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Sunday, September 27, 2015

What is RMS value of Alternating Current?

The RMS (Root Mean Square) value is 70.7% of the peak voltage and represents the amount of power that an AC wave can produce as compared to the equivalent DC voltage.

The international convention for specifying AC voltage is to express the RMS value of the wave unless it is otherwise defined. If you read voltage value as 100VAC, it means 100VAC (RMS). If you want to have peak value than it would be 141.14VAC peak.

If we apply a 110VAC(RMS) sine wave to a load then it will produce the same amount of power that a 110VDC steady voltage will produce. It shall be noted that the actual peak value is 155.5VAC. The alternating nature of sine wave produces less power than the direct current.

Saturday, September 26, 2015

Difference between Distance & Displacement

Displacement is a vector quantity as it is associated with direction,but distance is a scalar quantity which does not give any information about direction of motion.Distance and displacement magnitude are numerically equal only when the body travels in a straight line from A to B(i.e the shortest distance from a point to another). Also distance has always a non-zero magnitude,but displacement can be zero if the body returns to the point of origin at the end of motion.

Why do we hear a hissing sound when we open a Thumsup/Coke bottle?

The Coke is pumped with CO2 gas for carbonation process. Coke sealed under pressure condition. Partial gas gets dissolved in the liquid as bubbles and the remaining occupy the volume at the surface of the liquid and sealed cap. When the bottle is opened due to pressure difference, there is rush of gas at liquid surface to outside the bottle followed by dissolved gas rushing out from liquid making small explosions of liquid at surface leading to hissing sound and bubbles.

What is Carbon dating?

Radioactive C-14 is produced in our atmosphere by the bombardment of N-14 by neutrons produced by cosmic rays. The radioactive carbon-14 combines with oxygen to form carbon dioxide and is incorporated into the cycle of living things.

Carbon-14 decays with a half life of about 5730 years by the emission of an electron of energy 0.016 MeV. This changes the atomic number of the nucleus to 7, producing a nucleus of nitrogen-14. At equilibrium with the atmosphere, a gram of carbon shows an activity of about 15 decays per minute.

Presuming the rate of production of carbon-14 to be constant, the activity of a sample can be directly compared to the equilibrium activity of living matter and the age calculated. 

When living organisms die, their intake of 14C ceases, and the ratio of 14C / 12C   decreases as 14C decays. The period just before 9000 years ago had a higher 14C / 12C ratio by factor of about 1.5 than it does today.

Because the half-life of 14C is 5730 years, it is convenient to use the 14C / 12C ratio to determine the age of objects over a range up to 45,000 years ago.


Atoms touch each other along face diagonals. All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing.

ex: Al, Cu, Au, Pb, Ni, Pt, Ag

Coordination no = 12

4 atoms/unit cell: 6 face x 1/2 + 8 corners x 1/8

Atomic Packing Factor for a face-centered cubic structure = 0.74

Close-packed directions: length = 4R = sqrt (2a)

Unit cell contains: 6 x 1/2 + 8 x 1/8  = 4 atoms/unit cell