In the AC (Alternating Current) power supply, the direction of current flow changes periodically (for every fixed time length). For example, the current flow is in opposite direction, for every 1/100th second, for 50 Hz and 1/120th second, for 60 Hz power supplies. The power supply waveforms are sinusoidal in general and the model sinusoidal wave form (also called as sine wave) is shown here.
Why AC ?
Easy and convenient Power Transmission for long distances.
For the AC power supply, the voltages may be easily stepped-up or stepped-down, also economically, using transformer, which requires complex circuits for DC power supply.
AC generators are simpler in construction and maintenance than DC generators.
Rotating magnetic field, which is required for motors, can be easily generated with AC power supply, compared to DC power supply. This simplifies construction of motors.
Conversion process of AC to DC is simpler, which requires only rectifiers. Whereas conversion of DC to AC requires complex circuit(s).
110V or 220V ?
Some countries distribute 110V AC with 60Hz, whereas other countries distribute 220V AC with 50Hz, as power supply to houses and industries. As the voltage increases, the severity of electric shock increases and more dangerous when touched to the live wire(s). Our body resistance reduces with skin wetness. So, most of the cold counties adopt 110VAC to reduce the risk of electric shock at higher voltage.
For the same power requirement, for higher voltage, less current is consumed, whereas for lower voltage, higher current is consumed. The cross-section of wire requirement depends on quantity of current flow, which is directly proportional to it. So, for 110V power supply, the wire diameter is more (almost double cross-section), when compared to 220V power supply, for the same power requirement.
What is RMS ?
In the DC power supply, the Voltage or Current can be directly indicated, since the DC power supply is steady. Whereas in AC power supply, the Voltage or Current is periodically reversed. So, we can measure only Peak values, with special measuring equipment(s). So, in AC circuits, RMS values are used for measurement.
The word RMS stands for Root Mean Square value. The RMS value shall be applied to Alternating Current or Alternating Voltage in AC power supplies only. For example, the RMS value of AC is equivalent to actual DC value. i.e., the power generated or power consumed using a DC power supply is equal to RMS AC power supply. So, RMS value is always less than the Peak value (Voltage or Current).
Most of the electronic circuits does not require mains AC power supply as power source. Some circuits control the mains operated appliances, like motors, lighting system etc.
Touching the mains supply is dangerous and lethal. So, care should be taken, while working on mains power supply.