The negative half cycle flows from the neutral line to the live wire. Why does the live wire always come on when using a test pen?
The reason why the live wire is lit up when measured with a test pen is because the ground voltage of the live wire is 220V. During the measurement, the live wire passes through the resistance inside the pen and is grounded to the human body. At this time, there will be current flowing through, so the light will turn on. And the zero line has a ground voltage of zero volts, because the zero line itself is connected to the earth, and it is equipotential with the earth you are in, so there is no potential difference and no current can be formed, so it naturally does not light up.
Why is the zero line zero? It is because it is grounded, and the specified ground potential is zero, so the natural zero line potential is zero. This is also the reason why the human body won't get an electric shock when standing on the ground and grabbing the neutral wire.
The neutral wire is the line drawn from the neutral point of a transformer. It is the circuit from the live wire to the electrical appliance. When the neutral wire is drawn from the midpoint of the transformer, it will be grounded. Therefore, you can consider the neutral wire as the ground where the transformer is located in the distance, while the ground wire in your home is the protective ground where your home is located. Because the earth also has resistance, sometimes the neutral wire in the home can also be charged, but the voltage is very low and will not cause personal danger.
The potential of the positive half cycle live wire is higher than that of the neutral wire, and the potential of the negative half cycle live wire is lower than that of the neutral wire. Water flows towards the lower part, and the current is the same. Here, we need to correct a misconception. Many people believe that current can only flow from the positive electrode to the negative electrode, but in fact, current also flows from the positive electrode with a higher potential to the positive electrode with a lower potential
The electric potential mentioned here may not be understood by the person asking this question, and it can also be understood as voltage. In fact, it is not the same concept. Voltage is a potential difference. Assuming that electric potential is height, voltage is height difference. For example, there are two reservoirs on the mountain, one is 100 meters (electric potential) and the other is 150 meters (electric potential), and their height difference (voltage) is 50 meters. If the two reservoirs are connected, the higher water will flow towards the lower water. If they are both 150 meters (electric potential), it is impossible for them to flow (voltage is zero), even if their height (electric potential) is very high
If you understand above, the potential of the neutral wire is assumed to be at the well surface, with a fixed height, while the live wire is a water bucket filled with water, with a variable height. When the water bucket is above the well surface, the water will flow towards the well surface, and vice versa
Why does the measuring pen light up on the live wire but not on the neutral wire? It is because the top of the measuring pen needs to be connected to the human body and then connected to the ground, and the neutral wire is also connected to the ground, with the same high potential
