Pratik Barve, Author at PhysicStuff

What is Tripple point?

By Pratik Barve | March 22, 2018 | Comments 0 Comment

Tripple point is a term coined in 1873 by James Thomson, brother of Lord Kelvin. Tripple point of a substance means the temperature and pressure at which a substance exists in all the 3 states (gas, liquid, solid) simultaneously in thermodynamic equilibrium. Now that sounds pretty weird. For instance let us consider the substance to be water. Water boils at 373 K (100 degrees celcius), it is liquid above 273K (above 0 degrees Celcius) and it’s solid i.e ice below 273K (below 0 degrees Celcius). So how is it even possible that all three states are able to coexist.

First let’s go through some basic thermodynamics.
PV=nRT
This is the famous Ideal Gas equation.
P : Pressure
V : Volume
n : No. of mole i.e amount of substance
R : Gas constant
T : Temperature

Now when we try to achieve tripple point of any substance, the system or the container is isolated from the surrounding. So the volume remains constant, amount of substance is constant, the gas constant will always be a constant. So to keep the equation valid Pressure has to be directly proportional to Temperature.

Here things become interesting. So if we reduce the pressure in the system we actually reduce the boiling point of the substance. That’s why in vacuum chamber water boils instantly. So now all we have to do it carefully maintain pressure and temperature of the system in such a way that it freezes the substance into solid but low pressure starts boiling it and in between there will be liquid state. That’s how you obtain a tripple point of a substance.

And believe me it just looks too weird that a substance, for instance water, is boiling but freezing at the same time in the same container!
The tripple point for water is achieved at 273.16K (0.01 degrees celcius) and at the Pressure of 0.0060 atm i.e 0.611… kPa.

Take a look at this interesting video : https://youtu.be/r3zP9Rj7lnc

Just like water has tripple point at specific parameters of pressure and temperature other substances have thier own tripple points where they coexits in all 3 states.

What is the significance of tripple point?

What exactly is this parameter used for? Well triple points make ideal reference points for the calibration of thermometers. They can be realised by using a sealed, evacuated, cylindrical glass cell filled with the pure substance, with an axial re-entrant well for the insertion of the thermometer. This device is called tripple point cell used for calibration of thermometers.
The triple point of water has a unique place in metrology since it is the basis of the definition of the units of temperature, the kelvin and the degree Celsius. Its temperature is 273.16 K and 0.01 °C by definition. Additionally, the triple points of mercury and several gases – argon, oxygen, neon and hydrogen – are used as low temperature reference points on the ITS-90. Triple point cells containing organic substances can also be made. Ethylene carbonate has a triple point temperature of 36.315 °C which, being close to body temperature, makes it a highly useful reference point for the calibration of clinical thermometers, while benzoic acid has a triple point temperature of 122.33 °C, close to the sterilising temperature of medical drip solutions.

Triple point cells are so effective at achieving highly precise, reproducible temperatures, an international calibration standard for thermometers called ITS–90 relies upon triple point cells of hydrogen, neon, oxygen, argon, mercury, and water for delineating six of its defined temperature points.

How does lightning occur? | The physics of Lightning and Thunders

By Pratik Barve | March 7, 2018 | Comments 10 comments

The lightning we see during rains or storms are so fascinating in nature! A really huge spark of electrostatic discharge from clouds to the ground or sometimes within clouds itself. And after that bright flash follows a really loud bang which is called thunder. So what is it that really happens up above in the clouds that creates this huge electric discharge and a loud bang?

Branching of lightning bolt in slow motion.
Image credits: NOAA SciJinks

First we will understand how lightning works and then how thunder is generated.

Formation of Lightning

Lightning is the sudden discharge of electricity. Initially the charges get separated in the cloud formation itself. The primary source of lightning is the cloud type termed cumulonimbus, commonly referred to as the thundercloud. Due to the air currents inside the clouds and other factors the positive charges gets accumulated in the top region of clouds and negative charge is accumulated at the bottom region. Unfortunately we don’t know exactly why or how this charge separations occurs in the first place. One widely held theory is that the thunderstorm clouds have supercooled ice crystals and graupel (soft hail) and due to strong updraft they are pushed in the upper region of clouds and become positively charged.The heavier ice crystals which are negatively charged fall to the lower regions.

Now the lower region is accumulated with negative charge. As like charges repel, the negative charge on the ground is repelled and positive charge accumulates on the ground. Now unlike charges attract! When enough negative charge in clouds and positive charge on ground is accumulated, sudden discharge takes place which we see as a lightning strike. The negative charge starts flowing towards ground and the positive charge on ground starts travelling up towards the clouds. The point when these two flows meet is the point when we see a huge lightning bolt from cloud to the ground!

Negative charges from cloud (blue) and positive charges from ground (red) start moving towards each other eventually discharging suddenly creating a lightning. Image Credits: NOAA SciJinks

Its the same static discharge/zap we feel sometimes when we touch car doors or even other people, its just much larger in magnitude (and much lethal obviously).

Now the types of lightning:

Intra cloud: This type of lightning happens completely within clouds. It happens due to discharge between different charge regions within the clouds.
Intra-cloud lightning.
Image credits: NSSL, NOAA
Cloud to Cloud: This type of lightning strikes happen between two or more surrounding clouds.
Cloud to cloud lightning.
Image Credits: Wikimedia
Cloud to Ground: This is what we should be worried about! This is the type of lightning that strikes the ground.
Cloud to Ground lightning strike.

The branching can be clearly seen as the charges try to find the shortest possible path. This is the reason why tall buildings and trees are striked by lightning more often.
Cloud to air: This type of lightning occurs when there’s a discharge between clouds and oppositely charged air surrounding the clouds. The distribution of charges varies and depends on many factors so this type of discharge happens.
Cloud to Air Lightning.
Image Credits: NSSL NOAA

What causes Thunder?

Thunder is that very familiar loud rumbling sound we hear a few seconds after a lightning strike. We can actually determine how far away the lightning strike took place by counting the interval between flash and rumbling sound.

The average lightning bolt striking from cloud to ground contains roughly 1 Billion Joules of energy!! Now that’s just lot of energy and lightning strikes last for such a short duration. This immense energy superheats the air surrounding the lightning channel to plasma temperatures in a very short duration. Temperatures can go upto 30,000 Kelvins (53,540 degrees Fahrenheit). This creates a shockwave in the air rapidly and rumbling sound is generated. High frequency sound gets quickly absorbed by landscape so thunders due to distant lightnings sounds like low rumble.

How to survive a lightning strike?

If unfortunately you are in flat lands during a thunderstorm remember that you are one of the shortest possible routes for charges to discharge. Laying flat on ground won’t work as lightning can hit the ground and first and then your body. Avoid standing under trees, poles, any sort of tall conducting pointy things. It’s just safe to be indoors during a thunderstorm.

Staying inside a car or bus also might help as the frame or surface of vehicles act as a Faraday cage and lightning will flow on the surface and not through your body.

These are some tips just incase you are in the open during a thunderstorm.

Crouch down low like a baseball catcher. Get as low as you can. The nearer you are to the ground, the less likely you are to be struck by lightning. But never lie down!
If your hair begins to stand on end or your skin starts to tingle, a lightning strike is imminent. (You literally have a few seconds before you get hit). Immediately get into the crouching position. Lightning may strike without this warning, however.
Place hands over ears to minimize hearing loss from the loud clap of thunder that will boom very close to you.
The only thing touching the ground should be the front portion of your feet. Lightning can hit the ground first, and then enter your body. The more you minimize your contact with the ground, the less chance of electricity entering your body.
Touch the heels of your feet together. If electricity from a ground strike enters through your feet, this increases the chances of the electricity going in one foot and out the other, rather than into the rest of your body.
Don’t touch any possible conductors.
How to survive a Lightning Strike?
Image credits: artofmanliness.com

I hope you never get into such situation but knowing stuff is always good, ain’t it?

What is a Lunar Eclipse?

By Pratik Barve | February 2, 2018 | Comments 3 comments

There’s flood of images of blood moon on social media since 31st Jan. The popular term being used for the phenomenon is “Super Blue Blood Moon”. That phenomenon was nothing but a Total Lunar Eclipse. Now we will understand what happens and how it happens. First of all lets understand what is a lunar eclipse and then we will discuss its types.

What is a Lunar Eclipse?

The most common definition we hear is, when the sun earth and moon are aligned in the same line/ when the moon enters the Earth’s shadow. That all is fine, but the most natural question that pops up in our mind is; then why doesn’t eclipse happen every month? As it takes about 27.3 days for one complete revolution around Earth shouldn’t the sun, moon and earth line up twice in a month which would lead to one total solar eclipse and one total lunar eclipse every month? But that doesn’t happen right? Eclipses doesn’t happen that often. Well that’s because the Moon’s orbital plane is inclined by about 5.1° to the ecliptic plane (the plane in which Earth orbits around sun). So Eclipses occur only when the moon passes through that exact point where these two planes coincide. And this doesn’t happen that often.

By Peter Sobchak - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=35889221 — The Moon’s orbital plane differs a little than the Earth’s orbital plane.

Types of Lunar Eclipses

Umbra – The region where no sunlight reaches.
Penumbra – Region where sunlight reaches partially.

Penumbral Lunar Eclipse

Penumbral Lunar Eclipse happens when the moon passes through the penumbra region of the Earth’s shadow. This type of eclipse is usually very hard to notice as there is only slight change in the brightness of moon.

Partial Lunar Eclipse

When part of the moon passes through Umbra we get a partial Lunar eclipse. It is easy to notice as there is significant drop in Moon’s brightness. It might happen once or twice in a year.

Total Lunar Eclipse

Now this is something which doesn’t happen quite often. Total Lunar Eclipse happens when the moon passes through the umbra i.e all the sunlight is blocked by Earth. This event happened recently on 31st January 2018.

The types of Eclipses. Image credits: Addison Wesley.

So what was so special about the Lunar eclipse of 31st January? And why was it called Super Blue Blood Moon? Lets understand it word by word.

Super moon

The moon’s orbit around earth is not exact circle, its an ellipse. So there must be two points; when the moon is farthest from Earth and when it is closest to Earth. The farthest point is called apogee (406,300 km) and closest point is called perigee (356,700 km). Super moon is when its a full moon at perigee and its called super moon because it looks a little bit bigger than usual.

Comparison of Moon at Apogee and Perigee.

Blue Moon

This has nothing to do with the colour blue. When two full moons appear in one month then the second full moon of the month is called a blue moon! There is a phrase “once in a blue moon” so its evident that this is something rare. Blue moon happens once in 2 to 3 years. And whats more rare is a double blue moon which is two months in a year which has two full moons each! And this happens 3-5 times in a century!! The last double blue moon happened in 1999 and next will happen in 2037!

Blood Moon

Blood moon sounds so surreal! When a total lunar eclipse occurs all the sunlight is blocked by earth and moon is in the umbra region. Now Earth has a thick layer of atmosphere. Rayleigh’s scattering of sunlight in atmosphere is what gives sky its blue colour. The atmosphere scatters blue light more efficiently than red light (In better terms, the scattering is inversely proportional to the fourth power of wavelength). So the Red light passes through the atmosphere without getting scattered and enters the umbra region onto the moon! That’s what gives the moon a reddish hue! Hence the name blood moon.

The atmosphere bends red light towards the umbra region.
Image credits: lovebigisland.com

Super Blue Blood Moon

Now what are the odds of super moon, blue moon and blood moon happening at the same time !! That’s why people were hyped for the Super Blue Blood Moon!!! I got to watch the eclipse and also clicked images at different stages of eclipse. The first image is when the totality occurred, the moon was visibly red even with naked eyes. And then gradually it turned white into a full moon! Isn’t it beautiful!?

Composite image of different stages of Total Lunar Eclipse.

The next Blue Blood Moon will happen on 31st December 2028 and the next Super Blue Blood Moon will happen on 31st January 2037! So if you missed this event you need to have a lot of patience.