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Monthly Archives: January 2012

Working of Francis Turbine

Please explain the working of francis turbine? (Manish asked)


Francis Turbine is a water turbine developed by James B Francis and is mainly used for Electrical Power Production ranging from 10MW to  750 MW.  It is usually placed at the bottom of a dam where there is large pressure difference between the water entry level and exit level. As the water moves through the runner, its spinning radius decreases, further acting on the runner. At the exit, water acts on cup-shaped runner features, leaving with no swirl and very little kinetic or potential energy. The turbine’s exit tube is shaped to help decelerate the water flow and recover the pressure.


Can light do some mechanical work?

“We know that sound can cause some mechanical work(eg:when we keep our hand in front of a large speaker,we feel something hitting the back of our hand), but why this doesn’t happen with light??” -Manishankar asks.

Calorific Value

If I have the Calorific Value of a dry wood sample (17540 joules per gram of sample). how can I determine the percentage of carbon content of this wood sample? Assuming the sample is 75% Cellulose and Hemocellulose and 25% lignine?

Harmonic analysis problem

The figure shows a two-component complex tone deviation x dependence on time t. The values on deviation axis are in 1cm intervals and on the time axis, there are 2 cm intervals.

1) Find out the main tone frequency. (Ans. 25kHz, Explain How)
2) Main tone amplitude? (Ans. 7.5 cm , Explain How)
3)Whats the upper/overtone frequency? (Ans. 100kHz, Explain How)
4)Whats the upper/overtone amplitude? (should be 2 cm. , Explain How)

Black Holes

I have no background in physics though I am deeply fascinated by it. The THEORY OF EVERYTHING has always intrigued me. I have had no schooling per say, but I have watched a lot of documentaries, lectures, and have read many articles, papers and completely exhausted my printer from printing Wiki pages about everything encompassing physics, quantum mechanics, energy, gravity, black holes, matter, dark energy etc. My question is this: How can a black hole, which has a gravitational force so strong nothing can escape it, not even light, how is it possible for some black holes to eject material out from it? I have read many papers about how some super-massive black holes sometimes consume more material than they can actually ‘EAT’ and will sometimes eject that material out into space hundreds of light years? Isn’t the gravitational force supposed to catch that material and re-consume it? My only thought is that the material never crosses the event horizon and therefore can not be fully captured by the gravity of the blackhole. Please help. Your input is greatly appreciated.

(Asked Michael J. Garcia Jr. )

Electric field inside and outside the plates of a capacitor

How do you find the direction of electric field on the outer or inner surface of  2 parallel plate of a capacitor ?



The direction of electric field is from the positive plate to negative plate.

The electric field outside the plates is zero as the electric field due to each plate (E=Q/2ε0A) cancel out being equal in magnitude and opposite in direction.

In between the plates the two electric fields add up as they are in same direction. (From positive plate to negative plate).

The magnitude of electric field between the plates is twice the electric field due to either; i.e; E=2 x Q/2ε0A = Q/ε0A

Displacement – A Numerical Problem

A fly is sitting in the middle of a wall of a cubic room of side ’a’.If it flies and sit to one of opposite corner of the wall. Find its displacement?

(Harmeet Singh asked)
If the fly sit on the same wall on the horizontal, (from A to B in the diagram) the displacement is “a
If the fly sits on the diagonally opposite corner on the same wall, (from A to C in the diagram) the displacement is “√2 a
If the fly sits on the corner on the longest diagonal of the room,  (from A to H in the diagram) the displacement is “√3 a
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