The Van de Graaff Generator: Fig. 1 is a schematic diagram of a Van de Graaff ge

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The Van de Graaff Generator: Fig. 1 is a schematic diagram of a Van de Graaff generator. The high voltage terminal T is spherical in shape and is supported on an insulating column C. Charge is conveyed to it by an endless belt B running between upper and lower pulleys P_1 and P_2. The upper pulley is situated in the field-free space inside the metal terminal. Similarly, the lower pulley is located inside a grounded metal box. Charge is sprayed on the belt by the lower corona points S_2 and is then removed by the upper points S_1 located inside the terminal. An essential feature is that belt charging and charge removal are accomplished within field-free spaces. These functions are then independent of the potential difference that may exist between the high voltage terminal and ground. The Van de Graaff used in this lab is a small commercial model. Corona Discharge: Set the Van de Graaff into operation with the speed control in the counter clockwise (slow) position. Slowly turn the knob clockwise until the belt is running smoothly. When a sharp grounded object is brought near the terminal, a blue glow called a corona discharge will emanate from the object toward the sphere. For example, the grounded object can be a clip lead, one end of which is connected to the base of the Van de Graaff. The effect may also be noted if a finger is pointed in the direction of the terminal. You may wish to grasp a pointed conducting object, such as a screwdriver, and point that at the terminal rather than your bare finger. If a spark jumps directly onto you it may cause some discomfort. If the spark jumps onto a metal object which you are grasping firmly, the area of contact is large enough so that there is not much sensation. The corona discharge is a consequence of current flowing through ionized air which acts as a conductor. Ionization is the result of a high voltage gradient that surrounds the point of a sharp object and strips electrons from the air molecules leaving charged ions behind. The blue is the characteristic color of the light emitted when ionized air molecules recombine with electrons to form neutral molecules. Since the Van de Graaff terminal is negatively charged, electrons flow away from it and toward the point. It may be necessary to darken the room in order to observe the effect. Another manifestation of corona discharge is the electric wind. Attach a pin or needle to the terminal surface with a little clay. Turn the Van de Graaff on and when you bring your hand close to the pin you will feel a wind coming from it.

Explanation / Answer

As written above how corona discharge generated. We know that electric charge resides on the surface of conductor. Also charge is more concentrated on sharp points rather than flat surface.

Now here a needle ( as needle have sharp tip so charge will be concentrated on its tip) is brought near the Van de Graff then charge will induce on the needle. As needle has sharp tip so charge will be concentrated on tip, this charge will create high electric field. Now if this electric field magnitude exceeds the corona discharge inception voltage, then it ionises air around it. As this air have same polarity as that of needle. So this air will be repelled by needle. Also air molecules have same polarity so they repel each other. Due to this air will expand very rapidly. This ionises air emanates from the tip of needle called electric wind. This electric wind is followed by hissing noise.

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