Van De Graff generator is used to generate high potential differences of near about million volts. Then the generated potential differences are used to speed up the particles like ions etc. Two main principles of Van De Graff Generator are given below: • It is designed on the basis of the property that the charge given to a conductor conductor which is hollow inside, is transferred to the outer surface and scatters uniformly over it. • It is founded on the phenomenon of Corona Discharge.
Construction: There is a large spherical conductor having a few meters of radius. An insulating belt similar to conveyor belt moves on the two pulleys. Two combs , 1 is the spray comb which is responsible for spraying of charge and the other is collecting comb for collecting charges are named B1 and B2 respectively. These combs work on the principle of corona discharge or action of sharp points. Acceleration of ions take place in the discharge tube D. The whole apparatus is placed in a steel compartment. Compartment is filled with Nitrogen or Methane. The pressure inside the chamber is maintained very high. Working: The system is started by giving positive potential to B1 by a high tension source of 104V .Electric wind having a positive charge will be produced. Production of wind occurs due to the discharging di scharging of charge from the sharp edges. Belt moves continuously and will reach the sphere on moving. When the belt will reach the sphere an induced negative charge will be produced on the sharp edges of the comb B2. At once a induced positive charge will be produced on the other side of the comb B2.It will be transferred to sphere S. Similarly, due to the discharging action of the B2,
wind will be produced but this will be negatively charged. The negatively charged wind helps to make the positively charged belt neutral. After this the belt will be totally discharged. Again after rotation, the belt will come down. It will take the positive charge from the comb B1. Then again this charge will be taken by the comb B2. The above process repeats again and again. Due to the repetition the charge will start collecting on the sphere S. Charge carrying capacity of the sphere will be shown with the formula.= 4π ε0 R
Corona discharge is a process by which a current f lows from an electrode with a high potential into a neutral fluid, usually air, by ionizing that fluid so as to create a region of plasma around the electrode. The ions generated eventually pass charge to nearby areas of lower potential, or recombine to form neutral gas molecules. When the potential gradient (electric field) is large enough at a point in the fluid, the fluid at that point ionizes and it becomes conductive. If a charged object has a sharp point, the electric field strength around that point will be much higher than elsewhere. Air near the electrode can become ionized (partially conductive), while regions more distant do not. When the air near the point becomes conductive, it has the effect of increasing the apparent size of the conductor. Since the new conductive region is less sharp, the ionization may not extend past this local region. Outside this region of ionization and conductivity, the charged particles slowly find their way to an oppositely charged object and are neutralized. If the geometry and gradient are such that the i onized region continues to grow until it reaches another conductor at a lower potential, a low resistance conductive path between the two will be formed, resulting in an electric arc. Corona discharge usually forms at highly curved regions on electrodes, such as sharp corners, projecting points, edges of metal surfaces, or small diameter wires. The high curvature causes a high potential gradient at these locations, so that the air breaks down and forms plasma there first. In order to suppress corona formation, terminals on high voltage equipment are frequently designed with smooth large diameter rounded shapes like balls or toruses, and corona rings are often added to insulators of high voltage transmission li nes. Coronas may be positive or negative. This is determined by the polarity of the voltage on the highly-curved electrode. If the curved electrode is positive wit h respect to the flat electrode, it has a positive corona, if it negative, It it has a negative corona . (See below for more details.) The physics of positive and negative coronas are strikingly different. This asymmetry is a result of the great difference in mass between electrons and positively charged ions, with only the electron having the ability to undergo a significant degree of ionising inelastic collision at common temperatures and pressures.
Problems caused by corona discharges [edit] Coronas can generate audible and radio-frequency noise, particularly near electric power transmission lines. They also represent a power loss, and their action on atmospheric particulates, along with associated ozone and NOx production, can also be disadvantageous to human health where power lines run through built-up areas. Therefore, power transmission equipment is designed to minimise the formation of corona discharge. Corona discharge is generally undesirable in:
Electric power transmission, where it causes: Power loss o Audible noise o Electromagnetic interference o Purple glow o Ozone production o Insulation damage o Electrical components such as transformers, capacitors, electric motors and generators. Corona progressively damages the insulation inside these devices, leading to premature equipment failure. One form of attack is ozone cracking of elastomer items like O-rings. Situations where high voltages are in use, but ozone production is to be minimised Static electricity discharge
