Evan's Space

Wonders of Physics


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Fiery Re-entry into Earth’s Atmosphere (updated)

This post was updated following the first astronauts launched by SpaceX returned home safely on 3 Aug 2020.

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

First astronauts launched by SpaceX return to earth (3 Aug 2020)


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Black is a good emitter of infrared radiation

In theory, we have learned that

Dull (matt/rough) and black surface is a good emitter and good absorber of infrared radiation.

In this simple demonstration, the container sides are painted white matt, silver smooth and black matt.

Hot water is poured into the container and the thermal energy is conducted to the whole container. The temperature of the container is more or less uniform.

Using a infrared thermometer, we measure the temperature of the different surfaces.

From this simple demonstration, we can conclude that dull black surface is the best emitter of infrared radiation and smooth silver surface is the worst emitter of infrared radiation.

 


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Radiometer – Thermal Transfer

Radiometer is a device for measuring the amount of infrared radiation. It consists of freely pivoted rotor with four vanes perpendicular to one another. The vane surface is painted alternate black and silver. The rotor is enclosed in a partial vacuum glass bulb.

In general, when exposed to infrared radiation, the rotor will spin. The greater the amount of radiation, the faster the rotor spins.

The actual working principle is actually much more complicated. Refer to the videos below for detailed explanation.

For our context in O-level, we can briefly explained based on what we learned. As the vanes of the rotor are exposed to infrared radiation, the black side of the vane absorbs more radiation as it is a good absorber of radiation and hence its at higher temperature. The silver side reflects the radiation. The air molecules at the black side will get heated up and gain more kinetic energy. Hence rate of collision is higher and the air molecules collide on the black side with more force than the silver side. This results in a net force on the black surface and the rotor spins in a specific direction as shown in the video.

The more complicated theory how radiometer works

 


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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.


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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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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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Processes for Transfer of Thermal Energy

Three processes of thermal transfer: conduction, convection and radiation.

Conduction vs Convection:

Similarity:

  • both requires a medium for thermal transfer to take place.

Differences:

  • convection cannot take place in solid but conduction can.
  • conduction is due to vibration and collision of molecules while convection is due to fluid density changes.

Radiation vs Conduction & Convection

  • Radiation can take place in vacuum but both conduction and convection require a medium.


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Convection Current – demo

Convection is a process in which thermal energy is transferred within a fluid (liquid or gas) due to the difference in density which creates a current.

The video below shows the convection current in a heated tube filled with water. Colour dye is added to enable us to see the convection current through our naked eyes.

 

 


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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.

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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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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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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Why a cover is not needed?

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These 2 photos were taken a NTUC hypermart at Nex shopping centre. There are many such freezers or merely insulated containers chilled by ice. But wouldn’t it be better to have a cover or lid to prevent thermal energy to getting the food?

First concept to know, thermal energy flows from a region of hot to cold. As the NTUC is air-conditioned, the temperature difference is not too big. Hence the gain in thermal energy by the food and ice will not be great.

Secondly, in convention of fluid, warm air rises as it is less dense and cool air sinks as it is denser. So for this setting, the cool air around the food and the ice is denser. So the cool air will sink and stay inside the container, keeping the food cold and the ice will not melt rapidly.

Thirdly, a more practical reason, which is for the convenience for the customers to have access to the food items easily so as to promote sale. Especially for fast moving items or promotion items.

Now you understand?


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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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Sentosa-5573-1


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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]