool water sinks as it is denser and warm water rises as it is less dense, the bottom of the lake will always be lower temperature compared to near the surface. The bottom of the lake may not necessary be 4 degree Celsius.


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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.


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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 30oC

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

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]