AC is short for alternating current. This means that the direction of current flowing in a circuit is constantly being reversed back and forth. This is done with any type of AC current/voltage source.The electrical current in your house is alternating current.Alternating Current (AC) flows one way, then the other way, continually reversing direction.An AC voltage is continually changing between positive (+) and negative (-).The rate of changing direction is called the frequency of the AC and it is measured in hertz (Hz) which is the number of forwards-backwards cycles per second.Mains electricity in the India has a frequency of 50Hz.An AC supply is suitable for powering some devices such as lamps and heaters.

                 Properties of electrical signals

         An electrical signal is a voltage or current which conveys information, usually it means a voltage. The term can be used for any voltage or current in a circuit.

The voltage-time graph on the right shows various properties of an electrical signal. In addition to the properties labelled on the graph, there is frequency which is the number of cycles per second.

The diagram shows a sine wave but these properties apply to any signal with a constant shape.

  • Amplitude is the maximum voltage reached by the signal.

    It is measured in volts, V.

  • Peak voltage is another name for amplitude.

  • Peak-peak voltage is twice the peak voltage (amplitude). When reading an oscilloscope trace it is usual to measure peak-peak voltage.

  • Time period is the time taken for the signal to complete one cycle.

    It is measured in seconds (s), but time periods tend to be short so milliseconds (ms) and microseconds (µs) are often used. 1ms = 0.001s and 1µs = 0.000001s.

  • Frequency is the number of cycles per second.

    It is measured in hertz (Hz), but frequencies tend to be high so kilohertz (kHz) and megahertz (MHz)are often used. 1kHz = 1000Hz and 1MHz = 1000000Hz.

    frequency  =  



    time period  =  


    time period


    Mains electricity in the India has a frequency of 50Hz,
    so it has a time period of 1/50 = 0.02s = 20ms.

             Root Mean Square (RMS) Values

The value of an AC voltage is continually changing from zero up to the positive peak, through zero to the negative peak and back to zero again. Clearly for most of the time it is less than the peak voltage, so this is not a good measure of its real effect.

Instead we use the root mean square voltage (VRMS) which is 0.7 of the peak voltage (Vpeak):

VRMS = 0.7 × Vpeak   and   Vpeak = 1.4 × VRMS

These equations also apply to current.
They are only true for sine waves (the most common type of AC) because the 0.7 and 1.4 are different values for other shapes.

The RMS value is the effective value of a varying voltage or current. It is the equivalent steady DC (constant) value which gives the same effect.

                        AVERAGE VALUE

The AVERAGE value of an alternating current or voltage is the average of ALL the INSTANTANEOUS values during ONE alternation. Since the voltage increases from zero to peak value and decreases back to zero during one alternation, the average value must be some value between those two limits. You could determine the average value by adding together a series of instantaneous values of the alternation (between 0° and 180°), and then dividing the sum by the number of instantaneous values used. The computation would show that one alternation of a sine wave has an average value equal to 0.636 times the peak value. The formula for average voltage is

Eavg = 0.636 X Emax

where Eavg is the average voltage of one alternation, and Emax is the maximum or peak voltage. Similarly, the formula for average current is

Iavg = 0.636 X Imax

where Iavg is the average current in one alternation, and Imax is the maximum or peak current.

Do not confuse the above definition of an average value with that of the average value of a complete cycle. Because the voltage is positive during one alternation and negative during the other alternation, the average value of the voltage values occurring during the complete cycle is zero.

                          PEAK VALUES

During each complete cycle of ac there are always two maximum or peak values, one for the positive half-cycle and the other for the negative half-cycle. The difference between the peak positive value and the peak negative value is called the peak-to-peak value of the sine wave. This value is twice the maximum or peak value of the sine wave and is sometimes used for measurement of ac voltages.

      Advantages of AC electrical current over DC

  •  AC electrical distribution systems can easily allow changes in voltage using transformers. This gives AC electrical distribution systems a great advantage compared to DC.

  • By using transformers, AC power can be changed up to very high voltages for transmission and then changed down again to safer voltages for consumers to use, with comparatively very much less loss of power than DC.

  • AC has less copper loss (more heating effect) than DC and it’s easier and cheaper to produce too.

  • AC is usually used for transmission because DC cannot be run through a transformer.

  • Edison’s original system was DC. It required many power generating stations because the voltage couldn’t be stepped up/down like AC. This is why Westinghouse’s (Well, Tesla’s IIRC. Westinghouse bought it.) AC system lasted in the long run.

  • Sometimes high voltage DC is used for transmission, but it requires special equipment at both ends.

  • Semiconductor electronics require low voltage DC so your electronic devices have power supplies that change the high voltage AC to low voltage DC.

  • A battery used to store electricity can only store and deliver DC. This is why the basic electrical system used in vehicles is DC.