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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.



Limitations of Homogenity

Paraser asked

“What are the limitations of homogenity?”

The question is acutually”What are the limitations of dimensional analysis?”

The dimensional analysis is based on the principle of homogenity of dimensions. The question is already discussed earlier  HERE

Impulse – momentum and packing of glassware

Why are glassware packed with straw? Answer in terms of physics.

Asked Diya


This is due to the impulse momentum principle

Questions normally not answered by Students in Viva

Here are some questions which are not properly answered by students during Viva.

  1. Name the Physical quantities to be kept constant for Ohm’s law to be true.
  2. State the Principle of a potentiometer. (The students say that potential drop is proportional to length but the constant quantities are not mentioned)
  3. How can we increase the sensitivity of a potentiometer?
  4. Define figure of merit of a galvanometer.
  5. Which has more resistance – a galvanometer or a milliammeter?
  6. How does an LED emit light?
  7. What is the difference between an ordinary diode and an LED?
  8. Define principal axis of a convex lens?
  9. What happens to the focal length of a concave mirror if it is immersed in water?
  10. What are the factors affecting the intrernal resistance of a cell?
  11. What are the difference between primary and secondary cell?
  12. Why can’t we use a dry cell for starting a car?


  1. length, area of cross section,temperature
  2. The potential drop across any length of a conductor of uniform cross section and composition carrying a constant current is directly proportional to the length.
  3. increasing the length of potentiometer wire, decreasing the current, decreasing the potential gradient
  4. Current for unit deflection
  5. galvanometer
  6. The energy released during recombination of electrons and holes across the junction is responsible for the release of light by LED
  7. In ordinary diode the energy emitted during recombination of electrons and holes is in the invisible region of the em spectrum but in the case of LED, the energy is in the visible region.
  8. Straight line joining the centres of curvature of the lens..
  9. The focal length of mirror does not change by changing the medium. Their is a pure geometrical relationship between the radius of curvature and focal length and the relation does not include any term depending on refractive index.
  10. the nature of electrolyte, the concentration of electrolyte, temperature, distance between electrodes, area of electrodes
  11. Primary cell cannot be recharged, secondary cells can be recharged; secondary cells have less internal resistance than primary cells.
  12. Due to the high internal resistance of a dry cell, it won’t be able to provide the current sufficient to start the car

Principle of Homogenity of Dimensions

Binay Sharma asked:

“Explain the applications of the Principle of homogenity of Dimensions and its importance”

Principle of homogenity of dimensions states that “For an equation to be dimansionally correct, the dimensions of each term on LHS must be equal to the dimensions of each term on RHS.”


1. To determine the dimensions of an unknown quantity in an equation

2. To check the accuracy of an equation

3. To derive a formula connecting the given (or assumed) physical quantities

4. To convert a physical quantity from one system of unit to other

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