The question of plus and minus in a mains socket is repeated quite often, it is quite natural to associate the two poles of the socket and the two colors of the wires with the battery but wrong . In the case of a cell or battery, we have direct voltage (DC), the mains supply is alternating voltage (AC). You can find more about direct and alternating current here: AC vs DC .
In the network socket you will most often find: 1. Blue neutral wire (N) connected to ground potential, reference. 2. Phase wire (L) usually brown or black, red, gray, white - this is the second work wire on which the voltage changes with respect to the N wire. 3. The protective conductor (PE), usually yellow-green, which is grounded and connected to the protective pin in the socket, is an important element of electric shock protection.
Figure 1. How to connect the socket. L and N terminals may be interchanged.
The shape of the voltage changes with time on the conductor L is similar to a sinusoid. Value effective tension in Poland is 230 V. The frequency of the mains voltage in Poland is 50 Hz, i.e. within 1 second, the voltage on the L conductor changes its polarity "passing through zero" and reaching alternately voltage -325 V and +325 V (maximum voltage, amplitude sine wave). Or else, one cycle takes 0.02 seconds.
Figure 2. Line voltage waveform - voltage of the L (brown) wire in relation to the N (blue) wire.
Attention! The PE protective conductor connected to the metal parts of the device housing protects against the appearance of dangerous voltage on these conductive parts. In older buildings, it is possible to find an installation without a yellow-green PE wire, then the protective pins and the N terminal are connected to a common neutral wire. The subject of old installations is difficult, not every two-wire installation can combine the protective contact with a blue or black wire. You need to know your network layout.
In home installations, you can find circuits with DC power supplies, most often in the popular LED lighting. The power supplies are connected to an alternating mains voltage and provide direct current, the voltage at the terminals is from several to several dozen volts. What colors will be associated with "+" and "-"? The terms + and - are incorrect because it should be used in a symmetrical supply (there are two voltages, e.g.. +24 and -24 relative to the mass of the system). In DC voltage circuits, most often we encounter GND (i.e. the mass of the system), which is often somewhat incorrectly marked as "-" and the second terminal "+" where a specific voltage will be present in relation to GND. In this type of installations, the "+" wire will most often be red or, less often, yellow, while the GND wire insulation will be black or blue. Sometimes both wires are insulated with the same color (e.g. white), while the "+" wire has a strip, e.g. red. In addition to voltage power supplies, you can also find current power supplies, which, instead of a specific constant voltage, maintain a constant current, e.g. for LEDs. In the case of DC current power supplies, the polarity is also important. Current type power supply, how is it different from voltage supply?
[line: d0b7b15bc5] Please do not report the topic. This is a simple tutorial for those looking for basic information on AC and DC.