Definitions of Units of Physical quantities

There are 7 fundamental physical quantities that we use in our daily life. They are

1. length(m)
2. mass(kg)
3. time(s)
4. electric current(A)
5. temperature(K)
6. amount of substance(n)
7. luminous intensity(L)

A system called as the System International(SI) decided to define these fundamental quantities.

The definitions are as follows

1. Meter: The meter is the distance travelled by light in 1/29979248 seconds.
2. Kilogram: The Kilogram is the mass of a cylinder made of platinum iridium alloy kept at the International Bureau of Weights and Measures at Sevres, near Paris
3. Second: The second is the time taken by light of specified wavelength emitted by a cesium-133 atom to execute 9,192,631,770 vibrations.
4. Ampere: The Ampere is that current which when flowing in each of two infinitely long straight parallel conductors of negligible cross-section placed one meter apart in vacuum produces between the conductors, a force of 2 x 10^-7 newton per meter length of each conductor.
5. Kelvin: The kelvin is 1/273.16 of the thermodynamic temperature of triple point of water.
6. Mole: The mole is the amount of substance that contains as many elementary entities(atoms or molecules) as there are number of atoms in 0.012kg of carbon-12. This number is called Avogadro constant and its value is 6.02214199 x 10^23.
7. Candela: The candela is the luminous intensity of a black body of surface area 1/600 m² placed at the temperature of freezing platinum and at a pressure of 101,325 newton per square meter, in the direction perpendicular to its surface.
8. Radian: The radian is the angle subtended at the center of a circle by an arc whose length is equal to the radius of the circle.
9. Steradian: The steradian is the solid angle subtended at the center of a sphere by an area of its surface equal to the square of the radius of the sphere.

	Fundamental quantity	Symbol of the quantity	Name of the SI Unit	Symbol of the unit
1	Length	l	meter	m
2	Mass	m	kilogram	kg
3	Time	t	second	s
4	Electric Current	I	ampere	A
5	Temperature	T	kelvin	K
6	Amount of substance	n	mole	mol
7	Luminous Intensity	L	candela	cd

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A New Planet

New Planet Kepler-16b discovered by NASA in 2011. Scientists across the world are perplexed. The discovery of a star that orbits around two suns has stunned many. Others are amazed. But it may take years to verify the reports coming from a NASA satellite Kepler, launched two years ago that a star some two hundred light years away orbits around two suns.

Being called Kepler or Tatooine planet, it is made up of hard rocks where gasses are frozen all over it. Reports suggest that it is the size of Saturn.

While detailing about the Kepler Mission of NASA, its website says, “Considering that we want to find planets in the habitable zone of stars like the Sun, the time between transits is about one year. To reliably detect a sequence one needs four transits. Hence, the mission duration needs to be at least three and one half years. If the Kepler Mission continues for longer, it will be able to detect smaller, and more distant planets as well as a larger number of true Earth analogs”.

It further says, “The Kepler instrument is a specially designed 0.95-meter diameter telescope called a photometer or light meter. It has a very large field of view for an astronomical telescope 105 square degrees, which is comparable to the area of your hand held at arm’s length. The fields of view of most telescopes are less than one square degree. Kepler needs the large field of view in order to observe the large number of stars. It stares at the same star field for the entire mission and continuously and simultaneously monitors the brightnesses of more than 100,000 stars for at least 3.5 years, the initial length of the mission, which can be extended”.

But the new star is reported to be lifeless and made of hard rocks all over it. The two suns it is orbiting are smaller in size compared to our own sun. A NASA scientist who is associated with Kepler mission says, “This discovery confirms a new class of planetary systems that could harbor life…Given that most stars in our galaxy are part of a binary system, this means the opportunities for life are much broader than if planets form only around single stars. This milestone discovery confirms a theory that scientists have had for decades but could not prove until now.”

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Roche Lobe

Gravitational force holds each and every particle of the star within a range of distance. The pull of gravity is effective for some distance and after which the effect fades away. For example: The gravitational pull of the Earth is effective only for 1000 km until which the atmosphere extends. After that, the pull is no longer felt and an object can easily escape to the vacuum. The same applies for a star. The region of space around a star in a binary system within which orbiting material is gravitationally bound to that star is called as the Roche lobe.
If the star expands past its Roche lobe, then the material can escape the gravitational pull of the star. Roche limit is the distance at which an object held together only by gravity begins to break up due to tidal forces.The Roche lobe, Roche limit and Roche sphere are named after the French astronomer Édouard Roche.

 The droplet-shaped figures in the equipotential plot at the bottom of the figure are called the Roche lobes of each star

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