Several problems worthy of attention should be used "electric test pen"
Junior high school physics and electricity In "living electricity", students are required to know the function of the measuring pen and how to use it. Measuring pen is mainly used to distinguish whether the power transmission line is live or neutral. It is often made into a screwdriver or a pen. When in use, the hand should be in contact with the metal body at the end of the pen or the "pen clip", and the tip of the pen should be in contact with the wire to be tested or the conductor connected to the wire. If the neon tube glows, it means that it is in contact with the live wire; if the neon tube does not glow, it means that it is in contact with the neutral wire. When using the test pen, the conductor, the metal body of the pen tip, the resistor, the neon tube, the spring, the metal body of the pen tail and the earth form a current path. Therefore, a weak current flows through the neon tube and the human body, and the neon tube glows. However, in practical application, there are several issues worthy of attention:
1 Nib contact is live. Neon tubes don't glow
Touch the live wire with a test pen, and some students can't observe that the neon tube lights up. Possible reasons:
(1) The tip of the pen is not in good contact or the hand does not touch the metal body at the end of the pen at all. At this time, no path is formed, and the neon tube does not emit light.
(2) The neon tube or resistor inside the test pencil is damaged, causing an open circuit and the neon tube does not emit light.
(3) The light outside is too strong, and the neon tube emits light weakly. Students do not observe carefully and mistakenly think that the neon tube does not emit light.
2 The nib contact is the zero line. Neon tubes glow
When testing with a test pen, some students' pen tip is in contact with the neutral line, and the neon tube is bright. Why does this happen? The reason:
(1) The switch of the electrical appliance is connected to the neutral line or the remote end of the neutral line is disconnected. At this time, the line that should be connected to the neutral line is connected to the live line through an electrical appliance, so that the neon tube glows.
(2) There is a capacitor near the circuit. If there are electrical appliances with a lot of capacitance in the circuit (such as TV sets, refrigerators, etc.), charge the capacitors when the electrical appliances are in use. When the measuring pen touches the conductor, the capacitor discharges to the outside, making the neon tube glow.
(3) It is affected by static electricity and induced electricity. According to relevant statistics, in dry weather, the friction of sweaters on students' bodies can generate a voltage of 5,000 V or higher when they are walking. This phenomenon is less likely.
3 It is easy to misjudge the voltage level of the electric pen
Judge the voltage level in the circuit according to the brightness of the electric pen. The higher the brightness of the test pencil, the greater the voltage? The author has done an interesting experiment: take a test pencil and touch the same live wire, and the neon tube is much brighter when people stand on the ground than when they stand on a dry wooden bench. This shows that the brightness of the measuring pencil is not only related to the size of the voltage, but also related to the measurement conditions and environment. Therefore, the brightness of the measuring pencil is not directly proportional to the magnitude of the voltage. As for the person standing on a dry wooden bench, why does the neon tube still glow? The author thinks that it is affected by the surrounding air. There is a certain amount of water vapor containing dust and other impurities in the air, so the current passes through the human body and the surrounding air to form a current, which is sensitive. The neon tube emits a weak light when a very small current flows through it. And the more humid the air, the greater the brightness of the neon tube.
