General knowledge of electrostatic instruments
Electrostatic instruments usually work under the conditions of high voltage (on the order of 104V) and micro-electricity (on the order of 10-6 coulombs), so the requirements for insulation performance are very high. It can be said that the key to the success or failure of the electrostatic experiment is the insulation problem. The purpose of insulation is to minimize the dissipation of charge.
1. Dryness: Moisture can invalidate many electrostatic instruments. Because water molecules (and some ions in the air) adhere to the surface of the insulator, the insulation performance will be greatly reduced. This is especially obvious for certain hydrophilic substances. For example, when glass is moved from an environment with a relative humidity of 50% to an environment with a relative humidity of 80%, the surface resistance of the glass drops to one 75% of its original value. In a classroom full of students, the relative humidity can easily reach over 85%. This is why some instruments work well when preparing for class, but they are not easy to use when they arrive in class. The simplest way to dry the environment is to ventilate.
It is generally believed that the relative humidity not greater than 70% is a condition for the normal operation of electrostatic instruments. Ventilation often cannot guarantee this, so it is necessary to dry the surface of the instrument. No matter what heat source is used for drying, the surface temperature of the instrument should not exceed 50°C (it feels hot to the touch without being hot). The method of ventilation while baking can be adopted to make the instrument dry as soon as possible and shorten the drying time as much as possible.
2. Cleaning: Dust, fiber and sweat stains on the surface of the instrument will cause a lot of tiny dust, causing the electric charge to flow directly from the dust through the corona discharge to the air. Therefore, during use, the surface of the instrument should be cleaned carefully, and do not touch it directly with your hands.
3. When the instrument is used in conjunction with a high-voltage power supply such as a motor, it should be kept as far away as possible to prevent excessive gas ions caused by the high-voltage power supply during the discharge process from affecting the charged state of the instrument.
4. If you need to connect two instruments with a conductor in the experiment, it is best to use an electroscope connecting rod or a discharge fork. If you have to use a wire, you should also hang the wire with insulated wires such as nylon wire.
5. When observing the instrument, do not exhale directly into the instrument.
6. High-molecular insulating materials are easy to age, and the influence of ultraviolet and infrared radiation is *obvious. The aging of hard rubber products is firstly the formation of a greenish-brown film on the surface. At this time, the static electricity insulation has failed. Wipe off the film gently with fine sandpaper dipped in alcohol, and it can be reused. The aging of plastic is embrittlement and cannot be repaired. Therefore, the instrument should be kept away from direct sunlight and away from heat sources.
7. Selection of insulating materials: The fundamental reason for the poor use of a considerable number of electrostatic instruments is improper selection of insulating materials or improper handling. For example: Bakelite (phenolic resin) is an excellent insulating material in low-voltage electrical appliances, but it is not suitable for use in electrostatic equipment. Because bakelite powder often contains trace metal impurities. Some materials have good insulation properties, but some dyes are added in order to be beautiful or distinguish parts during the manufacturing process, which often reduces the insulation properties of the materials. Therefore, in addition to choosing excellent insulating properties, do not add dyes to the insulating materials of electrostatic instruments.
Among the commonly used insulating materials, polystyrene and rigid polyvinyl chloride have poor mechanical strength and are easy to age; low-pressure polyethylene and polypropylene have good toughness, but they are also easy to age; glass is anti-aging, but the surface is easy to moisture (strong hydrophilicity); Although polytetrafluoroethylene and industrial plexiglass are more expensive, they have better performance in all aspects. It is an ideal electrostatic insulating material.