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Working Principle of Van de Graff generator

What is the working principle of Van de Graff generator?


Akash Rai asked.


Schematic view of a classical Van de Graaff-ge...


Answer: This is a question asked by many of the students.


There are basically two principles involved. The principle stresses in NCERT text book is that,


Photograph of a Van de Graaff generator.
Photograph of a Van de Graaff generator. (Photo credit: Wikipedia)


If we consider two concentric spheres connected electrically, then any charge given to the inner conductor will immediately be transferred to the outer sphere, irrespective of the amount of charge already present on the outer sphere. This happens because, the inner sphere is always at a higher potential and charges flow from high potential to low potential.


Another principle involved is the action of sharp points and corona discharge.


When a conductor with sharp points are charged, there will be greater surface charge density near the sharp points and the electric field near them will be high. This can ionise the surrounding air and can cause the charge to leak away. Thus a charged metal with sharp points will get discharged by this action.; the corona discharge.



How do we see movies?

We are able to see the movie picture in a cinema hall. Define the phenomenon responsible for it.Explain how does this happen?


Asked Amogh A Joshi:




The phenomenon responsible here, which enables us to watch the movies and enjoy them is PERSISTENCE OF VISION. 


Animated horse, made by rotoscoping 19th centu...
Animated horse, made by rotoscoping 19th century photos by Eadweard Muybridge. Artistic license has been used to achieve the cartoony look. Animated by J-E Nyström, User:Janke, released under CC-BY-SA-2.5 (Photo credit: Wikipedia)


Persistence of vision is the property by virtue of which the image formed on the retina of human eye remains for about 1/15th of a second. So, if more than 15 frames are projected per second on screen one after the other in succession, we wont be seeing them as separate but we will be able to enjoy them as continuous action.


See the following video in which Gagnam Style Video is made out of a series of drawings.



Refraction through a prism

For an equilateral prism , find two angle of incidence differing by 20 degrees showing same deviation of 40 degrees?

Asked Atambir Singh


Use the formula \delta =i_{1}+i_{2}-A

Here A = 60 degree (Since equilateral prism)

deviation is 40 degree

This gives A+\delta =i_{1}+i_{2}=100


the values of angles of incidence are 60 degree and 40 degree


A problem from Gravitation

Three identical bodies each of mass m are located at the vertices of an equilateral triangle with side r. At what speed they move if they all revolve under the influence of one another’s gravitation in a circular orbit circumscribing the triangle while still preserving the equilateral triangle

Asked Nitin gupta (from our mobile website http://m.askphysics.com)

An equilateral triangle with equal sides and a...
An equilateral triangle with equal sides and angles marked

Please post your answer now!

Traveling in a flight from say India to USA

If I have to travel from India to the USA, I take flight. The flight flies and keeps traveling towards USA for about say 17 hours. Why does it kepps travelling? If it flies and stays still for the hours we assume USA to be behind India time, will it land there and reach USA? If no why?

United (States) Parcel Service.


Asked Ashwini Joshi

Are we creating a universe when we explode a nuclear bomb?

The big bang theory claims an explosion formed the universe.  Therefore we are living inside of an explosion.  However an explosion as we experience it is almost an instantaneous thing. To an observer outside the universe it would be over rather quickly.  Yet we perceive it as billions of years.

Are we creating a universe when we explode a nuclear bomb?

Answer: We are not creating universe by bomb explosion – but destructing it.

Explosion (Photo credit: gynti_46)

The big Bang is referred to as an explosion  because of the fast expansion it caused.

English: Shows slices of expansion of universe...
English: Shows slices of expansion of universe without an initial singularity (Photo credit: Wikipedia)

Equilibrium – Numerical Problem

SOLID HEMISPHERE OF RADII R EACH, are placed in contact with each other with each other with their flat faces on a rough horizontal surface. A sphere of mass m and radius R is placed symmetrically on top of them. The normal reaction between the top sphere and any hemisphere assuming the system to be in state equilibrium is

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